This library implements HTTP Digest Authentication as per RFC2617. Unlike Basic Authentication, digest authentication is based on challenge-reponse and therefore does not need to send the password over the (insecure) connection. In addition, it provides a count mechanism that ensure that old credentials cannot be reused, which prevents attackers from using old credentials with a new request. Digest authentication have the following advantages and disadvantages:

• Advantages
  • Authentication without exchanging the password
  • No re-use of authentication data
• Disadvantages
  • An extra round trip is needed for the first authentication
  • Server-side storage of the password is the MD5 hash of the user, realm and password. As MD5 hashes are quick to compute, one needs strong passwords. This fixed algorithm also allows for rainbow table attacks, although their value is limited because you need to precompute the rainbow table for every server (realm) and user.
  • The connection is sensitive to man-in-the-middle attack, where the attacker can both change the request and response.
  • Both client and server need to keep an administration of issued nonce values and associated nonce count values.

And, of course, the connection itself remains insecure. Digest based authentication is a viable alternative if HTTPS is not a good option and security of the data itself is not an issue.

This library acts as plugin for library(http/http_dispatch), where the registered handler (http_handler/3) can be given the option below to initiate digest authentication.

• authentication(digest(PasswdFile, Realm))

Above, PasswdFile is a file containing lines of the from below, where PasswordHash is computed using http_digest_password_hash/4. See also library(http/http_authenticate), http_read_passwd_file/2 and http_write_passwd_file/2.

User ":" PasswordHash (":" Extra)*

This library also hooks into library(http/http_open) if the option authorization(digest(User, Password)) is given.

See also

- https://tools.ietf.org/html/rfc2617

 http_digest_challenge(+Realm, +Options)//

Generate the content for a 401 WWW-Authenticate: Digest header field.

 http_parse_digest_challenge(+Challenge, -Fields) is det

Parse the value of an HTTP WWW-Authenticate header into a list of Name(Value) terms.

 http_digest_response(+Challenge, +User, +Password, -Reply, +Options)

Formulate a reply to a digest authentication request. Options:

path(+Path)

The request URI send along with the authentication. Defaults to /

method(+Method)

The HTTP method. Defaults to 'GET'

nc(+Integer)

The nonce-count as an integer. This is formatted as an 8 hex-digit string.

Arguments:

Challenge	- is a list Name(Value), normally from http_parse_digest_challenge/2. Must contain realm and nonce. Optionally contains opaque.
User	- is the user we want to authenticated
Password	- is the user's password
Options	- provides additional options

 http_digest_password_hash(+User, +Realm, +Password, -Hash) is det

Compute the password hash for the HTTP password file. Note that the HTTP digest mechanism does allow us to use a seeded expensive arbitrary hash function. Instead, the hash is defined as the MD5 of the following components:

<user>:<realm>:<password>.

The inexpensive MD5 algorithm makes the hash sensitive to brute force attacks while the lack of seeding make the hashes sensitive for rainbow table attacks, although the value is somewhat limited because the realm and user are part of the hash.

 http:authenticate(+Digest, +Request, -Fields)[multifile]

Plugin for library(http_dispatch) to perform basic HTTP authentication. Note that we keep the authentication details cached to avoid a `nonce-replay' error in the case that the application tries to verify multiple times.

This predicate throws http_reply(authorise(digest(Digest)))

Arguments:

Digest	- is a term digest(File, Realm, Options)
Request	- is the HTTP request
Fields	- describes the authenticated user with the option user(User) and with the option user_details(Fields) if the password file contains additional fields after the user and password.

 http:authenticate_client(+URL, +Action) is semidet[multifile]

This hooks is called by http_open/3 with the following Action value:

send_auth_header(+AuthData, +Out, +Options)

Called when sending the initial request. AuthData contains the value for the http_open/3 option authorization(AuthData) and Out is a stream on which to write additional HTTP headers.

auth_reponse(+Headers, +OptionsIn, -Options)

Called if the server replies with a 401 code, challenging the client. Our implementation adds a request_header(authorization=Digest) header to Options, causing http_open/3 to retry the request with the additional option.