Internet Engineering Task Force (IETF) R. Shekh-Yusef, Ed. Request for Comments: 7616 Avaya Obsoletes: 2617 D. Ahrens Category: Standards Track Independent ISSN: 2070-1721 S. Bremer Netzkonform September 2015 HTTP Digest Access Authentication
AbstractThe Hypertext Transfer Protocol (HTTP) provides a simple challenge- response authentication mechanism that may be used by a server to challenge a client request and by a client to provide authentication information. This document defines the HTTP Digest Authentication scheme that can be used with the HTTP authentication mechanism. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7616.
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. Syntax Convention . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Digest Access Authentication Scheme . . . . . . . . . . . . . 5 3.1. Overall Operation . . . . . . . . . . . . . . . . . . . . 5 3.2. Representation of Digest Values . . . . . . . . . . . . . 5 3.3. The WWW-Authenticate Response Header Field . . . . . . . 5 3.4. The Authorization Header Field . . . . . . . . . . . . . 9 3.4.1. Response . . . . . . . . . . . . . . . . . . . . . . 11 3.4.2. A1 . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4.3. A2 . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.4.4. Username Hashing . . . . . . . . . . . . . . . . . . 12 3.4.5. Parameter Values and Quoted-String . . . . . . . . . 12 3.4.6. Various Considerations . . . . . . . . . . . . . . . 13 3.5. The Authentication-Info and Proxy-Authentication-Info Header Fields . . . . . . . . . . . . . . . . . . . . . . 14 3.6. Digest Operation . . . . . . . . . . . . . . . . . . . . 15 3.7. Security Protocol Negotiation . . . . . . . . . . . . . . 16 3.8. Proxy-Authenticate and Proxy-Authorization . . . . . . . 17 3.9. Examples . . . . . . . . . . . . . . . . . . . . . . . . 18 3.9.1. Example with SHA-256 and MD5 . . . . . . . . . . . . 18 3.9.2. Example with SHA-512-256, Charset, and Userhash . . . 19 4. Internationalization Considerations . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . 23 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 Attacks . . . . . . . . . . . . . . . . 25 5.9. Chosen Plaintext Attacks . . . . . . . . . . . . . . . . 25 5.10. Precomputed Dictionary Attacks . . . . . . . . . . . . . 26 5.11. Batch Brute-Force Attacks . . . . . . . . . . . . . . . . 26 5.12. Parameter Randomness . . . . . . . . . . . . . . . . . . 26 5.13. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 26 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 6.1. Hash Algorithms for HTTP Digest Authentication . . . . . 27 6.2. Digest Scheme Registration . . . . . . . . . . . . . . . 28 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 28 7.1. Normative References . . . . . . . . . . . . . . . . . . 28 7.2. Informative References . . . . . . . . . . . . . . . . . 30 Appendix A. Changes from RFC 2617 . . . . . . . . . . . . . . . 31
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32 RFC2617]. See Appendix A for the new capabilities introduced by this specification. The details of the challenge-response authentication mechanism are specified in the "Hypertext Transfer Protocol (HTTP/1.1): Authentication" [RFC7235]. The combination of this document with the definition of the "Basic" authentication scheme [RFC7617], "HTTP Authentication-Info and Proxy- Authentication-Info Response Header Fields" [RFC7615], and "Hypertext Transfer Protocol (HTTP/1.1): Authentication" [RFC7235] obsolete [RFC2617]. RFC2119]. RFC5234] and the ABNF List Extension of [RFC7230].
RFC2617], the MD5 algorithm is still supported but NOT RECOMMENDED. The size of the digest depends on the algorithm used. The bits in the digest are converted from the most significant to the least significant bit, four bits at a time, to the ASCII representation as follows. Each sequence of four bits is represented by its familiar hexadecimal notation from the characters 0123456789abcdef; that is, binary 0000 is 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 calculate the digest, then the MD5 digest will be represented as 32 hexadecimal characters, while SHA-256 and SHA-512/256 are represented as 64 hexadecimal characters.
