MD5sess

MD5-sess is a variant of the MD5-based algorithm used in HTTP Digest Access Authentication. It was designed to improve security by deriving the credential from a hash of the password and by incorporating nonces into the calculation.

In Digest authentication, the server challenges the client with a nonce and a realm. When MD5-sess is negotiated with a quality of protection (qop) of auth or auth-int, the client computes the credentials as follows: first, H1 is the MD5 hash of username, realm, and password (that is, H1 = MD5(username:realm:password)). The A1 value is then formed by concatenating H1, the server nonce, and a client nonce (cnonce). The A2 value depends on qop and typically involves the MD5 of the method and the digest URI (and, for auth-int, may include the entity-body). The final response is MD5(A1: nonce: nc: cnonce: qop: A2). The password itself is not transmitted; instead a hash-derived value is used in the response.

MD5-sess is defined in RFC 7616 as an optional algorithm for HTTP Digest authentication. It is supported by many clients and servers but not universally. If MD5-sess is not available or negotiated, the traditional MD5-based credentials may be used instead.

Security considerations: while MD5-sess reduces some exposure by avoiding direct password transmission, it relies on MD5, which has known cryptographic weaknesses. Strong passwords, proper TLS protection, and consideration of newer authentication mechanisms are advised for modern deployments.