HA1

HA1 is a value used in HTTP Digest Access Authentication to represent a hashed form of user credentials. In typical deployments, HA1 is the MD5 hash of the string username:realm:password (denoted A1 in RFCs). The server stores HA1 instead of the plaintext password, and the client uses it indirectly in computing the response to a server challenge.

Computation: For the common MD5-based scheme, HA1 = MD5(username:realm:password). If the algorithm parameter is "MD5-sess," HA1 is defined as MD5(MD5(username:realm:password):nonce:cnonce). The actual authentication response sent by the client is derived from HA1, the nonce, qop, nc, cnonce, the HTTP method, and the requested URI.

Use and purpose: The HA1 value helps avoid sending the user's password over the network, reducing exposure to eavesdroppers. It is a server-side credential representation and allows stateless verification by recomputing the expected response from HA1 and the other values.

Variants and definitions: The concept of HA1 is defined in RFC 2617 and updated in RFC 7616; it is central to Digest authentication and not used in basic authentication. The exact form depends on the chosen algorithm (MD5 or MD5-sess). It is distinct from the digest HA2 value, which covers the request method and URI.

Security considerations: If an attacker obtains HA1, they may attempt to impersonate the user to a Digest-authenticated service using the same realm and algorithm, especially if nonces are not properly bound. The use of MD5-sess and strong nonce management mitigates some risks, but compromising hash storage remains a potential vulnerability.

See also: HTTP Digest Access Authentication, MD5, MD5-sess, RFC 7616, RFC 2617.