example is "email@example.com". (See Section 2.2 of [RFC7235] for more details.) domain A quoted, space-separated list of URIs, as specified in [RFC3986], that define the protection space. If a URI is a path-absolute, it is relative to the canonical root URL. (See Section 2.2 of [RFC7235].) An absolute-URI in this list may refer to a different server than the web-origin [RFC6454]. The client can use this list to determine the set of URIs for which the same authentication information may be sent: any URI that has a URI in this list as a prefix (after both have been made absolute) MAY be assumed to be in the same protection space. If this parameter is omitted or its value is empty, the client SHOULD assume that the protection space consists of all URIs on the web-origin. This parameter is not meaningful in Proxy-Authenticate header fields, for which the protection space is always the entire proxy; if present, it MUST be ignored. nonce A server-specified string which should be uniquely generated each time a 401 response is made. It is advised that this string be Base64 or hexadecimal data. Specifically, since the string is passed in the header field lines as a quoted string, the double- quote character is not allowed, unless suitably escaped. The contents of the nonce are implementation dependent. The quality of the implementation depends on a good choice. A nonce might, for example, be constructed as the Base64 encoding of timestamp H(timestamp ":" ETag ":" secret-data) where timestamp is a server-generated time, which preferably includes micro- or nanoseconds, or other non-repeating values; ETag is the value of the HTTP ETag header field associated with the requested entity; and secret-data is data known only to the server. With a nonce of this form, a server would recalculate the hash portion after receiving the client authentication header field and reject the request if it did not match the nonce from that header field or if the timestamp value is not recent enough. 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 version of the resource. Including the IP address of the client 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. However, that would break because requests from a single user often go through different proxies. Also, IP address spoofing is not that hard. An implementation might choose not to accept a previously used nonce or a previously used digest, in order to protect against a replay attack. Or, an implementation might choose to use one-time nonces or digests for POST or PUT requests and a timestamp for GET requests. For more details on the issues involved, see Section 5 of this document. The nonce is opaque to the client. opaque A string of data, specified by the server, that SHOULD be returned by the client unchanged in the Authorization header field of subsequent requests with URIs in the same protection space. It is RECOMMENDED that this string be Base64 or hexadecimal data. stale A case-insensitive flag indicating that the previous request from the client was rejected because the nonce value was stale. If stale is true, the client may wish to simply retry the request with a new encrypted response, without re-prompting the user for a new username and password. The server SHOULD only set stale to true if it receives a request for which the nonce is invalid. If stale is false, or anything other than true, or the stale parameter is not present, the username and/or password are invalid, and new values MUST be obtained. algorithm A string indicating an algorithm used to produce the digest and an unkeyed digest. If this is not present, it is assumed to be "MD5". If the algorithm is not understood, the challenge SHOULD 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 algorithm to the data "data" with secret "secret" will be denoted by KD(secret, data), and the string obtained by applying the
unkeyed digest algorithm to the data "data" will be denoted H(data). KD stands for Keyed Digest, and the notation unq(X) means the value of the quoted-string X without the surrounding quotes and with quoting slashes removed. For "<algorithm>" and "<algorithm>-sess" H(data) = <algorithm>(data) and KD(secret, data) = H(concat(secret, ":", data)) For example: For the "SHA-256" and "SHA-256-sess" algorithms H(data) = SHA-256(data) i.e., the digest is the "<algorithm>" of the secret concatenated with a colon concatenated with the data. The "<algorithm>-sess" is intended to allow efficient third-party authentication servers; for the difference in usage, see the description in Section 3.4.2. qop This parameter MUST be used by all implementations. It is a quoted string of one or more tokens indicating the "quality of protection" values supported by the server. The value "auth" indicates authentication; the value "auth-int" indicates authentication with integrity protection. See the descriptions below for calculating the response parameter value for the application of this choice. Unrecognized options MUST be ignored. charset This is an OPTIONAL parameter that is used by the server to indicate the encoding scheme it supports. The only allowed value is "UTF-8". userhash This is an OPTIONAL parameter that is used by the server to indicate that it supports username hashing. Valid values are: "true" or "false". Default value is "false".
