draft-ietf-httpauth-digest-09.txt   draft-ietf-httpauth-digest-10.txt 
HTTPAuth Working Group R. Shekh-Yusef, Ed. HTTPAuth R. Shekh-Yusef, Ed.
Internet-Draft Avaya Internet-Draft Avaya
Obsoletes: 2617 (if approved) D. Ahrens Obsoletes: 2617 (if approved) D. Ahrens
Intended Status: Standards Track Independent Intended status: Standards Track Independent
Expires: June 12, 2015 S. Bremer Expires: July 14, 2015 S. Bremer
Netzkonform Netzkonform
December 9, 2014 January 10, 2015
HTTP Digest Access Authentication HTTP Digest Access Authentication
draft-ietf-httpauth-digest-09 draft-ietf-httpauth-digest-10
Abstract Abstract
HTTP provides a simple challenge-response authentication mechanism HTTP provides a simple challenge-response authentication mechanism
that may be used by a server to challenge a client request and by a that may be used by a server to challenge a client request and by a
client to provide authentication information. This document defines client to provide authentication information. This document defines
the HTTP Digest Authentication scheme that may be used with the the HTTP Digest Authentication scheme that can be used with the HTTP
authentication mechanism. authentication mechanism.
Status of this Memo Editorial Note (To be removed by RFC Editor before publication)
This Internet-Draft is submitted to IETF in full conformance with the Discussion of this draft takes place on the HTTPAuth working group
mailing list (http-auth@ietf.org), which is archived at [1].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Copyright and License Notice Copyright Notice
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Table of Contents Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2 Syntax Convention . . . . . . . . . . . . . . . . . . . . . . . 4 2. Syntax Convention . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Algorithm Variants . . . . . . . . . . . . . . . . . . . . . 4 2.2. ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Digest Access Authentication Scheme . . . . . . . . . . . . . 4
3 Digest Access Authentication Scheme . . . . . . . . . . . . . . 5 3.1. Overall Operation . . . . . . . . . . . . . . . . . . . . 4
3.1 Overall Operation . . . . . . . . . . . . . . . . . . . . . 5 3.2. Representation of Digest Values . . . . . . . . . . . . . 4
3.2 Representation of Digest Values . . . . . . . . . . . . . . 5 3.3. The WWW-Authenticate Response Header . . . . . . . . . . 5
3.3 The WWW-Authenticate Response Header . . . . . . . . . . . . 5 3.4. The Authorization Request Header Field . . . . . . . . . 8
3.4 The Authorization Request Header . . . . . . . . . . . . . . 9 3.4.1. Response . . . . . . . . . . . . . . . . . . . . . . 10
3.4.1 Response . . . . . . . . . . . . . . . . . . . . . . . . 10 3.4.2. A1 . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4.2 A1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.4.3. A2 . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4.3 A2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4.4. Username Hashing . . . . . . . . . . . . . . . . . . 11
3.4.4 Username Hashing . . . . . . . . . . . . . . . . . . . . 11 3.4.5. Parameter Values and Quoted-String . . . . . . . . . 11
3.4.5 Parameter Values and Quoted-String . . . . . . . . . . . 12 3.4.6. Various Considerations . . . . . . . . . . . . . . . 12
3.4.6 Various Considerations . . . . . . . . . . . . . . . . . 12 3.5. The Authentication-Info Header . . . . . . . . . . . . . 13
3.5 The Authentication-Info Header . . . . . . . . . . . . . . . 13 3.5.1. Digest Usage of Authentication-Info . . . . . . . . . 13
3.5.1 Digest Usage of Authentication-Info . . . . . . . . . . 14 3.6. Digest Operation . . . . . . . . . . . . . . . . . . . . 15
3.6 Digest Operation . . . . . . . . . . . . . . . . . . . . . . 15 3.7. Security Protocol Negotiation . . . . . . . . . . . . . . 16
3.7 Security Protocol Negotiation . . . . . . . . . . . . . . . 16 3.8. Proxy-Authenticate and Proxy-Authorization . . . . . . . 16
3.8 Proxy-Authenticate and Proxy-Authorization . . . . . . . . . 17 3.9. Examples . . . . . . . . . . . . . . . . . . . . . . . . 17
3.9 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.9.1. Example with SHA-256 and MD5 . . . . . . . . . . . . 17
3.9.1 Example with SHA-256 and MD5 . . . . . . . . . . . . . . 17 3.9.2. Example with SHA-512-256, Charset, and Userhash . . . 19
3.9.2 Example with SHA-512-256, Charset, and Userhash . . . . 19
4 Internationalization . . . . . . . . . . . . . . . . . . . . . . 20
5 Security Considerations . . . . . . . . . . . . . . . . . . . . 21
5.1 Limitations . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2 Storing passwords . . . . . . . . . . . . . . . . . . . . . 21
5.3 Authentication of Clients using Digest Authentication . . . 22
5.4 Limited Use Nonce Values . . . . . . . . . . . . . . . . . . 22
5.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . . 23
5.6 Weakness Created by Multiple Authentication Schemes . . . . 24
5.7 Online dictionary attacks . . . . . . . . . . . . . . . . . 24
5.8 Man in the Middle . . . . . . . . . . . . . . . . . . . . . 24
5.9 Chosen plaintext attacks . . . . . . . . . . . . . . . . . . 25
5.10 Precomputed dictionary attacks . . . . . . . . . . . . . . 25
5.11 Batch brute force attacks . . . . . . . . . . . . . . . . . 26
5.12 Spoofing by Counterfeit Servers . . . . . . . . . . . . . . 26
5.13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 27
6 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 28
6.1 HTTP Digest Hash Algorithms Registry . . . . . . . . . . . 28
6.2 Digest Scheme Registration . . . . . . . . . . . . . . . . 28
6.3 Authentication-Info Header Registration . . . . . . . . . . 29
7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 29
8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 30
8.2 Informative References . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
1 Introduction 4. Internationalization . . . . . . . . . . . . . . . . . . . . 20
5. Security Considerations . . . . . . . . . . . . . . . . . . . 20
5.1. Limitations . . . . . . . . . . . . . . . . . . . . . . . 20
5.2. Storing passwords . . . . . . . . . . . . . . . . . . . . 21
5.3. Authentication of Clients using Digest Authentication . . 21
5.4. Limited Use Nonce Values . . . . . . . . . . . . . . . . 22
5.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 23
5.6. Weakness Created by Multiple Authentication Schemes . . . 23
5.7. Online dictionary attacks . . . . . . . . . . . . . . . . 24
5.8. Man in the Middle . . . . . . . . . . . . . . . . . . . . 24
5.9. Chosen plaintext attacks . . . . . . . . . . . . . . . . 25
5.10. Precomputed dictionary attacks . . . . . . . . . . . . . 25
5.11. Batch brute force attacks . . . . . . . . . . . . . . . . 25
5.12. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 26
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
6.1. HTTP Digest Hash Algorithms Registry . . . . . . . . . . 26
6.2. Digest Scheme Registration . . . . . . . . . . . . . . . 27
6.3. Authentication-Info Header Registration . . . . . . . . . 27
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1. Normative References . . . . . . . . . . . . . . . . . . 28
8.2. Informative References . . . . . . . . . . . . . . . . . 29
1. Introduction
HTTP provides a simple challenge-response authentication mechanism HTTP provides a simple challenge-response authentication mechanism
that may be used by a server to challenge a client request and by a that may be used by a server to challenge a client request and by a
client to provide authentication information. This document defines client to provide authentication information. This document defines
the HTTP Digest Authentication scheme that may be used with the the HTTP Digest Authentication scheme that can be used with the HTTP
authentication mechanism. authentication mechanism.
The details of the challenge-response authentication mechanism are The details of the challenge-response authentication mechanism are
specified in the [RFC7235] document. specified in the "Hypertext Transfer Protocol (HTTP/1.1):
Authentication" [RFC7235].
The combination of this document with Basic [BASIC] and [RFC7235] The combination of this document with the definition of the "Basic"
obsolete RFC2617. authentication scheme [BASIC] and [RFC7235] obsolete RFC 2617.
1.1 Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2 Syntax Convention 2. Syntax Convention
2.1 Examples 2.1. Examples
In the interest of clarity and readability, the extended parameters In the interest of clarity and readability, the extended parameters
or the header fields and parameters in the examples in this document or the header fields and parameters in the examples in this document
might be broken into multiple lines. Any line that is indented in might be broken into multiple lines. Any line that is indented in
this document is a continuation of the preceding line. this document is a continuation of the preceding line.
2.2 Algorithm Variants 2.2. ABNF
When used with the Digest mechanism, each one of the algorithms has
two variants: Session variant and non-Session variant.
The non-Session variant is denoted by "<algorithm>", e.g. "SHA-256",
and the Session variant is denoted by "<algorithm>-sess", e.g. "SHA-
256-sess".
2.3 ABNF
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234]. notation of [RFC5234], and the ABNF List Extension of [RFC7230].
3 Digest Access Authentication Scheme 3. Digest Access Authentication Scheme
3.1 Overall Operation 3.1. Overall Operation
The Digest scheme is based on a simple challenge-response paradigm. The Digest scheme is based on a simple challenge-response paradigm.
The Digest scheme challenges using a nonce value, and might indicate The Digest scheme challenges using a nonce value, and might indicate
that username hashing is supported. A valid response contains a that username hashing is supported. A valid response contains a
checksum of the username, the password, the given nonce value, the checksum of the username, the password, the given nonce value, the
HTTP method, and the requested URI. In this way, the password is HTTP method, and the requested URI. In this way, the password is
never sent in the clear, and the username can be hashed, depending on never sent in the clear, and the username can be hashed, depending on
the indication received from the server. The username and password the indication received from the server. The username and password
must be prearranged in some fashion not addressed by this document. must be prearranged in some fashion not addressed by this document.
The security of this protocol is critically dependent on the The security of this protocol is critically dependent on the
randomness of the randomly chosen parameters, such as client and randomness of the randomly chosen parameters, such as client and
server nonces. These should be generated by a strong random or server nonces. These should be generated by a strong random or
properly seeded pseudorandom source (see [RFC4086]). properly seeded pseudorandom source (see [RFC4086]).
3.2 Representation of Digest Values Some or all of the parameters used in the various headers fields used
by this document can be sent using the [RFC5987] encoding.
3.2. Representation of Digest Values
An optional header field allows the server to specify the algorithm An optional header field allows the server to specify the algorithm
used to create the checksum or digest. This documents adds SHA-256 used to create the checksum or digest. This documents adds SHA-256
and SHA-512/256 algorithms. To maintain backwards compatibility with and SHA-512/256 algorithms. To maintain backwards compatibility with
[RFC2617], the MD5 algorithm is still supported but NOT RECOMMENDED. [RFC2617], the MD5 algorithm is still supported but NOT RECOMMENDED.
