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2. Access Authentication Framework

2.1 Challenge and Response​

HTTP provides a simple challenge-response authentication framework that can be used by a server to challenge a client request and by a client to provide authentication information. It uses a case-insensitive token as a means to identify the authentication scheme, followed by additional information necessary for achieving authentication via that scheme. The latter can be either a comma-separated list of parameters or a single sequence of characters capable of holding base64-encoded information.

Authentication parameters are name=value pairs, where the name token is matched case-insensitively, and each parameter name MUST only occur once per challenge.

auth-scheme = token

auth-param = token BWS "=" BWS ( token / quoted-string )

token68 = 1*( ALPHA / DIGIT /
             "-" / "." / "_" / "~" / "+" / "/" ) *"="

The token68 syntax allows the 66 unreserved URI characters ([RFC3986]), plus a few others, so that it can hold a base64, base64url (URL and filename safe alphabet), base32, or base16 (hex) encoding, with or without padding, but excluding whitespace ([RFC4648]).

A 401 (Unauthorized) response message is used by an origin server to challenge the authorization of a user agent, including a WWW-Authenticate header field containing at least one challenge applicable to the requested resource.

A 407 (Proxy Authentication Required) response message is used by a proxy to challenge the authorization of a client, including a Proxy-Authenticate header field containing at least one challenge applicable to the proxy for the requested resource.

challenge = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

Note: Many clients fail to parse a challenge that contains an unknown scheme. A workaround for this problem is to list well-supported schemes (such as "basic") first.

A user agent that wishes to authenticate itself with an origin server -- usually, but not necessarily, after receiving a 401 (Unauthorized) -- can do so by including an Authorization header field with the request.

A client that wishes to authenticate itself with a proxy -- usually, but not necessarily, after receiving a 407 (Proxy Authentication Required) -- can do so by including a Proxy-Authorization header field with the request.

Both the Authorization field value and the Proxy-Authorization field value contain the client's credentials for the realm of the resource being requested, based upon a challenge received in a response (possibly at some point in the past). When creating their values, the user agent ought to do so by selecting the challenge with what it considers to be the most secure auth-scheme that it understands, obtaining credentials from the user as appropriate. Transmission of credentials within header field values implies significant security considerations regarding the confidentiality of the underlying connection, as described in Section 6.1.

credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

Upon receipt of a request for a protected resource that omits credentials, contains invalid credentials (e.g., a bad password) or partial credentials (e.g., when the authentication scheme requires more than one round trip), an origin server SHOULD send a 401 (Unauthorized) response that contains a WWW-Authenticate header field with at least one (possibly new) challenge applicable to the requested resource.

Likewise, upon receipt of a request that omits proxy credentials or contains invalid or partial proxy credentials, a proxy that requires authentication SHOULD generate a 407 (Proxy Authentication Required) response that contains a Proxy-Authenticate header field with at least one (possibly new) challenge applicable to the proxy.

A server that receives valid credentials that are not adequate to gain access ought to respond with the 403 (Forbidden) status code (Section 6.5.3 of [RFC7231]).

HTTP does not restrict applications to this simple challenge-response framework for access authentication. Additional mechanisms can be used, such as authentication at the transport level or via message encapsulation, and with additional header fields specifying authentication information. However, such additional mechanisms are not defined by this specification.

2.2 Protection Space (Realm)​

The "realm" authentication parameter is reserved for use by authentication schemes that wish to indicate a scope of protection.

A protection space is defined by the canonical root URI (the scheme and authority components of the effective request URI; see Section 5.5 of [RFC7230]) of the server being accessed, in combination with the realm value if present. These realms allow the protected resources on a server to be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm value is a string, generally assigned by the origin server, that can have additional semantics specific to the authentication scheme. Note that a response can have multiple challenges with the same auth-scheme but with different realms.

The protection space determines the domain over which credentials can be automatically applied. If a prior request has been authorized, the user agent MAY reuse the same credentials for all other requests within that protection space for a period of time determined by the authentication scheme, parameters, and/or user preferences (such as a configurable inactivity timeout). Unless specifically allowed by the authentication scheme, a single protection space cannot extend outside the scope of its server.

For historical reasons, a sender MUST only generate the quoted-string syntax. Recipients might have to support both token and quoted-string syntax for maximum interoperability with existing clients that have been accepting both notations for a long time.