For historical reasons, a sender MUST only generate the quoted string syntax values for the following parameters: realm, domain, nonce, opaque, and qop. For historical reasons, a sender MUST NOT generate the quoted string syntax values for the following parameters: stale and algorithm. RFC5987]. realm See "realm" definition in Section 3.3. uri The Effective Request URI (Section 5.5 of [RFC7230]) of the HTTP request; duplicated here because proxies are allowed to change the request target ("request-target", Section 3.1.1 of [RFC7230]) in transit.
qop Indicates what "quality of protection" the client has applied to the message. Its value MUST be one of the alternatives the server indicated it supports in the WWW-Authenticate header field. These values affect the computation of the response. Note that this is a single token, not a quoted list of alternatives as in WWW- Authenticate. cnonce This parameter MUST be used by all implementations. The cnonce value is an opaque quoted ASCII-only string value provided by the client and used by both client and server to avoid chosen plaintext attacks, to provide mutual authentication, and to provide some message integrity protection. See the descriptions below of the calculation of the rspauth and response values. nc This parameter MUST be used by all implementations. The nc parameter stands for "nonce count". The nc value is the hexadecimal count of the number of requests (including the current request) that the client has sent with the nonce value in this request. For example, in the first request sent in response to a given nonce value, the client sends "nc=00000001". The purpose of this parameter is to allow the server to detect request replays by maintaining its own copy of this count -- if the same nc value is seen twice, then the request is a replay. See the description below of the construction of the response value. userhash This OPTIONAL parameter is used by the client to indicate that the 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 missing, the proper response is a 4xx error code. If the response is invalid, then a login failure SHOULD be logged, since repeated login failures from a single client may indicate an attacker attempting to
guess passwords. The server implementation SHOULD be careful with the information being logged so that it won't put a cleartext password (e.g., entered into the username field) into the log. The definition of the response above indicates the encoding for its value. The following definitions show how the value is computed. Section 3.6.) Because the server needs only use the hash of the user credentials in order to create the A1 value, this construction could
be used in conjunction with a third-party authentication service so that the web server would not need the actual password value. The specification of such a protocol is beyond the scope of this specification.
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 strings or entity body whose contents make up the string to be digested. For example, the string A1 illustrated above must be Mufasa:firstname.lastname@example.org:Circle Of Life 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 other strings digested by H() must not have white space on either side of the colons that delimit their fields, unless that white space was in the quoted strings or entity body being digested. 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 -- it is computed before any transfer encoding is applied by the sender and after it has been removed by the recipient. Note that this includes multipart boundaries and embedded header fields in each part of any multipart content-type. Section 3.1.1 of [RFC7230]. The "request-target" value is the request-target from the request line as specified in Section 3.1.1 of [RFC7230]. This MAY be "*", an "absolute-URI", or an "absolute-path" as specified in Section 2.7 of [RFC7230], but it MUST agree with the request-target. In particular, it MUST be an "absolute-URI" if the request-target is an "absolute-URI". The cnonce value is a client-chosen value whose purpose is to foil chosen plaintext attacks. The authenticating server MUST assure that the resource designated by 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 error. (Since this may be a symptom of an attack, server implementers may want to consider logging such errors.) The purpose of duplicating information from the request URL in this field is to deal with the possibility that an intermediate proxy may alter the client's Request-Line. This altered (but presumably semantically equivalent) request would not result in the same digest as that calculated by the client. Implementers should be aware of how authenticated transactions need to interact with shared caches (see [RFC7234]).
RFC7615] are generic fields that MAY be used by a server to communicate some information regarding the successful authentication of a client response. The Digest Authentication scheme MAY add the Authentication-Info header field in the confirmation request and include parameters from the following list: nextnonce The value of the nextnonce parameter is the nonce the server wishes the client to use for a future authentication response. The server MAY send the Authentication-Info header field with a nextnonce field as a means of implementing one-time nonces or otherwise changing nonces. If the nextnonce field is present, the client SHOULD use it when constructing the Authorization header field for 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". Server implementations SHOULD carefully consider the performance implications of the use of this mechanism; pipelined requests will not be possible if every response includes a nextnonce parameter that MUST be used on the next request received by the server. Consideration SHOULD be given to the performance vs. security tradeoffs of allowing an old nonce value to be used for a limited time to permit request pipelining. Use of the nc parameter can retain most of the security advantages of a new server nonce without the deleterious effects on pipelining. qop Indicates the "quality of protection" options applied to the response by the server. The value "auth" indicates authentication; the value "auth-int" indicates authentication with integrity protection. The server SHOULD use the same value for the qop parameter in the response as was sent by the client in the corresponding request.
rspauth The optional response digest in the rspauth parameter supports mutual authentication -- the server proves that it knows the user's secret, and with qop=auth-int also provides limited integrity protection of the response. The rspauth value is calculated as for the response in the Authorization header field, except that if qop is set to "auth" or is not specified in the Authorization header field for the request, A2 is A2 = ":" request-uri and if "qop=auth-int", then A2 is A2 = ":" request-uri ":" H(entity-body) cnonce and nc The cnonce value and nc value MUST be the ones for the client request to which this message is the response. The rspauth, cnonce, and nc parameters MUST be present if "qop=auth" or "qop=auth-int" is specified. The Authentication-Info header field is allowed in the trailer of an HTTP message transferred via chunked transfer coding. 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.