The size of the digest depends on the algorithm used. The bits in the The size of the digest depends on the algorithm used. The bits in
digest are converted from the most significant to the least the digest are converted from the most significant to the least
significant bit, four bits at a time to the ASCII representation as significant bit, four bits at a time to the ASCII representation as
follows. Each four bits is represented by its familiar hexadecimal follows. Each four bits is represented by its familiar hexadecimal
notation from the characters 0123456789abcdef, that is binary 0000 is notation from the characters 0123456789abcdef, that is binary 0000 is
represented by the character '0', 0001 by '1' and so on up to the represented by the character '0', 0001 by '1' and so on up to the
representation of 1111 as 'f'. If the MD5 algorithm is used to representation of 1111 as 'f'. If the MD5 algorithm is used to
calculate the digest, then the MD5 digest will be represented as 32 calculate the digest, then the MD5 digest will be represented as 32
hexadecimal characters, while SHA-256 and SHA-512/256 are represented hexadecimal characters, while SHA-256 and SHA-512/256 are represented
as 64 hexadecimal characters. as 64 hexadecimal characters.
3.3 The WWW-Authenticate Response Header 3.3. The WWW-Authenticate Response Header
If a server receives a request for an access-protected object, and an If a server receives a request for an access-protected object, and an
acceptable Authorization header field is not sent, the server acceptable Authorization header field is not sent, the server
responds with a "401 Unauthorized" status code and a WWW-Authenticate responds with a "401 Unauthorized" status code and a WWW-Authenticate
header field with Digest scheme as per the framework defined above. header field with Digest scheme as per the framework defined above.
The value of the header includes some or all of the following The value of the header field can include parameters from the
parameters: following list:
realm realm
A string to be displayed to users so they know which username and A string to be displayed to users so they know which username and
password to use. This string should contain at least the name of password to use. This string should contain at least the name of
the host performing the authentication and might additionally the host performing the authentication and might additionally
indicate the collection of users who might have access. An example indicate the collection of users who might have access. An
might be "registered_users@gotham.news.com". (See section 2.2 of example might be "registered_users@gotham.news.com". (See
[RFC7235] for more details). Section 2.2 of [RFC7235] for more details).
domain domain
A quoted, space-separated list of URIs, as specified in RFC 3986 A quoted, space-separated list of URIs, as specified in RFC 3986
[RFC3986], that define the protection space. If a URI is an [RFC3986], that define the protection space. If a URI is an
abs_path, it is relative to the canonical root URL of the web- abs_path, it is relative to the canonical root URL (See
origin. An absolute-URI in this list may refer to a different Section 2.2 of [RFC7235]) of the web-origin [RFC6454]. An
server than the web-origin. The client can use this list to absolute-URI in this list may refer to a different server than the
determine the set of URIs for which the same authentication web-origin. The client can use this list to determine the set of
information may be sent: any URI that has a URI in this list as a URIs for which the same authentication information may be sent:
prefix (after both have been made absolute) may be assumed to be any URI that has a URI in this list as a prefix (after both have
in the same protection space. If this parameter is omitted or its been made absolute) MAY be assumed to be in the same protection
value is empty, the client SHOULD assume that the protection space space. If this parameter is omitted or its value is empty, the
consists of all URIs on the web-origin. All URIs in this list client SHOULD assume that the protection space consists of all
SHOULD use the same scheme (https or http); mixing them is a bad URIs on the web-origin. All URIs in this list SHOULD use the same
idea. scheme (https or http); mixing them is a bad idea.
This parameter is not meaningful in Proxy-Authenticate header This parameter is not meaningful in Proxy-Authenticate header
fields, for which the protection space is always the entire proxy; fields, for which the protection space is always the entire proxy;
if present it should be ignored. if present it MUST be ignored.
nonce nonce
A server-specified data string which should be uniquely generated A server-specified data string which should be uniquely generated
each time a 401 response is made. It is recommended that this each time a 401 response is made. It is advised that this string
string be base64 or hexadecimal data. Specifically, since the be base64 or hexadecimal data. Specifically, since the string is
string is passed in the header field lines as a quoted string, the passed in the header field lines as a quoted string, the double-
double-quote character is not allowed. quote character is not allowed, unless suitably escaped.
The contents of the nonce are implementation dependent. The The contents of the nonce are implementation dependent. The
quality of the implementation depends on a good choice. A nonce quality of the implementation depends on a good choice. A nonce
might, for example, be constructed as the base 64 encoding of might, for example, be constructed as the base 64 encoding of
time-stamp H(time-stamp ":" ETag ":" secret-data) time-stamp H(time-stamp ":" ETag ":" secret-data)
where time-stamp is a server-generated time, which preferably where time-stamp is a server-generated time, which preferably
includes micro or nano seconds, or other non-repeating value, ETag includes micro or nano seconds, or other non-repeating values,
is the value of the HTTP ETag header field associated with the ETag is the value of the HTTP ETag header field associated with
requested entity, and secret-data is data known only to the the requested entity, and secret-data is data known only to the
server. With a nonce of this form a server would recalculate the server. With a nonce of this form a server would recalculate the
hash portion after receiving the client authentication header hash portion after receiving the client authentication header
field and reject the request if it did not match the nonce from field and reject the request if it did not match the nonce from
that header field or if the time-stamp value is not recent enough. that header field or if the time-stamp value is not recent enough.
In this way the server can limit the time of the nonce's validity. In this way the server can limit the time of the nonce's validity.
The inclusion of the ETag prevents a replay request for an updated The inclusion of the ETag prevents a replay request for an updated
version of the resource. Including the IP address of the client in version of the resource. Including the IP address of the client
the nonce would appear to offer the server the ability to limit in the nonce would appear to offer the server the ability to limit
the reuse of the nonce to the same client that originally got it. the reuse of the nonce to the same client that originally got it.
However, that would break proxy farms, where requests from a However, that would break when requests from a single user often
single user often go through different proxies in the farm. Also, go through different proxies. Also, IP address spoofing is not
IP address spoofing is not that hard. that hard.
An implementation might choose not to accept a previously used An implementation might choose not to accept a previously used
nonce or a previously used digest, in order to protect against a nonce or a previously used digest, in order to protect against a
replay attack. Or, an implementation might choose to use one-time replay attack. Or, an implementation might choose to use one-time
nonces or digests for POST or PUT requests and a time-stamp for nonces or digests for POST or PUT requests and a time-stamp for
GET requests. For more details on the issues involved see section GET requests. For more details on the issues involved see
5 of this document. Section 5 of this document.
The nonce is opaque to the client. The nonce is opaque to the client.
opaque opaque
A string of data, specified by the server, which SHOULD be A string of data, specified by the server, which SHOULD be
returned by the client unchanged in the Authorization header field returned by the client unchanged in the Authorization header field
of subsequent requests with URIs in the same protection space. It of subsequent requests with URIs in the same protection space. It
is RECOMMENDED that this string be base64 or hexadecimal data. is RECOMMENDED that this string be base64 or hexadecimal data.
stale stale
A case-insensitive flag indicating that the previous request from A case-insensitive flag indicating that the previous request from
the client was rejected because the nonce value was stale. If the client was rejected because the nonce value was stale. If
stale is TRUE, the client MAY wish to simply retry the request stale is TRUE, the client may wish to simply retry the request
with a new encrypted response, without re-prompting the user for a with a new encrypted response, without re-prompting the user for a
new username and password. The server SHOULD only set stale to new username and password. The server SHOULD only set stale to
TRUE if it receives a request for which the nonce is invalid. If TRUE if it receives a request for which the nonce is invalid. If
stale is FALSE, or anything other than TRUE, or the stale stale is FALSE, or anything other than TRUE, or the stale
parameter is not present, the username and/or password are parameter is not present, the username and/or password are
invalid, and new values MUST be obtained. invalid, and new values MUST be obtained.
algorithm algorithm
A string indicating a pair of algorithms used to produce the A string indicating a pair of algorithms used to produce the
digest and a checksum. If this is not present it is assumed to be digest and a checksum. If this is not present it is assumed to be
"MD5". If the algorithm is not understood, the challenge should be "MD5". If the algorithm is not understood, the challenge SHOULD
ignored (and a different one used, if there is more than one). be ignored (and a different one used, if there is more than one).
When used with the Digest mechanism, each one of the algorithms
has two variants: Session variant and non-Session variant. The
non-Session variant is denoted by "<algorithm>", e.g. "SHA-256",
and the Session variant is denoted by "<algorithm>-sess", e.g.
"SHA-256-sess".
In this document the string obtained by applying the digest In this document the string obtained by applying the digest
algorithm to the data "data" with secret "secret" will be denoted algorithm to the data "data" with secret "secret" will be denoted
by KD(secret, data), and the string obtained by applying the by KD(secret, data), and the string obtained by applying the
checksum algorithm to the data "data" will be denoted H(data). The checksum algorithm to the data "data" will be denoted H(data).
notation unq(X) means the value of the quoted-string X without the The notation unq(X) means the value of the quoted-string X without
surrounding quotes and with quoting slashes removed. the surrounding quotes and with quoting slashes removed.
For "<algorithm>" and "<algorithm>-sess" For "<algorithm>" and "<algorithm>-sess"
H(data) = <algorithm>(data) H(data) = <algorithm>(data)
and and
KD(secret, data) = H(concat(secret, ":", data)) KD(secret, data) = H(concat(secret, ":", data))
For example: For example:
For the "SHA-256" and "SHA-256-sess" algorithms For the "SHA-256" and "SHA-256-sess" algorithms
H(data) = SHA-256(data) H(data) = SHA-256(data)
i.e., the digest is the "<algorithm>" of the secret concatenated i.e., the digest is the "<algorithm>" of the secret concatenated
with a colon concatenated with the data. The "<algorithm>-sess" with a colon concatenated with the data. The "<algorithm>-sess"
algorithm is intended to allow efficient 3rd party authentication algorithm is intended to allow efficient 3rd party authentication
servers; for the difference in usage, see the description in servers; for the difference in usage, see the description in
section 3.4.2. section 3.4.2.
qop qop
This parameter MUST be used by all implementations compliant with This parameter MUST be used by all implementations. It is a
this version of the Digest scheme. It is a quoted string of one or quoted string of one or more tokens indicating the "quality of
more tokens indicating the "quality of protection" values protection" values supported by the server. The value "auth"
supported by the server. The value "auth" indicates indicates authentication; the value "auth-int" indicates
authentication; the value "auth-int" indicates authentication with authentication with integrity protection; see the descriptions
integrity protection; see the descriptions below for calculating below for calculating the response parameter value for the
the response parameter value for the application of this choice. application of this choice. Unrecognized options MUST be ignored.
Unrecognized options MUST be ignored.
charset charset
This is an OPTIONAL parameter that is used by the server to This is an OPTIONAL parameter that is used by the server to
indicate the encoding scheme it supports. indicate the encoding scheme it supports.
userhash userhash
This is an OPTIONAL parameter that is used by the server to This is an OPTIONAL parameter that is used by the server to
indicate that it supports username hashing. Valid values are: indicate that it supports username hashing. Valid values are:
"true" or "false". "true" or "false". Default value is "false".
3.4 The Authorization Request Header For historical reasons, a sender MUST only generate the quoted-string
syntax values for the following parameters: realm, domain, nonce,
opaque, and qop.
The client is expected to retry the request, passing an For historical reasons, a sender MUST NOT generate the quoted-string
Authorization header field line with Digest scheme, which is syntax values for the following parameters: stale and algorithm.
defined according to the framework above. The values of the opaque
and algorithm fields must be those supplied in the WWW-
Authenticate response header field for the entity being requested.
The request includes some or all of the following parameters: 3.4. The Authorization Request Header Field
The client is expected to retry the request, passing an Authorization
header field line with Digest scheme, which is defined according to
the framework above. The values of the opaque and algorithm fields
must be those supplied in the WWW-Authenticate response header field
for the entity being requested.
The request can include parameters from the following list:
response response
A string of the hex digits computed as defined below, which proves A string of the hex digits computed as defined below, which proves
that the user knows a password. that the user knows a password.
username username
The user's name in the specified realm. The user's name in the specified realm.
uri uri
The URI from request-target of the Request-Line; duplicated here The Effective Request URI [RFC7230] from request-target of the
because proxies are allowed to change the Request-Line in transit. Request-Line; duplicated here because proxies are allowed to
change the Request-Line in transit.
qop qop
Indicates what "quality of protection" the client has applied to Indicates what "quality of protection" the client has applied to
the message. Its value MUST be one of the alternatives the server the message. Its value MUST be one of the alternatives the server
indicated it supports in the WWW-Authenticate header field. These indicated it supports in the WWW-Authenticate header field. These
values affect the computation of the response. Note that this is a values affect the computation of the response. Note that this is
single token, not a quoted list of alternatives as in WWW- a single token, not a quoted list of alternatives as in WWW-
Authenticate. Authenticate.
cnonce cnonce
This parameter MUST be used by all implementations compliant with
this version of the Digest scheme. The cnonce value is an opaque This parameter MUST be used by all implementations. The cnonce
quoted ASCII-only string value provided by the client and used by value is an opaque quoted ASCII-only string value provided by the
both client and server to avoid chosen plaintext attacks, to client and used by both client and server to avoid chosen
provide mutual authentication, and to provide some message plaintext attacks, to provide mutual authentication, and to
integrity protection. See the descriptions below of the provide some message integrity protection. See the descriptions
calculation of the rspauth and response values. below of the calculation of the rspauth and response values.
nc nc
This parameter MUST be used by all implementations compliant with
this version of the Digest scheme. The "nc" parameter stands for This parameter MUST be used by all implementations. The "nc"
"nonce count". The nc value is the hexadecimal count of the number parameter stands for "nonce count". The nc value is the
of requests (including the current request) that the client has hexadecimal count of the number of requests (including the current
sent with the nonce value in this request. For example, in the request) that the client has sent with the nonce value in this
first request sent in response to a given nonce value, the client request. For example, in the first request sent in response to a
sends "nc=00000001". The purpose of this parameter is to allow the given nonce value, the client sends "nc=00000001". The purpose of
server to detect request replays by maintaining its own copy of this parameter is to allow the server to detect request replays by
this count - if the same nc value is seen twice, then the request maintaining its own copy of this count - if the same nc value is
is a replay. See the description below of the construction of the seen twice, then the request is a replay. See the description
response value. below of the construction of the response value.
userhash userhash
This OPTIONAL parameter is used by the client to indicate that the This OPTIONAL parameter is used by the client to indicate that the
username has been hashed. Valid values are: "true" or "false". username has been hashed. Valid values are: "true" or "false".
Default value is "false".
For historical reasons, a sender MUST only generate the quoted-string
syntax for the following parameters: username, realm, nonce, uri,
response, cnonce, and opaque.
For historical reasons, a sender MUST NOT generate the quoted-string
syntax for the following parameters: algorithm, qop, and nc.
If a parameter or its value is improper, or required parameters are If a parameter or its value is improper, or required parameters are
missing, the proper response is 400 Bad Request. If the request- missing, the proper response is a 4xx error code. If the response is
digest is invalid, then a login failure should be logged, since invalid, then a login failure SHOULD be logged, since repeated login
repeated login failures from a single client may indicate an attacker failures from a single client may indicate an attacker attempting to
attempting to guess passwords. The server implementation should be guess passwords. The server implementation SHOULD be careful with
careful with the information being logged so that it won't put a the information being logged so that it won't put a cleartext
cleartext password (e.g. entered into the username field) into the password (e.g. entered into the username field) into the log.
log.
The definition of response above indicates the encoding for its The definition of the response above indicates the encoding for its
value. The following definitions show how the value is computed. value. The following definitions show how the value is computed.
3.4.1 Response 3.4.1. Response
If the "qop" value is "auth" or "auth-int": If the "qop" value is "auth" or "auth-int":
response = <"> < KD ( H(A1), unq(nonce) response = <"> < KD ( H(A1), unq(nonce)
":" nc ":" nc
":" unq(cnonce) ":" unq(cnonce)
":" unq(qop) ":" unq(qop)
":" H(A2) ":" H(A2)
) <"> ) <">
See below for the definitions for A1 and A2. See below for the definitions for A1 and A2.
3.4.2 A1 3.4.2. A1
If the "algorithm" parameter's value is "<algorithm>", e.g. "SHA- If the "algorithm" parameter's value is "<algorithm>", e.g. "SHA-
256", then A1 is: 256", then A1 is:
A1 = unq(username) ":" unq(realm) ":" passwd A1 = unq(username) ":" unq(realm) ":" passwd
where where
passwd = < user's password > passwd = < user's password >
If the "algorithm" parameter's value is "<algorithm>-sess", e.g. If the "algorithm" parameter's value is "<algorithm>-sess", e.g.
"SHA-256-sess", then A1 is calculated using the nonce value provided "SHA-256-sess", then A1 is calculated using the nonce value provided
in the challenge from the server, and cnounce value from the request in the challenge from the server, and cnounce value from the request
by the client following receipt of a WWW-Authenticate challenge from by the client following receipt of a WWW-Authenticate challenge from
the server. It uses the server nonce from that challenge, herein the server. It uses the server nonce from that challenge, herein
called nonce-prime, and the client nonce value from the response, called nonce-prime, and the client nonce value from the response,
herein called cnonce-prime, to construct A1 as follows: herein called cnonce-prime, to construct A1 as follows:
A1 = H( unq(username) ":" unq(realm) A1 = H( unq(username) ":" unq(realm)
":" passwd ) ":" passwd )
":" unq(nonce-prime) ":" unq(cnonce-prime) ":" unq(nonce-prime) ":" unq(cnonce-prime)
This creates a 'session key' for the authentication of subsequent This creates a "session key" for the authentication of subsequent
requests and responses which is different for each "authentication requests and responses which is different for each "authentication
session", thus limiting the amount of material hashed with any one session", thus limiting the amount of material hashed with any one
key. (Note: see further discussion of the authentication session in key. (Note: see further discussion of the authentication session in
section 3.6.) Because the server need only use the hash of the user section 3.6.) Because the server need only use the hash of the user
credentials in order to create the A1 value, this construction could credentials in order to create the A1 value, this construction could
be used in conjunction with a third party authentication service so be used in conjunction with a third party authentication service so
that the web server would not need the actual password value. The that the web server would not need the actual password value. The
specification of such a protocol is beyond the scope of this specification of such a protocol is beyond the scope of this
specification. specification.
3.4.3 A2 3.4.3. A2
If the "qop" parameter's value is "auth" or is unspecified, then A2 If the "qop" parameter's value is "auth" or is unspecified, then A2
is: is:
A2 = Method ":" request-uri A2 = Method ":" request-uri
If the "qop" value is "auth-int", then A2 is: If the "qop" value is "auth-int", then A2 is:
A2 = Method ":" request-uri ":" H(entity-body) A2 = Method ":" request-uri ":" H(entity-body)
3.4.4 Username Hashing 3.4.4. Username Hashing
To protect the transport of the username from the client to the To protect the transport of the username from the client to the
server, the server SHOULD set the "userhash" parameter with the value server, the server SHOULD set the "userhash" parameter with the value
of "true" in the WWW-Authentication header field. of "true" in the WWW-Authentication header field.
If the client supports the "userhash" parameter, and the "userhash" If the client supports the "userhash" parameter, and the "userhash"
parameter value in the WWW-Authentication header field is set to parameter value in the WWW-Authentication header field is set to
"true", then the client MUST calculate a hash of the username after "true", then the client MUST calculate a hash of the username after
any other hash calculation and include the "userhash" parameter with any other hash calculation and include the "userhash" parameter with
the value of "true" in the Authorization Request Header field. If the the value of "true" in the Authorization Request Header field. If
client does not provide the "username" as a hash value or the the client does not provide the "username" as a hash value or the
"userhash" parameter with the value of "true", the server MAY reject "userhash" parameter with the value of "true", the server MAY reject
the request. the request.
The following is the operation that the client will take to hash the The following is the operation that the client will take to hash the
username, using the same algorithm used to hash the credentials: username, using the same algorithm used to hash the credentials:
username = H( unq(username) ":" unq(realm) ) username = H( unq(username) ":" unq(realm) )
3.4.5 Parameter Values and Quoted-String 3.4.5. Parameter Values and Quoted-String
Note that the value of many of the parameters, such as "username" Note that the value of many of the parameters, such as "username"
value, are defined as a "quoted-string". However, the "unq" notation value, are defined as a "quoted-string". However, the "unq" notation
indicates that surrounding quotation marks are removed in forming the indicates that surrounding quotation marks are removed in forming the
string A1. Thus if the Authorization header field includes the fields string A1. Thus if the Authorization header field includes the
fields
username="Mufasa", realm=myhost@testrealm.com username="Mufasa", realm=myhost@testrealm.com
and the user Mufasa has password "Circle Of Life" then H(A1) would be and the user Mufasa has password "Circle Of Life" then H(A1) would be
H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks
in the digested string. in the digested string.
No white space is allowed in any of the strings to which the digest No white space is allowed in any of the strings to which the digest
function H() is applied unless that white space exists in the quoted function H() is applied unless that white space exists in the quoted
strings or entity body whose contents make up the string to be strings or entity body whose contents make up the string to be
digested. For example, the string A1 illustrated above must be digested. For example, the string A1 illustrated above must be
Mufasa:myhost@testrealm.com:Circle Of Life Mufasa:myhost@testrealm.com:Circle Of Life
with no white space on either side of the colons, but with the white with no white space on either side of the colons, but with the white
space between the words used in the password value. Likewise, the space between the words used in the password value. Likewise, the
other strings digested by H() must not have white space on either other strings digested by H() must not have white space on either
side of the colons which delimit their fields unless that white space side of the colons which delimit their fields unless that white space
was in the quoted strings or entity body being digested. was in the quoted strings or entity body being digested.
Also note that if integrity protection is applied (qop=auth-int), the Also note that if integrity protection is applied (qop=auth-int), the
H(entity-body) is the hash of the entity body, not the message body - H(entity-body) is the hash of the entity body, not the message body -
it is computed before any transfer encoding is applied by the sender it is computed before any transfer encoding is applied by the sender
and after it has been removed by the recipient. Note that this and after it has been removed by the recipient. Note that this
includes multipart boundaries and embedded header fields in each part includes multipart boundaries and embedded header fields in each part
of any multipart content-type. of any multipart content-type.
3.4.6 Various Considerations 3.4.6. Various Considerations
The "Method" value is the HTTP request method, in all-uppercase US- The "Method" value is the HTTP request method, in US-ASCII letters,
ASCII letters, as specified in section 3.1.1 of [RFC7230]. The as specified in section 3.1.1 of [RFC7230]. The "request-target"
"request-target" value is the request-target from the request line as value is the request-target from the request line as specified in
specified in section 3.1.1 of [RFC7230]. This MAY be "*", an section 3.1.1 of [RFC7230]. This MAY be "*", an "absolute-URI" or an
"absolute-URI" or an "absolute-path" as specified in section 2.7 of "absolute-path" as specified in section 2.7 of [RFC7230], but it MUST
[RFC7230], but it MUST agree with the request-target. In particular, agree with the request-target. In particular, it MUST be an
it MUST be an "absolute-URI" if the request-target is an "absolute- "absolute-URI" if the request-target is an "absolute-URI". The
URI". The "cnonce" value is a client-chosen value whose purpose is to "cnonce" value is a client-chosen value whose purpose is to foil
foil chosen plaintext attacks. chosen plaintext attacks.
The authenticating server MUST assure that the resource designated by The authenticating server MUST assure that the resource designated by
the "uri" parameter is the same as the resource specified in the the "uri" parameter is the same as the resource specified in the
Request-Line; if they are not, the server SHOULD return a 400 Bad Request-Line; if they are not, the server SHOULD return a 400 Bad
Request error. (Since this may be a symptom of an attack, server Request error. (Since this may be a symptom of an attack, server
implementers may want to consider logging such errors.) The purpose implementers may want to consider logging such errors.) The purpose
of duplicating information from the request URL in this field is to of duplicating information from the request URL in this field is to
deal with the possibility that an intermediate proxy may alter the deal with the possibility that an intermediate proxy may alter the
client's Request-Line. This altered (but presumably semantically client's Request-Line. This altered (but presumably semantically
equivalent) request would not result in the same digest as that equivalent) request would not result in the same digest as that
calculated by the client. calculated by the client.
Implementers should be aware of how authenticated transactions Implementers should be aware of how authenticated transactions need
interact with shared caches. The HTTP/1.1 protocol specifies that to interact with shared caches. The HTTP protocol specifies that
when a shared cache (see [RFC7234]) has received a request containing when a shared cache (see [RFC7234]) has received a request containing
an Authorization header field and a response from relaying that an Authorization header field and a response from relaying that
request, it MUST NOT return that response as a reply to any other request, it MUST NOT return that response as a reply to any other
request, unless one of two Cache-Control (see section 3.2 of request, unless one of two Cache-Control (see section 3.2 of
[RFC7234]) directive was present in the response. If the original [RFC7234]) directive was present in the response. If the original
response included the "must-revalidate" Cache-Control directive, the response included the "must-revalidate" Cache-Control directive, the
cache MAY use the entity of that response in replying to a subsequent cache MAY use the entity of that response in replying to a subsequent
request, but MUST first revalidate it with the origin server, using request, but MUST first revalidate it with the origin server, using
the request header fields from the new request to allow the origin the request header fields from the new request to allow the origin
server to authenticate the new request. Alternatively, if the server to authenticate the new request. Alternatively, if the
original response included the "public" Cache-Control directive, the original response included the "public" Cache-Control directive, the
response entity MAY be returned in reply to any subsequent request. response entity MAY be returned in reply to any subsequent request.
3.5 The Authentication-Info Header 3.5. The Authentication-Info Header
The Authentication-Info header field is a generic field that MAY be The Authentication-Info header field is a generic field that MAY be
used by a server to communicate some information regarding the used by a server to communicate some information regarding the
successful authentication of a client response. The following is the successful authentication of a client response. The following is the
syntax of the header: syntax of the header:
Authentication-Info = auth-info Authentication-Info = auth-info
auth-info = *auth-param auth-info = #auth-param
The auth-param is defined in [RFC7235]. The auth-param is defined in [RFC7235].
3.5.1 Digest Usage of Authentication-Info 3.5.1. Digest Usage of Authentication-Info
The Digest authentication scheme MAY add the Authentication-Info The Digest authentication scheme MAY add the Authentication-Info
header field in the confirmation request with some or all of the header field in the confirmation request and include parameters from
following parameters: the following list:
nextnonce nextnonce
The value of the nextnonce parameter is the nonce the server The value of the nextnonce parameter is the nonce the server
wishes the client to use for a future authentication response. wishes the client to use for a future authentication response.
The server MAY send the Authentication-Info header field with a The server MAY send the Authentication-Info header field with a
nextnonce field as a means of implementing one-time or otherwise nextnonce field as a means of implementing one-time or otherwise
changing nonces. If the nextnonce field is present the client changing nonces. If the nextnonce field is present the client
SHOULD use it when constructing the Authorization header field for SHOULD use it when constructing the Authorization header field for
its next request. Failure of the client to do so MAY result in a its next request. Failure of the client to do so MAY result in a
request to re-authenticate from the server with the "stale=TRUE". request to re-authenticate from the server with the "stale=TRUE".
Server implementations SHOULD carefully consider the Server implementations SHOULD carefully consider the
performance implications of the use of this mechanism; performance implications of the use of this mechanism;
pipelined requests will not be possible if every response pipelined requests will not be possible if every response
includes a nextnonce parameter that MUST be used on the next includes a nextnonce parameter that MUST be used on the next
request received by the server. Consideration SHOULD be given request received by the server. Consideration SHOULD be given
to the performance vs. security tradeoffs of allowing an old to the performance vs. security tradeoffs of allowing an old
nonce value to be used for a limited time to permit request nonce value to be used for a limited time to permit request
pipelining. Use of the "nc" parameter can retain most of the pipelining. Use of the "nc" parameter can retain most of the
security advantages of a new server nonce without the security advantages of a new server nonce without the
deleterious affects on pipelining. deleterious affects on pipelining.
qop qop
Indicates the "quality of protection" options applied to the Indicates the "quality of protection" options applied to the
response by the server. The value "auth" indicates authentication; response by the server. The value "auth" indicates
the value "auth-int" indicates authentication with integrity authentication; the value "auth-int" indicates authentication with
protection. The server SHOULD use the same value for the qop integrity protection. The server SHOULD use the same value for
parameter in the response as was sent by the client in the the qop parameter in the response as was sent by the client in the
corresponding request. corresponding request.
rspauth rspauth
The optional response digest in the "rspauth" parameter supports The optional response digest in the "rspauth" parameter supports
mutual authentication -- the server proves that it knows the mutual authentication -- the server proves that it knows the
user's secret, and with qop=auth-int also provides limited user's secret, and with qop=auth-int also provides limited
integrity protection of the response. The "rspauth" value is integrity protection of the response. The "rspauth" value is
calculated as for the response in the Authorization header field, calculated as for the response in the Authorization header field,
except that if "qop=auth" or is not specified in the Authorization except that if "qop=auth" or is not specified in the Authorization
header field for the request, A2 is header field for the request, A2 is
A2 = ":" request-uri
and if "qop=auth-int", then A2 is A2 = ":" request-uri
A2 = ":" request-uri ":" H(entity-body) and if "qop=auth-int", then A2 is
A2 = ":" request-uri ":" H(entity-body)
cnonce and nc cnonce and nc
The "cnonce" value and "nc" value MUST be the ones for the client The "cnonce" value and "nc" value MUST be the ones for the client
request to which this message is the response. The "rspauth", request to which this message is the response. The "rspauth",
"cnonce", and "nc" parameters MUST be present if "qop=auth" or "cnonce", and "nc" parameters MUST be present if "qop=auth" or
"qop=auth-int" is specified. "qop=auth-int" is specified.
The Authentication-Info header field is allowed in the trailer of an The Authentication-Info header field is allowed in the trailer of an
HTTP message transferred via chunked transfer-coding. HTTP message transferred via chunked transfer-coding.
3.6 Digest Operation For historical reasons, a sender MUST only generate the quoted-string
syntax for the following parameters: nextnonce, rspauth, and cnonce.
For historical reasons, a sender MUST NOT generate the quoted-string
syntax for the following parameters: qop and nc.
For historical reasons, the nc value MUST be exactly 8 hexadecimal
digits.
3.6. Digest Operation
Upon receiving the Authorization header field, the server MAY check Upon receiving the Authorization header field, the server MAY check
its validity by looking up the password that corresponds to the its validity by looking up the password that corresponds to the
submitted username. Then, the server MUST perform the same digest submitted username. Then, the server MUST perform the same digest
operation (e.g. MD5, SHA-256) performed by the client, and compare operation (e.g. MD5, SHA-256) performed by the client, and compare
the result to the given response value. the result to the given response value.
Note that the HTTP server does not actually need to know the user's Note that the HTTP server does not actually need to know the user's
cleartext password. As long as H(A1) is available to the server, the cleartext password. As long as H(A1) is available to the server, the
validity of an Authorization header field MAY be verified. validity of an Authorization header field can be verified.
The client response to a WWW-Authenticate challenge for a protection The client response to a WWW-Authenticate challenge for a protection
space starts an authentication session with that protection space. space starts an authentication session with that protection space.
The authentication session lasts until the client receives another The authentication session lasts until the client receives another
WWW-Authenticate challenge from any server in the protection space. A WWW-Authenticate challenge from any server in the protection space.
client SHOULD remember the username, password, nonce, nonce count and A client SHOULD remember the username, password, nonce, nonce count
opaque values associated with an authentication session to use to and opaque values associated with an authentication session to use to
construct the Authorization header field in future requests within construct the Authorization header field in future requests within
that protection space. The Authorization header field MAY be included that protection space. The Authorization header field MAY be
preemptively; doing so improves server efficiency and avoids extra included preemptively; doing so improves server efficiency and avoids
round trips for authentication challenges. The server MAY choose to extra round trips for authentication challenges. The server MAY
accept the old Authorization header field information, even though choose to accept the old Authorization header field information, even
the nonce value included might not be fresh. Alternatively, the though the nonce value included might not be fresh. Alternatively,
server MAY return a 401 response with a new nonce value, causing the the server MAY return a 401 response with a new nonce value, causing
client to retry the request; by specifying stale=TRUE with this the client to retry the request; by specifying stale=TRUE with this
response, the server tells the client to retry with the new nonce, response, the server tells the client to retry with the new nonce,
but without prompting for a new username and password. but without prompting for a new username and password.
Because the client is required to return the value of the opaque Because the client is REQUIRED to return the value of the opaque
parameter given to it by the server for the duration of a session, parameter given to it by the server for the duration of a session,
the opaque data MAY be used to transport authentication session state the opaque data can be used to transport authentication session state
information. (Note that any such use can also be accomplished more information. (Note that any such use can also be accomplished more
easily and safely by including the state in the nonce.) For example, easily and safely by including the state in the nonce.) For example,
a server could be responsible for authenticating content that a server could be responsible for authenticating content that
actually sits on another server. It would achieve this by having the actually sits on another server. It would achieve this by having the
first 401 response include a domain parameter whose value includes a first 401 response include a domain parameter whose value includes a
URI on the second server, and an opaque parameter whose value URI on the second server, and an opaque parameter whose value
contains the state information. The client will retry the request, at contains the state information. The client will retry the request,
which time the server might respond with a 301/302 redirection, at which time the server might respond with "HTTP Redirection
pointing to the URI on the second server. The client will follow the (Section 6.4 of [RFC7231]), pointing to the URI on the second server.
redirection, and pass an Authorization header field, including the
<opaque> data.
As with the basic scheme, proxies MUST be completely transparent in The client will follow the redirection, and pass an Authorization
the Digest access authentication scheme. That is, they MUST forward header field, including the <opaque> data.
the WWW-Authenticate, Authentication-Info and Authorization header
fields untouched. If a proxy wants to authenticate a client before a Proxies MUST be completely transparent in the Digest access
authentication scheme. That is, they MUST forward the WWW-
Authenticate, Authentication-Info and Authorization header fields
untouched. If a proxy wants to authenticate a client before a
request is forwarded to the server, it can be done using the Proxy- request is forwarded to the server, it can be done using the Proxy-
Authenticate and Proxy-Authorization header fields described in Authenticate and Proxy-Authorization header fields described in
section 3.8 below. section 3.8 below.
3.7 Security Protocol Negotiation 3.7. Security Protocol Negotiation
It is useful for a server to be able to know which security schemes a It is useful for a server to be able to know which security schemes a
client is capable of handling. client is capable of handling.
It is possible that a server MAY want to require Digest as its It is possible that a server wants to require Digest as its
authentication method, even if the server does not know that the authentication method, even if the server does not know that the
client supports it. A client is encouraged to fail gracefully if the client supports it. A client is encouraged to fail gracefully if the
server specifies only authentication schemes it cannot handle. server specifies only authentication schemes it cannot handle.
When a server receives a request to access a resource, the server When a server receives a request to access a resource, the server
might challenge the client by responding with "401 Unauthorized" might challenge the client by responding with "401 Unauthorized"
status code, and include one or more WWW-Authenticate header fields. response, and include one or more WWW-Authenticate header fields. If
If the server challenges with multiple Digest header fields, then the server responds with multiple challenges, then each one of these
each one of these header fields MUST use a different digest challenges MUST use a different digest algorithm. The server MUST
algorithm. The server MUST add these Digest header fields to the add these challenges to the response in order of preference, starting
response in order of preference, starting with the most preferred with the most preferred algorithm, followed by the less preferred
header field, followed by the less preferred header fields. algorithm.
This specification defines the following algorithms: This specification defines the following algorithms:
* SHA2-256 (mandatory to implement) o SHA2-256 (mandatory to implement)
* SHA2-512/256 (as a backup algorithm)
* MD5 (for backward compatibility).
When the client receives the response it SHOULD use the topmost o SHA2-512/256 (as a backup algorithm)
header field that it supports, unless a local policy dictates
otherwise. The client SHOULD ignore any challenge it does not o MD5 (for backward compatibility).
When the client receives the first challenge it SHOULD use the
topmost header field that it supports, unless a local policy dictates
otherwise. The client MUST ignore any challenge it does not
understand. understand.
3.8 Proxy-Authenticate and Proxy-Authorization 3.8. Proxy-Authenticate and Proxy-Authorization
The digest authentication scheme MAY also be used for authenticating The digest authentication scheme can also be used for authenticating
users to proxies, proxies to proxies, or proxies to origin servers by users to proxies, proxies to proxies, or proxies to origin servers by
use of the Proxy-Authenticate and Proxy-Authorization header fields. use of the Proxy-Authenticate and Proxy-Authorization header fields.
These header fields are instances of the Proxy-Authenticate and These header fields are instances of the Proxy-Authenticate and
Proxy- Authorization header fields specified in sections 4.2 and 4.3 Proxy-Authorization header fields specified in sections 4.2 and 4.3
of the HTTP/1.1 specification [RFC7235] and their behavior is subject of the HTTP/1.1 specification [RFC7235] and their behavior is subject
to restrictions described there. The transactions for proxy to restrictions described there. The transactions for proxy
authentication are very similar to those already described. Upon authentication are very similar to those already described. Upon
receiving a request which requires authentication, the proxy/server receiving a request which requires authentication, the proxy/server
MUST issue the "407 Proxy Authentication Required" response with a MUST issue the "407 Proxy Authentication Required" response with a
"Proxy-Authenticate" header field. The digest-challenge used in the "Proxy-Authenticate" header field. The digest-challenge used in the
Proxy-Authenticate header field is the same as that for the WWW- Proxy-Authenticate header field is the same as that for the WWW-
Authenticate header field as defined above in section 3.2.1. Authenticate header field as defined above in Section 3.3.
The client/proxy MUST then re-issue the request with a Proxy- The client/proxy MUST then re-issue the request with a Proxy-
Authorization header field, with parameters as specified for the Authorization header field, with parameters as specified for the
Authorization header field in section 3.4 above. Authorization header field in section 3.4 above.
On subsequent responses, the server sends Proxy-Authenticate-Info On subsequent responses, the server sends Proxy-Authenticate-Info
with parameters the same as those for the Authentication-Info header with parameters the same as those for the Authentication-Info header
field. field.
Note that in principle a client could be asked to authenticate itself Note that in principle a client could be asked to authenticate itself
to both a proxy and an end-server, but never in the same response. to both a proxy and an end-server, but never in the same response.
3.9 Examples 3.9. Examples
3.9.1 Example with SHA-256 and MD5 3.9.1. Example with SHA-256 and MD5
The following example assumes that an access protected document is The following example assumes that an access protected document is
being requested from the server via a GET request. The URI of the being requested from the server via a GET request. The URI of the
document is http://www.nowhere.org/dir/index.html". Both client and document is http://www.example.org/dir/index.html". Both client and
server know that the username for this document is "Mufasa" and the server know that the username for this document is "Mufasa" and the
password is "Circle of Life" ( with one space between each of the password is "Circle of Life" ( with one space between each of the
three words). three words).
The first time the client requests the document, no Authorization The first time the client requests the document, no Authorization
header field is sent, so the server responds with: header field is sent, so the server responds with:
HTTP/1.1 401 Unauthorized HTTP/1.1 401 Unauthorized
WWW-Authenticate: Digest WWW-Authenticate: Digest
realm="http-auth@example.org", realm="http-auth@example.org",
qop="auth, auth-int", qop="auth, auth-int",
algorithm="SHA-256", algorithm=SHA-256,
nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v", nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS" opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS",
WWW-Authenticate: Digest WWW-Authenticate: Digest
realm="http-auth@example.org", realm="http-auth@example.org",
qop="auth, auth-int", qop="auth, auth-int",
algorithm="MD5", algorithm=MD5,
nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v", nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS" opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"
The client MAY prompt the user for their username and password, after The client can prompt the user for their username and password, after
which it will respond with a new request, including the following which it will respond with a new request, including the following
Authorization header field if the client chooses MD5 digest: Authorization header field if the client chooses MD5 digest:
Authorization:Digest username="Mufasa", Authorization: Digest username="Mufasa",
realm="http-auth@example.org", realm="http-auth@example.org",
uri="/dir/index.html", uri=/dir/index.html,
algorithm=MD5, algorithm=MD5,
nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v", nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
nc=00000001, nc=00000001,
cnonce="f2/wE4q74E6zIJEtWaHKaf5wv/H5QzzpXusqGemxURZJ", cnonce="f2/wE4q74E6zIJEtWaHKaf5wv/H5QzzpXusqGemxURZJ",
qop=auth, qop=auth,
response="8ca523f5e9506fed4657c9700eebdbec", response="8ca523f5e9506fed4657c9700eebdbec",
opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS" opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"
If the client chooses to use the SHA-256 algorithm for calculating If the client chooses to use the SHA-256 algorithm for calculating
the response, the client responds with a new request including the the response, the client responds with a new request including the
following Authorization header field: following Authorization header field:
Authorization:Digest username="Mufasa", Authorization: Digest username="Mufasa",
realm="http-auth@example.org", realm="http-auth@example.org",
uri="/dir/index.html", uri="/dir/index.html",
algorithm=SHA-256, algorithm=SHA-256,
nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v", nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
nc=00000001, nc=00000001,
cnonce="f2/wE4q74E6zIJEtWaHKaf5wv/H5QzzpXusqGemxURZJ", cnonce="f2/wE4q74E6zIJEtWaHKaf5wv/H5QzzpXusqGemxURZJ",
qop=auth, qop=auth,
response="753927fa0e85d155564e2e272a28d1802ca10daf449 response="753927fa0e85d155564e2e272a28d1802ca10daf449
6794697cf8db5856cb6c1", 6794697cf8db5856cb6c1",
opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS" opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"
3.9.2 Example with SHA-512-256, Charset, and Userhash 3.9.2. Example with SHA-512-256, Charset, and Userhash
The following example assumes that an access protected document is The following example assumes that an access protected document is
being requested from the server via a GET request. The URI for the being requested from the server via a GET request. The URI for the
request is "http://api.example.org/doe.json". Both client and server request is "http://api.example.org/doe.json". Both client and server
know the userhash of the username, support the UTF-8 charset, and use know the userhash of the username, support the UTF-8 character
the SHA-512-256 algorithm. The username for the request is "Jason encoding scheme, and use the SHA-512-256 algorithm. The username for
Doe" and the password is "Secret, or not?". the request is "Jaesoen Doe" and the password is "Secret, or not?".
The first time the client requests the document, no Authorization The first time the client requests the document, no Authorization
header field is sent, so the server responds with: header field is sent, so the server responds with:
HTTP/2.0 401 Unauthorized HTTP/1.1 401 Unauthorized
WWW-Authenticate: Digest WWW-Authenticate: Digest
realm="api@example.org", realm="api@example.org",
qop=auth, qop=auth,
algorithm=SHA-512-256, algorithm=SHA-512-256,
nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK", nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK",
opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS", opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS",
charset=UTF-8, charset=UTF-8,
userhash=true userhash=true
The client MAY prompt the user for the required credentials and send The client can prompt the user for the required credentials and send
a new request with following Authorization header field: a new request with following Authorization header field:
Authorization: Digest Authorization: Digest
username="488869477bf257147b804c45308cd62ac4e25eb717 username="488869477bf257147b804c45308cd62ac4e25eb717
b12b298c79e62dcea254ec", b12b298c79e62dcea254ec",
realm="api@example.org", realm="api@example.org",
uri="/doe.json", uri="/doe.json",
algorithm=SHA-512-256, algorithm=SHA-512-256,
nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK", nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK",
nc=00000001, nc=00000001,
cnonce="NTg6RKcb9boFIAS3KrFK9BGeh+iDa/sm6jUMp2wds69v", cnonce="NTg6RKcb9boFIAS3KrFK9BGeh+iDa/sm6jUMp2wds69v",
qop=auth, qop=auth,
response="ae66e67d6b427bd3f120414a82e4acff38e8ecd9101d response="ae66e67d6b427bd3f120414a82e4acff38e8ecd9101d
6c861229025f607a79dd", 6c861229025f607a79dd",
opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS", opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS",
userhash=true userhash=true
If the client can not provide a hashed username for any reason, the If the client can not provide a hashed username for any reason, the
client MAY try a request with this Authorization header field: client can try a request with this Authorization header field:
Authorization: Digest Authorization: Digest
username*=UTF-8''J%C3%A4s%C3%B8n%20Doe, username*=UTF-8''J%C3%A4s%C3%B8n%20Doe,
realm="api@example.org", realm="api@example.org",
uri="/doe.json", uri="/doe.json",
algorithm=SHA-512-256, algorithm=SHA-512-256,
nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK", nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK",
nc=00000001, nc=00000001,
cnonce="NTg6RKcb9boFIAS3KrFK9BGeh+iDa/sm6jUMp2wds69v", cnonce="NTg6RKcb9boFIAS3KrFK9BGeh+iDa/sm6jUMp2wds69v",
qop=auth, qop=auth,
response="ae66e67d6b427bd3f120414a82e4acff38e8ecd9101d6 response="ae66e67d6b427bd3f120414a82e4acff38e8ecd9101d6
c861229025f607a79dd", c861229025f607a79dd",
opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS", opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS",
userhash=false userhash=false
4 Internationalization 4. Internationalization
In challenges, servers SHOULD use the "charset" authentication In challenges, servers SHOULD use the "charset" authentication
parameter (case-insensitive) to express the character encoding they parameter (case-insensitive) to express the character encoding they
expect the user agent to use when generating A1 (see section 3.4.2) expect the user agent to use when generating A1 (see section 3.4.2)
and username hashing (see section 3.4.4). and username hashing (see section 3.4.4).
The only allowed value is "UTF-8", to be matched case-insensitively The only allowed value is "UTF-8", to be matched case-insensitively
(see [RFC2978], Section 2.3). It indicates that the server expects (see [RFC2978], Section 2.3). It indicates that the server expects
user name and password to be converted to Unicode Normalization Form user name and password to be converted to Unicode Normalization Form
C ("NFC", see Section 3 of [RFC5198]) and to be encoded into octets C ("NFC", see Section 3 of [RFC5198]) and to be encoded into octets
using the UTF-8 character encoding scheme ([RFC3629]). using the UTF-8 character encoding scheme ([RFC3629]), ), and percent
escaped in extended notation ([RFC5987]).
For the username, recipients MUST support all characters defined in For the username, recipients MUST support all characters defined in
the "UsernameCasePreserved" profile defined in in Section 3.3 of the "UsernameCasePreserved" profile defined in in Section 3.3 of
[PRECIS], with the exception of the colon (":") character. [PRECIS], with the exception of the colon (":") character.
For the password, recipients MUST support all characters defined in For the password, recipients MUST support all characters defined in
the "OpaqueString" profile defined in in Section 4.2 of [PRECIS]. the "OpaqueString" profile defined in in Section 4.2 of [PRECIS].
If the user agent does not support the encoding indicated by the If the user agent does not support the encoding indicated by the
server, it MUST fail the request. server, it can fail the request.
5 Security Considerations 5. Security Considerations
5.1 Limitations 5.1. Limitations
HTTP Digest authentication, when used with human-memorable passwords, HTTP Digest authentication, when used with human-memorable passwords,
is vulnerable to dictionary attacks. Such attacks are much easier is vulnerable to dictionary attacks. Such attacks are much easier
than cryptographic attacks on any widely used algorithm, including than cryptographic attacks on any widely used algorithm, including
those that are no longer considered secure. In other words, algorithm those that are no longer considered secure. In other words,
agility does not make this usage any more secure. algorithm agility does not make this usage any more secure.
As a result, Digest authentication SHOULD be used only with passwords As a result, Digest authentication SHOULD be used only with passwords
that have a reasonable amount of entropy, e.g. 128-bit or more. Such that have a reasonable amount of entropy, e.g. 128-bit or more. Such
passwords typically cannot be memorized by humans but can be used for passwords typically cannot be memorized by humans but can be used for
automated web services. automated web services.
Digest authentication SHOULD be used over a secure channel like HTTPS Digest authentication SHOULD be used over a secure channel like HTTPS
[RFC2818]. [RFC2818].
5.2 Storing passwords 5.2. Storing passwords
Digest authentication requires that the authenticating agent (usually Digest authentication requires that the authenticating agent (usually
the server) store some data derived from the user's name and password the server) store some data derived from the user's name and password
in a "password file" associated with a given realm. Normally this in a "password file" associated with a given realm. Normally this
might contain pairs consisting of username and H(A1), where H(A1) is might contain pairs consisting of username and H(A1), where H(A1) is
the digested value of the username, realm, and password as described the digested value of the username, realm, and password as described
above. above.
The security implications of this are that if this password file is The security implications of this are that if this password file is
compromised, then an attacker gains immediate access to documents on compromised, then an attacker gains immediate access to documents on
the server using this realm. Unlike, say a standard UNIX password the server using this realm. Unlike, say a standard UNIX password
file, this information need not be decrypted in order to access file, this information need not be decrypted in order to access
documents in the server realm associated with this file. On the other documents in the server realm associated with this file. On the
hand, decryption, or more likely a brute force attack, would be other hand, decryption, or more likely a brute force attack, would be
necessary to obtain the user's password. This is the reason that the necessary to obtain the user's password. This is the reason that the
realm is part of the digested data stored in the password file. It realm is part of the digested data stored in the password file. It
means that if one Digest authentication password file is compromised, means that if one Digest authentication password file is compromised,
it does not automatically compromise others with the same username it does not automatically compromise others with the same username
and password (though it does expose them to brute force attack). and password (though it does expose them to brute force attack).
There are two important security consequences of this. First the There are two important security consequences of this. First the
password file must be protected as if it contained unencrypted password file must be protected as if it contained unencrypted
passwords, because for the purpose of accessing documents in its passwords, because for the purpose of accessing documents in its
realm, it effectively does. realm, it effectively does.
A second consequence of this is that the realm string SHOULD be A second consequence of this is that the realm string SHOULD be
unique among all realms which any single user is likely to use. In unique among all realms which any single user is likely to use. In
particular a realm string SHOULD include the name of the host doing particular a realm string SHOULD include the name of the host doing
the authentication. The inability of the client to authenticate the the authentication. The inability of the client to authenticate the
server is a weakness of Digest Authentication. server is a weakness of Digest Authentication.
5.3 Authentication of Clients using Digest Authentication 5.3. Authentication of Clients using Digest Authentication
Digest Authentication does not provide a strong authentication Digest Authentication does not provide a strong authentication
mechanism, when compared to public key based mechanisms, for example. mechanism, when compared to public key based mechanisms, for example.
However, it is significantly stronger than (e.g.) CRAM-MD5, which has However, it is significantly stronger than (e.g.) CRAM-MD5, which
been proposed for use with LDAP [RFC4513], POP and IMAP (see has been proposed for use with LDAP [RFC4513], POP and IMAP (see
[RFC2195]). It was intended to replace the much weaker and even more
[RFC2195]). It was intended to replace the much weaker and even more
dangerous Basic mechanism. dangerous Basic mechanism.
Digest Authentication offers no confidentiality protection beyond Digest Authentication offers no confidentiality protection beyond
protecting the actual username and password. All of the rest of the protecting the actual username and password. All of the rest of the
request and response are available to an eavesdropper. request and response are available to an eavesdropper.
Digest Authentication offers only limited integrity protection for Digest Authentication offers only limited integrity protection for
the messages in either direction. If qop=auth-int mechanism is used, the messages in either direction. If qop=auth-int mechanism is used,
those parts of the message used in the calculation of the WWW- those parts of the message used in the calculation of the WWW-
Authenticate and Authorization header field response parameter values Authenticate and Authorization header field response parameter values
(see section 3.2 above) are protected. Most header fields and their (see section 3.2 above) are protected. Most header fields and their
values could be modified as a part of a man-in-the-middle attack. values could be modified as a part of a man-in-the-middle attack.
Many needs for secure HTTP transactions cannot be met by Digest Many needs for secure HTTP transactions cannot be met by Digest
Authentication. For those needs TLS is more appropriate protocol. In Authentication. For those needs TLS is more appropriate protocol.
particular Digest authentication cannot be used for any transaction In particular Digest authentication cannot be used for any
requiring confidentiality protection. Nevertheless many functions transaction requiring confidentiality protection. Nevertheless many
remain for which Digest authentication is both useful and functions remain for which Digest authentication is both useful and
appropriate. appropriate.
5.4 Limited Use Nonce Values 5.4. Limited Use Nonce Values
The Digest scheme uses a server-specified nonce to seed the The Digest scheme uses a server-specified nonce to seed the
generation of the response value (as specified in section 3.4.1 generation of the response value (as specified in section 3.4.1
above). As shown in the example nonce in section 3.2.1, the server is above). As shown in the example nonce in section 3.3, the server is
free to construct the nonce such that it MAY only be used from a free to construct the nonce such that it MAY only be used from a
particular client, for a particular resource, for a limited period of particular client, for a particular resource, for a limited period of
time or number of uses, or any other restrictions. Doing so time or number of uses, or any other restrictions. Doing so
strengthens the protection provided against, for example, replay strengthens the protection provided against, for example, replay
attacks (see 4.5). However, it should be noted that the method chosen attacks (see 4.5). However, it should be noted that the method
for generating and checking the nonce also has performance and chosen for generating and checking the nonce also has performance and
resource implications. For example, a server MAY choose to allow each resource implications. For example, a server MAY choose to allow
nonce value to be used only once by maintaining a record of whether each nonce value to be used only once by maintaining a record of
or not each recently issued nonce has been returned and sending a whether or not each recently issued nonce has been returned and
next-nonce parameter in the Authentication-Info header field of every sending a next-nonce parameter in the Authentication-Info header
response. This protects against even an immediate replay attack, but field of every response. This protects against even an immediate
has a high cost checking nonce values, and perhaps more important replay attack, but has a high cost checking nonce values, and perhaps
will cause authentication failures for any pipelined requests more important will cause authentication failures for any pipelined
(presumably returning a stale nonce indication). Similarly, requests (presumably returning a stale nonce indication). Similarly,
incorporating a request-specific element such as the Etag value for a incorporating a request-specific element such as the Etag value for a
resource limits the use of the nonce to that version of the resource resource limits the use of the nonce to that version of the resource
and also defeats pipelining. Thus it MAY be useful to do so for and also defeats pipelining. Thus it MAY be useful to do so for
methods with side effects but have unacceptable performance for those methods with side effects but have unacceptable performance for those
that do not. that do not.
5.5 Replay Attacks 5.5. Replay Attacks
A replay attack against Digest authentication would usually be A replay attack against Digest authentication would usually be
pointless for a simple GET request since an eavesdropper would pointless for a simple GET request since an eavesdropper would
already have seen the only document he could obtain with a replay. already have seen the only document he could obtain with a replay.
This is because the URI of the requested document is digested in the This is because the URI of the requested document is digested in the
client request and the server will only deliver that document. By client request and the server will only deliver that document. By
contrast under Basic Authentication once the eavesdropper has the contrast under Basic Authentication once the eavesdropper has the
user's password, any document protected by that password is open to user's password, any document protected by that password is open to
him. him.
Thus, for some purposes, it is necessary to protect against replay Thus, for some purposes, it is necessary to protect against replay
attacks. A good Digest implementation can do this in various ways. attacks. A good Digest implementation can do this in various ways.
The server created "nonce" value is implementation dependent, but if The server created "nonce" value is implementation dependent, but if
it contains a digest of the client IP, a time-stamp, the resource it contains a digest of the client IP, a time-stamp, the resource
ETag, and a private server key (as recommended above) then a replay ETag, and a private server key (as recommended above) then a replay
attack is not simple. An attacker must convince the server that the attack is not simple. An attacker must convince the server that the
request is coming from a false IP address and must cause the server request is coming from a false IP address and must cause the server
to deliver the document to an IP address different from the address to deliver the document to an IP address different from the address
to which it believes it is sending the document. An attack can only to which it believes it is sending the document. An attack can only
succeed in the period before the time-stamp expires. Digesting the succeed in the period before the time-stamp expires. Digesting the
client IP and time-stamp in the nonce permits an implementation which client IP and time-stamp in the nonce permits an implementation which
does not maintain state between transactions. does not maintain state between transactions.
For applications where no possibility of replay attack can be For applications where no possibility of replay attack can be
tolerated the server can use one-time nonce values which will not be tolerated the server can use one-time nonce values which will not be
honored for a second use. This requires the overhead of the server honored for a second use. This requires the overhead of the server
remembering which nonce values have been used until the nonce time- remembering which nonce values have been used until the nonce time-
stamp (and hence the digest built with it) has expired, but it stamp (and hence the digest built with it) has expired, but it
effectively protects against replay attacks. effectively protects against replay attacks.
An implementation must give special attention to the possibility of An implementation must give special attention to the possibility of
replay attacks with POST and PUT requests. Unless the server employs replay attacks with POST and PUT requests. Unless the server employs
one-time or otherwise limited-use nonces and/or insists on the use of one-time or otherwise limited-use nonces and/or insists on the use of
the integrity protection of qop=auth-int, an attacker could replay the integrity protection of qop=auth-int, an attacker could replay
valid credentials from a successful request with counterfeit form valid credentials from a successful request with counterfeit form
data or other message body. Even with the use of integrity protection data or other message body. Even with the use of integrity
most metadata in header fields is not protected. Proper nonce protection most metadata in header fields is not protected. Proper
generation and checking provides some protection against replay of nonce generation and checking provides some protection against replay
previously used valid credentials, but see 4.8. of previously used valid credentials, but see 4.8.
5.6 Weakness Created by Multiple Authentication Schemes 5.6. Weakness Created by Multiple Authentication Schemes
An HTTP/1.1 server MAY return multiple challenges with a 401 An HTTP/1.1 server MAY return multiple challenges with a 401
(Authenticate) response, and each challenge MAY use a different auth- (Authenticate) response, and each challenge MAY use a different auth-
scheme. A user agent MUST choose to use the strongest auth- scheme it scheme. A user agent MUST choose to use the strongest auth-scheme it
understands and request credentials from the user based upon that understands and request credentials from the user based upon that
challenge. challenge.
Note that many browsers will only recognize Basic and will require Note that many browsers will only recognize Basic and will require
that it be the first auth-scheme presented. Servers should only that it be the first auth-scheme presented. Servers SHOULD only
include Basic if it is minimally acceptable. include Basic if it is minimally acceptable.
When the server offers choices of authentication schemes using the When the server offers choices of authentication schemes using the
WWW-Authenticate header field, the strength of the resulting WWW-Authenticate header field, the strength of the resulting
authentication is only as good as that of the of the weakest of the authentication is only as good as that of the of the weakest of the
authentication schemes. See section 5.7 below for discussion of authentication schemes. See Section 5.7 below for discussion of
particular attack scenarios that exploit multiple authentication particular attack scenarios that exploit multiple authentication
schemes. schemes.
5.7 Online dictionary attacks 5.7. Online dictionary attacks
If the attacker can eavesdrop, then it can test any overheard If the attacker can eavesdrop, then it can test any overheard nonce/
nonce/response pairs against a list of common words. Such a list is response pairs against a list of common words. Such a list is
usually much smaller than the total number of possible passwords. The usually much smaller than the total number of possible passwords.
cost of computing the response for each password on the list is paid The cost of computing the response for each password on the list is
once for each challenge. paid once for each challenge.
The server can mitigate this attack by not allowing users to select The server can mitigate this attack by not allowing users to select
passwords that are in a dictionary. passwords that are in a dictionary.
5.8 Man in the Middle 5.8. Man in the Middle
Both Basic and Digest authentication are vulnerable to "man in the Digest authentication is vulnerable to "man in the middle" (MITM)
middle" (MITM) attacks, for example, from a hostile or compromised attacks, for example, from a hostile or compromised proxy. Clearly,
proxy. Clearly, this would present all the problems of eavesdropping. this would present all the problems of eavesdropping. But it also
But it also offers some additional opportunities to the attacker. offers some additional opportunities to the attacker.
A possible man-in-the-middle attack would be to add a weak A possible man-in-the-middle attack would be to add a weak
authentication scheme to the set of choices, hoping that the client authentication scheme to the set of choices, hoping that the client
will use one that exposes the user's credentials (e.g. password). For will use one that exposes the user's credentials (e.g. password).
this reason, the client SHOULD always use the strongest scheme that For this reason, the client SHOULD always use the strongest scheme
it understands from the choices offered. that it understands from the choices offered.
An even better MITM attack would be to remove all offered choices, An even better MITM attack would be to remove all offered choices,
replacing them with a challenge that requests only Basic replacing them with a challenge that requests only Basic
authentication, then uses the cleartext credentials from the Basic authentication, then uses the cleartext credentials from the Basic
authentication to authenticate to the origin server using the authentication to authenticate to the origin server using the
stronger scheme it requested. A particularly insidious way to mount stronger scheme it requested. A particularly insidious way to mount
such a MITM attack would be to offer a "free" proxy caching service such a MITM attack would be to offer a "free" proxy caching service
to gullible users. to gullible users.
User agents should consider measures such as presenting a visual User agents should consider measures such as presenting a visual
indication at the time of the credentials request of what indication at the time of the credentials request of what
authentication scheme is to be used, or remembering the strongest authentication scheme is to be used, or remembering the strongest
authentication scheme ever requested by a server and produce a authentication scheme ever requested by a server and produce a
warning message before using a weaker one. It might also be a good warning message before using a weaker one. It might also be a good
idea for the user agent to be configured to demand Digest idea for the user agent to be configured to demand Digest
authentication in general, or from specific sites. authentication in general, or from specific sites.
Or, a hostile proxy might spoof the client into making a request the Or, a hostile proxy might spoof the client into making a request the
attacker wanted rather than one the client wanted. Of course, this is attacker wanted rather than one the client wanted. Of course, this
still much harder than a comparable attack against Basic is still much harder than a comparable attack against Basic
Authentication. Authentication.
5.9 Chosen plaintext attacks 5.9. Chosen plaintext attacks
With Digest authentication, a MITM or a malicious server can With Digest authentication, a MITM or a malicious server can
arbitrarily choose the nonce that the client will use to compute the arbitrarily choose the nonce that the client will use to compute the
response. This is called a "chosen plaintext" attack. The ability to response. This is called a "chosen plaintext" attack. The ability
choose the nonce is known to make cryptanalysis much easier. to choose the nonce is known to make cryptanalysis much easier.
However, no way to analyze the one-way functions used by Digest using However, no way to analyze the one-way functions used by Digest using
chosen plaintext is currently known. chosen plaintext is currently known.
The countermeasure against this attack is for clients to use the The countermeasure against this attack is for clients to use the
"cnonce" parameter; this allows the client to vary the input to the "cnonce" parameter; this allows the client to vary the input to the
hash in a way not chosen by the attacker. hash in a way not chosen by the attacker.
5.10 Precomputed dictionary attacks 5.10. Precomputed dictionary attacks
With Digest authentication, if the attacker can execute a chosen With Digest authentication, if the attacker can execute a chosen
plaintext attack, the attacker can precompute the response for many plaintext attack, the attacker can precompute the response for many
common words to a nonce of its choice, and store a dictionary of common words to a nonce of its choice, and store a dictionary of
(response, password) pairs. Such precomputation can often be done in (response, password) pairs. Such precomputation can often be done in
parallel on many machines. It can then use the chosen plaintext parallel on many machines. It can then use the chosen plaintext
attack to acquire a response corresponding to that challenge, and attack to acquire a response corresponding to that challenge, and
just look up the password in the dictionary. Even if most passwords just look up the password in the dictionary. Even if most passwords
are not in the dictionary, some might be. Since the attacker gets to are not in the dictionary, some might be. Since the attacker gets to
pick the challenge, the cost of computing the response for each pick the challenge, the cost of computing the response for each
password on the list can be amortized over finding many passwords. A password on the list can be amortized over finding many passwords. A
dictionary with 100 million password/response pairs would take about dictionary with 100 million password/response pairs would take about
3.2 gigabytes of disk storage. 3.2 gigabytes of disk storage.
The countermeasure against this attack is to for clients to use the The countermeasure against this attack is to for clients to use the
"cnonce" parameter. "cnonce" parameter.
5.11 Batch brute force attacks 5.11. Batch brute force attacks
With Digest authentication, a MITM can execute a chosen plaintext With Digest authentication, a MITM can execute a chosen plaintext
attack, and can gather responses from many users to the same nonce. attack, and can gather responses from many users to the same nonce.
It can then find all the passwords within any subset of password It can then find all the passwords within any subset of password
space that would generate one of the nonce/response pairs in a single space that would generate one of the nonce/response pairs in a single
pass over that space. It also reduces the time to find the first pass over that space. It also reduces the time to find the first
password by a factor equal to the number of nonce/response pairs password by a factor equal to the number of nonce/response pairs
gathered. This search of the password space can often be done in gathered. This search of the password space can often be done in
parallel on many machines, and even a single machine can search large parallel on many machines, and even a single machine can search large
subsets of the password space very quickly -- reports exist of subsets of the password space very quickly -- reports exist of
searching all passwords with six or fewer letters in a few hours. searching all passwords with six or fewer letters in a few hours.
The countermeasure against this attack is to for clients to use of The countermeasure against this attack is to for clients to use of
the "cnonce" parameter. the "cnonce" parameter.
5.12 Spoofing by Counterfeit Servers 5.12. Summary
Basic Authentication is vulnerable to spoofing by counterfeit
servers. If a user can be led to believe that she is connecting to a
host containing information protected by a password she knows, when
in fact she is connecting to a hostile server, then the hostile
server can request a password, store it away for later use, and feign
an error. This type of attack is more difficult with Digest
Authentication -- but the client must know to demand that Digest
authentication be used, perhaps using some of the techniques
described above to counter "man-in-the-middle" attacks. Again, the
user can be helped in detecting this attack by a visual indication of
the authentication mechanism in use with appropriate guidance in
interpreting the implications of each scheme.
5.13 Summary
By modern cryptographic standards Digest Authentication is weak. But By modern cryptographic standards Digest Authentication is weak. But
for a large range of purposes it is valuable as a replacement for for a large range of purposes it is valuable as a replacement for
Basic Authentication. It remedies some, but not all, weaknesses of Basic Authentication. It remedies some, but not all, weaknesses of
Basic Authentication. Its strength may vary depending on the Basic Authentication. Its strength may vary depending on the
implementation. In particular the structure of the nonce (which is implementation. In particular the structure of the nonce (which is
dependent on the server implementation) may affect the ease of dependent on the server implementation) may affect the ease of
mounting a replay attack. A range of server options is appropriate mounting a replay attack. A range of server options is appropriate
since, for example, some implementations may be willing to accept the since, for example, some implementations may be willing to accept the
server overhead of one-time nonces or digests to eliminate the server overhead of one-time nonces or digests to eliminate the
possibility of replay. Others may satisfied with a nonce like the one possibility of replay. Others may satisfied with a nonce like the
recommended above restricted to a single IP address and a single ETag one recommended above restricted to a single IP address and a single
or with a limited lifetime. ETag or with a limited lifetime.
The bottom line is that *any* compliant implementation will be The bottom line is that *any* compliant implementation will be
relatively weak by cryptographic standards, but *any* compliant relatively weak by cryptographic standards, but *any* compliant
implementation will be far superior to Basic Authentication. implementation will be far superior to Basic Authentication.
6 IANA Considerations 6. IANA Considerations
6.1 HTTP Digest Hash Algorithms Registry 6.1. HTTP Digest Hash Algorithms Registry
This specification creates a new IANA registry named "HTTP Digest This specification creates a new IANA registry named "HTTP Digest
Hash Algorithms". When registering a new hash algorithm, the Hash Algorithms". When registering a new hash algorithm, the
following information MUST be provided: following information MUST be provided:
o Hash Algorithm Hash Algorithm
The textual name of the hash algorithm. The textual name of the hash algorithm.
o Digest Size Digest Size
The size of the algorithm's output in bits. The size of the algorithm's output in bits.
o Reference Reference
A reference to the specification that describes the new algorithm. A reference to the specification that describes the new algorithm.
The update policy for this registry shall be Specification Required. The update policy for this registry shall be Specification Required.
The initial registry will contain the following entries: The initial registry will contain the following entries:
Hash Algorithm Digest Size Reference +----------------+-------------+-----------+
-------------- ----------- --------- | Hash Algorithm | Digest Size | Reference |
"MD5" 128 RFC XXXX +----------------+-------------+-----------+
"SHA-512-256" 256 RFC XXXX | "MD5" | 128 | RFC XXXX |
"SHA-256" 256 RFC XXXX | "SHA-512-256" | 256 | RFC XXXX |
| "SHA-256" | 256 | RFC XXXX |
+----------------+-------------+-----------+
Each one of the algorithms defined in the registry might have a -sess Each one of the algorithms defined in the registry might have a -sess
variant, e.g. MD5-sess, SHA-256-sess, etc. variant, e.g. MD5-sess, SHA-256-sess, etc.
6.2 Digest Scheme Registration 6.2. Digest Scheme Registration
This specification registers the Digest scheme with the This specification registers the Digest scheme with the
Authentication Scheme Registry. Authentication Scheme Registry.
Authentication Scheme Name: Digest Authentication Scheme Name: Digest
Pointer to specification text: RFCXXX Pointer to specification text: this specification
6.3 Authentication-Info Header Registration 6.3. Authentication-Info Header Registration
This specification registers the Authentication-Info Header field This specification registers the Authentication-Info Header field
with the Message Header Field Registry. with the Message Header Field Registry.
Header Field Name: Authentication-Info Header Field Name: Authentication-Info
Protocol: http Protocol: http
Status: standard Status: standard
Reference: RFCXXXX, Section 3.5 Reference: RFCXXXX, Section 3.5
7 Acknowledgments 7. Acknowledgments
The authors of this document would like to thank the authors of The authors of this document would like to thank the authors of
RFC2617, as this document heavily borrows text from their document to [RFC2617], as this document heavily borrows text from their document
provide a complete description of the digest scheme and its to provide a complete description of the digest scheme and its
operations. operations.
Special thanks to Julian Reschke for his reviews, comments, Special thanks to Julian Reschke for his reviews, comments,
suggestions, and text provided to various areas in this document. suggestions, and text provided to various areas in this document.
The authors would like to thank Stephen Farrell, Yoav Nir, Phillip The authors would like to thank Stephen Farrell, Yoav Nir, Phillip
Hallam-Baker, Manu Sporny, Paul Hoffman, Yaron Sheffer, Sean Turner, Hallam-Baker, Manu Sporny, Paul Hoffman, Yaron Sheffer, Sean Turner,
Geoff Baskwill, Eric Cooper, Bjoern Hoehrmann, Martin Durst, Peter Geoff Baskwill, Eric Cooper, Bjoern Hoehrmann, Martin Durst, Peter
Saint-Andre, Michael Sweet, Daniel Stenberg, Brett Tate, Paul Leach, Saint-Andre, Michael Sweet, Daniel Stenberg, Brett Tate, Paul Leach,
Ilari Liusvaara, and Gary Mort, Alexey Melnikov, and Benjamin Kaduk Ilari Liusvaara, and Gary Mort, Alexey Melnikov, and Benjamin Kaduk
for their careful review and comments. for their careful review and comments.
The authors would like to thank Jonathan Stoke, Nico Williams, Harry The authors would like to thank Jonathan Stoke, Nico Williams, Harry
Halpin, and Phil Hunt for their comments on the mailing list when Halpin, and Phil Hunt for their comments on the mailing list when
discussing various aspects of this document. discussing various aspects of this document.
The authors would like to thank Paul Kyzivat and Dale Worley for The authors would like to thank Paul Kyzivat and Dale Worley for
their careful review and feedback on some aspects of this document. their careful review and feedback on some aspects of this document.
8 References 8. References
8.1 Normative References 8.1. Normative References
[PRECIS] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", draft-ietf-precis-
saslprepbis-12 (work in progress), December 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000. Procedures", BCP 19, RFC 2978, October 2000.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003. 10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, Resource Identifier (URI): Generic Syntax", STD 66, RFC
RFC 3986, January 2005. 3986, January 2005.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
[RFC4513] Harrison, R., Ed., "Lightweight Directory Access Protocol [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
(LDAP): Authentication Methods and Security Mechanisms", Requirements for Security", BCP 106, RFC 4086, June 2005.
RFC 4513, June 2006.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008. Interchange", RFC 5198, March 2008.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Syntax Specifications: ABNF", STD 68, RFC 5234, January Specifications: ABNF", STD 68, RFC 5234, January 2008.
2008.
[RFC7230] Fielding, R., Reschke, J., "Hypertext Transfer Protocol [RFC5987] Reschke, J., "Character Set and Language Encoding for
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June Hypertext Transfer Protocol (HTTP) Header Field
2014. Parameters", RFC 5987, August 2010.
[RFC7234] Fielding, R., Nottingham, M., Reschke, J., "Hypertext [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June 2011.
2014.
[RFC7235] Fielding, R., Reschke, J., "Hypertext Transfer Protocol [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
(HTTP/1.1): Authentication", RFC 7235, June 2014. Protocol (HTTP/1.1): Message Syntax and Routing", RFC
7230, June 2014.
[BASIC] Reschke, J., "The 'Basic' HTTP Authentication Scheme", [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
draft-ietf-httpauth-basicauth-update (Work in Progress), Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
September 2013. June 2014.
[PRECIS] Saint-Andre, P. and A. Melnikov, "Preparation, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Enforcement, and Comparison of Internationalized Strings Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
Representing Usernames and Passwords", draft-ietf-precis- RFC 7234, June 2014.
saslprepbis-12 (work in progress), December 2014.
8.2 Informative References [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.
8.2. Informative References
[BASIC] Reschke, J., "The 'Basic' HTTP Authentication Scheme",
draft-ietf-httpauth-basicauth-update-04 (work in
progress), December 2014.
[RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP [RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
AUTHorize Extension for Simple Challenge/Response", AUTHorize Extension for Simple Challenge/Response", RFC
RFC 2195, September 1997. 2195, September 1997.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4513] Harrison, R., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, June 2006.
Authors' Addresses Authors' Addresses
Rifaat Shekh-Yusef (Editor) Rifaat Shekh-Yusef (editor)
Avaya Avaya
250 Sydney Street 250 Sidney Street
Belleville, Ontario Belleville, Ontario
Canada Canada
Phone: +1-613-967-5267 Phone: +1-613-967-5267
Email: rifaat.ietf@gmail.com EMail: rifaat.ietf@gmail.com
David Ahrens David Ahrens
Independent Independent
California California
USA USA
EMail: ahrensdc@gmail.com EMail: ahrensdc@gmail.com
Sophie Bremer Sophie Bremer
Netzkonform Netzkonform
Germany Germany
Email: sophie.bremer@netzkonform.de EMail: sophie.bremer@netzkonform.de
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