The Hypertext Transfer Protocol (HTTP) Authentication-Info Header Field

This specification defines the "Authentication-Info" and "Proxy-Authentication-Info" header fields for use in HTTP authentication schemes () which need to return additional information during or after authentication. Both were previously defined in , defining the HTTP "Digest" authentication scheme. This document generalizes the description for use not only in "Digest" (), but also other future schemes that might have the same requirements for carrying additional information during authentication.

This specification uses the Augmented Backus-Naur Form (ABNF) notation of with a list extension, defined in , that allows for compact definition of comma-separated lists using a '#' operator (similar to how the '*' operator indicates repetition). The ABNF production for "auth-param" is defined in .

HTTP authentication schemes can use the Authentication-Info response header field to return additional information applicable to the authentication currently in use. The field value is a list of parameters (name/value pairs), using the "auth-param" syntax defined in . This specification only describes the generic format; authentication schemes using "Authentication-Info" will define the individual parameters. The "Digest" Authentication Scheme, for instance, defines multiple parameters in .

Authentication-Info = #auth-param The Authentication-Info header field can be used in any HTTP response, independently of request method and status code. Its semantics are defined by the applicable authentication scheme. Intermediaries are not allowed to modify the field value in any way. Authentication-Info can be used inside trailers ().

Parameter values can be expressed either as "token" or as "quoted-string" (). Authentication scheme definitions need to allow both notations, both for senders and recipients. This allows recipients to use generic parsing components, independent of the authentication scheme in use. For backwards compatibility, authentication scheme definitions can restrict the format for senders to one of the two variants. This can be important when it is known that deployed implementations will fail when encountering one of the two formats.

The Proxy-Authentication-Info header field is equivalent to Authentication-Info, except that it applies to proxy authentication ():

Proxy-Authentication-Info = #auth-param

Adding information to HTTP responses can affect security and privacy. The presence of the header fields alone indicates that HTTP authentication is in use. Additional information could be exposed by the contents of the authentication-scheme specific parameters; this will have to be considered in the definitions of these schemes.

HTTP header fields are registered within the "Message Headers" registry located at , as defined by . This document updates the definitions of the "Authentication-Info" and "Proxy-Authentication-Info" header fields, so the "Permanent Message Header Field Names" registry shall be updated accordingly: Header Field Name Protocol Status Reference Authentication-Info http standard of this document Proxy-Authentication-Info http standard of this document