The client response to a WWW-Authenticate challenge for a protection space starts an authentication session with that protection space. The authentication session lasts until the client receives another WWW-Authenticate challenge from any server in the protection space. A client SHOULD remember the username, password, nonce, nonce count, and opaque values associated with an authentication session to use to construct the Authorization header field in future requests within that protection space. The Authorization header field MAY be included preemptively; doing so improves server efficiency and avoids extra round trips for authentication challenges. The server MAY choose to accept the old Authorization header field information, even though the nonce value included might not be fresh. Alternatively, the server MAY return a 401 response with a new nonce value in the WWW-Authenticate header field, causing the client to retry the request; by specifying "stale=true" with this response, the server tells the client to retry with the new nonce, but without prompting for a new username and password. 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, the opaque data can be used to transport authentication session state information. (Note that any such use can also be accomplished more easily and safely by including the state in the nonce.) For example, a server could be responsible for authenticating content that actually sits on another server. It would achieve this by having the first 401 response include a domain parameter whose value includes a URI on the second server, and an opaque parameter whose value contains the state information. The client will retry the request, at which time the server might respond with "HTTP Redirection" (Section 6.4 of [RFC7231]), pointing to the URI on the second server. The client will follow the redirection and pass an Authorization header field, including the <opaque> data. 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- Authenticate and Proxy-Authorization header fields described in Section 3.8 below.
client supports it. A client is encouraged to fail gracefully if the server specifies only authentication schemes it cannot handle. When a server receives a request to access a resource, the server might challenge the client by responding with "401 Unauthorized" response and include one or more WWW-Authenticate header fields. If the server responds with multiple challenges, then each one of these challenges MUST use a different digest algorithm. The server MUST add these challenges to the response in order of preference, starting with the most preferred algorithm, followed by the less preferred algorithm. This specification defines the following algorithms: o SHA2-256 (mandatory to implement) o SHA2-512/256 (as a backup algorithm) o MD5 (for backward compatibility). When the client receives the first challenge, it SHOULD use the first challenge it supports, unless a local policy dictates otherwise. Sections 4.3 and 4.4 of the HTTP/1.1 specification [RFC7235], and their behavior is subject to restrictions described there. The transactions for proxy authentication are very similar to those already described. Upon receiving a request that requires authentication, the proxy/server MUST issue the "407 Proxy Authentication Required" response with a "Proxy-Authenticate" header field. The digest-challenge used in the Proxy-Authenticate header field is the same as that for the WWW- Authenticate header field as defined above in Section 3.3. The client/proxy MUST then reissue the request with a Proxy- Authorization header field, with parameters as specified for the Authorization header field in Section 3.4 above. On subsequent responses, the server sends Proxy-Authentication-Info with parameters the same as those for the Authentication-Info header field.
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.
If the client chooses to use the SHA-256 algorithm for calculating the response, the client responds with a new request including the following Authorization header field: Authorization: Digest username="Mufasa", realm="email@example.com", uri="/dir/index.html", algorithm=SHA-256, nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v", nc=00000001, cnonce="f2/wE4q74E6zIJEtWaHKaf5wv/H5QzzpXusqGemxURZJ", qop=auth, response="753927fa0e85d155564e2e272a28d1802ca10daf449 6794697cf8db5856cb6c1", opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"
The client can prompt the user for the required credentials and send a new request with following Authorization header field: Authorization: Digest username="488869477bf257147b804c45308cd62ac4e25eb717 b12b298c79e62dcea254ec", realm="firstname.lastname@example.org", uri="/doe.json", algorithm=SHA-512-256, nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK", nc=00000001, cnonce="NTg6RKcb9boFIAS3KrFK9BGeh+iDa/sm6jUMp2wds69v", qop=auth, response="ae66e67d6b427bd3f120414a82e4acff38e8ecd9101d 6c861229025f607a79dd", opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS", userhash=true If the client cannot provide a hashed username for any reason, the client can try a request with this Authorization header field: Authorization: Digest username*=UTF-8''J%C3%A4s%C3%B8n%20Doe, realm="email@example.com", uri="/doe.json", algorithm=SHA-512-256, nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK", nc=00000001, cnonce="NTg6RKcb9boFIAS3KrFK9BGeh+iDa/sm6jUMp2wds69v", qop=auth, response="ae66e67d6b427bd3f120414a82e4acff38e8ecd9101d 6c861229025f607a79dd", opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS", userhash=false Section 3.4.2) and username hashing (see Section 3.4.4). The only allowed value is "UTF-8", to be matched case-insensitively (see Section 2.3 in [RFC2978]). It indicates that the server expects the username and password to be converted to Unicode Normalization Form C ("NFC", see Section 3 of [RFC5198]) and to be encoded into octets using the UTF-8 character encoding scheme [RFC3629].
For the username, recipients MUST support all characters defined in the "UsernameCasePreserved" profile defined in Section 3.3 of [RFC7613], with the exception of the colon (":") character. For the password, recipients MUST support all characters defined in the "OpaqueString" profile defined in Section 4.2 of [RFC7613]. If the user agent does not support the encoding indicated by the server, it can fail the request. When usernames cannot be sent hashed and include non-ASCII characters, clients can include the username* parameter instead (using the value encoding defined in [RFC5987]). RFC2818].
realm is part of the digested data stored in the password file. It means that if one Digest Authentication password file is compromised, it does not automatically compromise others with the same username and password (though it does expose them to brute-force attack). There are two important security consequences of this. First, the password file must be protected as if it contained unencrypted passwords, because, for the purpose of accessing documents in its realm, it effectively does. A second consequence of this is that the realm string SHOULD be unique among all realms that any single user is likely to use. In particular, a realm string SHOULD include the name of the host doing the authentication. The inability of the client to authenticate the server is a weakness of Digest Authentication. RFC4513] and IMAP/POP (see [RFC2195]). It was intended to replace the much weaker and even more dangerous Basic mechanism. Digest Authentication offers no confidentiality protection beyond protecting the actual username and password. All of the rest of the request and response are available to an eavesdropper. Digest Authentication offers only limited integrity protection for the messages in either direction. If the "qop=auth-int" mechanism is used, those parts of the message used in the calculation of the WWW- Authenticate and Authorization header field response parameter values (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. Many needs for secure HTTP transactions cannot be met by Digest Authentication. For those needs, TLS is a more appropriate protocol. In particular, Digest Authentication cannot be used for any transaction requiring confidentiality protection. Nevertheless, many functions remain for which Digest Authentication is both useful and appropriate.
Section 3.4.1 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 particular client, for a particular resource, for a limited period of time or number of uses, or any other restrictions. Doing so strengthens the protection provided against, for example, replay attacks (see Section 5.5). However, it should be noted that the method chosen for generating and checking the nonce also has performance and resource implications. For example, a server MAY choose to allow each nonce value to be used only once by maintaining a record of whether or not each recently issued nonce has been returned and sending a next-nonce parameter in the Authentication- Info header field of every response. This protects against even an immediate replay attack, but it has a high cost due to checking nonce values; perhaps more important, it will cause authentication failures for any pipelined requests (presumably returning a stale nonce indication). Similarly, 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 and also defeats pipelining. Thus, it MAY be useful to do so for methods with side effects but have unacceptable performance for those that do not.
For applications where no possibility of replay attack can be tolerated, the server can use one-time nonce values that will not be honored for a second use. This requires the overhead of the server remembering which nonce values have been used until the nonce timestamp (and hence the digest built with it) has expired, but it effectively protects against replay attacks. An implementation must give special attention to the possibility of 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 the integrity protection of "qop=auth-int", an attacker could replay valid credentials from a successful request with counterfeit data or other message body. Even with the use of integrity protection, most metadata in header fields is not protected. Proper nonce generation and checking provides some protection against replay of previously used valid credentials, but see Section 5.8. Section 5.7 below for discussion of particular attack scenarios that exploit multiple authentication schemes.
dependent on the server implementation) may affect the ease of mounting a replay attack. A range of server options is appropriate since, for example, some implementations may be willing to accept the server overhead of one-time nonces or digests to eliminate the possibility of replay. Others may be satisfied with a nonce like the one recommended above, i.e., restricted to a single IP address and a single ETag or with a limited lifetime. The bottom line is that *any* compliant implementation will be relatively weak by cryptographic standards, but *any* compliant implementation will be far superior to Basic Authentication. RFC5226].
