Subject: Type 3 -- The DIGEST-MD5 Digest-Response
Author: SteveHB
In response to: Type 2 -- The DIGEST-MD5 digest-challenge
Posted on: 06/13/2006 01:34:35 PM
Structure:
digest-response = 1#( username | realm | nonce | cnonce |
nonce-count | qop | digest-uri | response |
maxbuf | charset | cipher | authzid |
auth-param )
username = "username" "=" <"> username-value <">
username-value = qdstr-val
cnonce = "cnonce" "=" <"> cnonce-value <">
cnonce-value = qdstr-val
nonce-count = "nc" "=" nc-value
nc-value = 8LHEX
qop = "qop" "=" qop-value
digest-uri = "digest-uri" "=" <"> digest-uri-value <">
digest-uri-value = serv-type "/" host [ "/" serv-name ]
serv-type = 1*ALPHA
host = 1*( ALPHA | DIGIT | "-" | "." )
serv-name = host
response = "response" "=" response-value
response-value = 32LHEX
LHEX = "0" | "1" | "2" | "3" |
"4" | "5" | "6" | "7" |
"8" | "9" | "a" | "b" |
"c" | "d" | "e" | "f"
cipher = "cipher" "=" cipher-value
authzid = "authzid" "=" <"> authzid-value <">
authzid-value = qdstr-val
username The user's name in the specified realm, encoded according to the value of the "charset" directive. This directive is required and MUST be present exactly once; otherwise, authentication fails.
realm The realm containing the user's account. This directive is required if the server provided any realms in the "digest-challenge", in which case it may appear exactly once and its value SHOULD be one of those realms. If the directive is missing, "realm-value" will set to the empty string when computing A1 (see below for details).
nonce The server-specified data string received in the preceding digest-challenge. This directive is required and MUST be present exactly once; otherwise, authentication fails.
cnonce A client-specified data string which MUST be different each time a digest-response is sent as part of initial authentication. The cnonce-value is an opaque quoted string value provided by the client and used by both client and server to avoid chosen plaintext attacks, and to provide mutual authentication. The security of the implementation depends on a good choice. It is RECOMMENDED that it contain at least 64 bits of entropy. This directive is required and MUST be present exactly once; otherwise, authentication fails.
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 directive 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. This directive may appear at most once; if multiple instances are present, the client should abort the authentication exchange.
qop Indicates what "quality of protection" the client accepted. If present, it may appear exactly once and its value MUST be one of the alternatives in qop-options. If not present, it defaults to "auth". These values affect the computation of the response. Note that this is a single token, not a quoted list of alternatives.
serv-type Indicates the type of service, such as "www" for web service, "ftp" for FTP service, "smtp" for mail delivery service, etc. The service name as defined in the SASL profile for the protocol see section 4 of [RFC 2222], registered in the IANA registry of "service" elements for the GSSAPI host-based service name form [RFC 2078].
host The DNS host name or IP address for the service requested. The DNS host name must be the fully-qualified canonical name of the host. The DNS host name is the preferred form; see notes on server processing of the digest-uri.
serv-name Indicates the name of the service if it is replicated. The service is considered to be replicated if the client's service-location process involves resolution using standard DNS lookup operations, and if these operations involve DNS records (such as SRV, or MX) which resolve one DNS name into a set of other DNS names. In this case, the initial name used by the client is the "serv-name", and the final name is the "host" component. For example, the incoming mail service for "example.com" may be replicated through the use of MX records stored in the DNS, one of which points at an SMTP server called "mail3.example.com"; it's "serv-name" would be "example.com", it's "host" would be "mail3.example.com". If the service is not replicated, or the serv-name is identical to the host, then the serv-name component MUST be omitted.
digest-uri Indicates the principal name of the service with which the client wishes to connect, formed from the serv-type, host, and serv-name. For example, the FTP service on "ftp.example.com" would have a "digest-uri" value of "ftp/ftp.example.com"; the SMTP server from the example above would have a "digest-uri" value of "smtp/mail3.example.com/example.com".
Servers SHOULD check that the supplied value is correct. This will detect accidental connection to the incorrect server. It is also so that clients will be trained to provide values that will work with implementations that use a shared back-end authentication service that can provide server authentication.
The serv-type component should match the service being offered. The host component should match one of the host names of the host on which the service is running, or it's IP address. Servers SHOULD NOT normally support the IP address form, because server authentication by IP address is not very useful; they should only do so if the DNS is unavailable or unreliable. The serv-name component should match one of the service's configured service names.
This directive may appear at most once; if multiple instances are present, the client should abort the authentication exchange.
Note: In the HTTP use of Digest authentication, the digest-uri is the URI (usually a URL) of the resource requested -- hence the name of the directive.
response A string of 32 hex digits computed as defined below, which proves that the user knows a password. This directive is required and MUST be present exactly once; otherwise, authentication fails.
maxbuf A number indicating the size of the largest buffer the client is able to receive. If this directive is missing, the default value is 65536. This directive may appear at most once; if multiple instances are present, the server should abort the authentication exchange.
charset This directive, if present, specifies that the client has used UTF-8 encoding for the username and password. If not present, the username and password must be encoded in ISO 8859-1 (of which US-ASCII is a subset). The client should send this directive only if the server has indicated it supports UTF-8. The directive is needed for backwards compatibility with HTTP Digest, which only supports ISO 8859-1.