The initial registry contains the following entries: +----------------+-------------+-----------+ | Hash Algorithm | Digest Size | Reference | +----------------+-------------+-----------+ | "MD5" | 128 | RFC 7616 | | "SHA-512-256" | 256 | RFC 7616 | | "SHA-256" | 256 | RFC 7616 | +----------------+-------------+-----------+ Each one of the algorithms defined in the registry might have a "-sess" variant, e.g., MD5-sess, SHA-256-sess, etc. To clarify the purpose of the existing "HTTP Digest Algorithm Values" registry and to avoid confusion between the two registries, IANA has added the following description to the existing "HTTP Digest Algorithm Values" registry: This registry lists the algorithms that can be used when creating digests of an HTTP message body, as specified in RFC 3230. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978, October 2000, <http://www.rfc-editor.org/info/rfc2978>. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <http://www.rfc-editor.org/info/rfc3629>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <http://www.rfc-editor.org/info/rfc3986>. [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005, <http://www.rfc-editor.org/info/rfc4086>. [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008, <http://www.rfc-editor.org/info/rfc5198>. [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <http://www.rfc-editor.org/info/rfc5234>. [RFC5987] Reschke, J., "Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010, <http://www.rfc-editor.org/info/rfc5987>. [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011, <http://www.rfc-editor.org/info/rfc6454>. [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>. [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, <http://www.rfc-editor.org/info/rfc7231>. [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014, <http://www.rfc-editor.org/info/rfc7234>. [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Authentication", RFC 7235, DOI 10.17487/RFC7235, June 2014, <http://www.rfc-editor.org/info/rfc7235>.
[RFC7613] Saint-Andre, P. and A. Melnikov, "Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords", RFC 7613, DOI 10.17487/RFC7613, August 2015, <http://www.rfc-editor.org/info/rfc7613>. [RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy- Authentication-Info Response Header Fields", RFC 7615, DOI 10.17487/RFC7615, September 2015, <http://www.rfc-editor.org/info/rfc7615>. [RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP AUTHorize Extension for Simple Challenge/Response", RFC 2195, DOI 10.17487/RFC2195, September 1997, <http://www.rfc-editor.org/info/rfc2195>. [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, DOI 10.17487/RFC2617, June 1999, <http://www.rfc-editor.org/info/rfc2617>. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000, <http://www.rfc-editor.org/info/rfc2818>. [RFC4513] Harrison, R., Ed., "Lightweight Directory Access Protocol (LDAP): Authentication Methods and Security Mechanisms", RFC 4513, DOI 10.17487/RFC4513, June 2006, <http://www.rfc-editor.org/info/rfc4513>. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme", RFC 7617, DOI 10.17487/RFC7617, September 2015, <http://www.rfc-editor.org/info/rfc7617>.
RFC 2069. RFC2617]. The authors of this document would like to thank John Franks, Phillip M. Hallam-Baker, Jeffery L. Hostetler, Scott D. Lawrence, Paul J. Leach, Ari Luotonen, and Lawrence C. Stewart for their work on that specification. Special thanks to Julian Reschke for his many reviews, comments, suggestions, and text provided to various areas in this document. The authors would like to thank Stephen Farrell, Yoav Nir, Phillip Hallam-Baker, Manu Sporny, Paul Hoffman, Yaron Sheffer, Sean Turner, Geoff Baskwill, Eric Cooper, Bjoern Hoehrmann, Martin Durst, Peter Saint-Andre, Michael Sweet, Daniel Stenberg, Brett Tate, Paul Leach, Ilari Liusvaara, Gary Mort, Alexey Melnikov, Benjamin Kaduk, Kathleen Moriarty, Francis Dupont, Hilarie Orman, and Ben Campbell for their careful review and comments. The authors would like to thank Jonathan Stoke, Nico Williams, Harry Halpin, and Phil Hunt for their comments on the mailing list when discussing various aspects of this document. The authors would like to thank Paul Kyzivat and Dale Worley for their careful review and feedback on some aspects of this document.
The authors would like to thank Barry Leiba for his help with the registry.