LHEX 32 hex digits, where the alphabetic characters MUST be lower case, because MD5 is not case insensitive.
cipher The cipher chosen by the client. This directive MUST appear exactly once if "auth-conf" is negotiated; if required and not present, authentication fails.
authzid The "authorization ID" as per RFC 2222, encoded in UTF-8. This directive is optional. If present, and the authenticating user has sufficient privilege, and the server supports it, then after authentication the server will use this identity for making all accesses and access checks. If the client specifies it, and the server does not support it, then the response-value will be incorrect, and authentication will fail.
The size of a digest-response MUST be less than 4096 bytes.
>
> On 06/13/2006 01:17:32 PM
SteveHB wrote:
Structure:
digest-challenge =
1#( realm | nonce | qop-options | stale | maxbuf | charset
algorithm | cipher-opts | auth-param )
realm = "realm" "=" <"> realm-value <">
realm-value = qdstr-val
nonce = "nonce" "=" <"> nonce-value <">
nonce-value = qdstr-val
qop-options = "qop" "=" <"> qop-list <">
qop-list = 1#qop-value
qop-value = "auth" | "auth-int" | "auth-conf" |
token
stale = "stale" "=" "true"
maxbuf = "maxbuf" "=" maxbuf-value
maxbuf-value = 1*DIGIT
charset = "charset" "=" "utf-8"
algorithm = "algorithm" "=" "md5-sess"
cipher-opts = "cipher" "=" <"> 1#cipher-value <">
cipher-value = "3des" | "des" | "rc4-40" | "rc4" |
"rc4-56" | token
auth-param = token "=" ( token | quoted-string )
The meanings of the values of the directives used above are as follows:
realm
Mechanistically, a string which can enable users to know which username and password to use, in case they might have different ones for different servers. Conceptually, it is the name of a collection of accounts that might include the user's account. This string should contain at least the name of the host performing the authentication and might additionally indicate the collection of users who might have access. An example might be "registered_users@gotham.news.example.com". This directive is optional; if not present, the client SHOULD solicit it from the user or be able to compute a default; a plausible default might be the realm supplied by the user when they logged in to the client system. Multiple realm directives are allowed, in which case the user or client must choose one as the realm for which to supply to username and password.
nonce
A server-specified data string which MUST be different each time a digest-challenge is sent as part of initial authentication. It is recommended that this string be base64 or hexadecimal data. Note that since the string is passed as a quoted string, the double-quote character is not allowed unless escaped. The contents of the nonce are implementation dependent. The security of the implementation depends on a good choice. It is RECOMMENDED that it contain at least 64 bits of entropy. The nonce is opaque to the client. This directive is required and MUST appear exactly once; if not present, or if multiple instances are present, the client should abort the authentication exchange.
qop-options
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; the value "auth-conf" indicates authentication with integrity protection and encryption. This directive is optional; if not present it defaults to "auth". The client MUST ignore unrecognized options; if the client recognizes no option, it should abort the authentication exchange.
stale
The "stale" directive is not used in initial authentication. This directive may appear at most once; if multiple instances are present, the client should abort the authentication exchange.
maxbuf
A number indicating the size of the largest buffer the server is able to receive when using "auth-int" or "auth-conf". If this directive is missing, the default value is 65536. This directive may appear at most once; if multiple instances are present, the client should abort the authentication exchange.
charset
This directive, if present, specifies that the server supports UTF-8 encoding for the username and password. If not present, the username and password must be encoded in ISO 8859-1 (of which US-ASCII is a subset). The directive is needed for backwards compatibility with HTTP Digest, which only supports ISO 8859-1. This directive may appear at most once; if multiple instances are present, the client should abort the authentication exchange.
algorithm
This directive is required for backwards compatibility with HTTP Digest., which supports other algorithms. . This directive is required and MUST appear exactly once; if not present, or if multiple instances are present, the client should abort the authentication exchange.
cipher-opts
A list of ciphers that the server supports. This directive must be present exactly once if "auth-conf" is offered in the "qop-options" directive, in which case the "3des" and "des" modes are mandatory-to-implement. The client MUST ignore unrecognized options; if the client recognizes no option, it should abort the authentication exchange.
des
the Data Encryption Standard (DES) cipher [FIPS] in cipher block chaining (CBC) mode with a 56 bit key.
3des
the "triple DES" cipher in CBC mode with EDE with the same key for each E stage (aka "two keys mode") for a total key length of 112 bits.
rc4, rc4-40, rc4-56
the RC4 cipher with a 128 bit, 40 bit, and 56 bit key, respectively.
auth-param
This construct allows for future extensions; it may appear more than once. The client MUST ignore any unrecognized directives.
For use as a SASL mechanism, note that the following changes are made to "digest-challenge" from HTTP: the following Digest options (called "directives" in HTTP terminology) are unused (i.e., MUST NOT be sent, and MUST be ignored if received):
opaque
domain
The size of a digest-challenge MUST be less than 2048 bytes.
References:
