CRAMMD5

CRAM-MD5 is a challenge–response authentication mechanism that uses the HMAC-MD5 hash and is defined for use with SASL (Simple Authentication and Security Layer). It is designed to let a client prove knowledge of a password without sending the password over the network. It has been used with email protocols such as IMAP, POP3, and SMTP, typically within SASL-enabled sessions.

How CRAM-MD5 works: During authentication, the server issues a challenge string, usually base64-encoded. The client decodes the challenge and computes an HMAC-MD5 of this challenge using the user’s password as the key. The resulting 128-bit digest is represented as a hexadecimal value. The client then sends back a response that includes the username and the hex digest, often encoded for transport (for example, base64). The server, which knows the user’s password, performs the same HMAC-MD5 calculation on the challenge and compares the digests. A match authenticates the user.

Security considerations: Because CRAM-MD5 does not encrypt the entire session, it only protects the password from being sent in clear during the authentication step. The resulting digest can still be subject to offline dictionary or guessing attacks if the password is weak and the challenge is not sufficiently random. It does not provide mutual authentication on its own and is not forward-secure. For stronger security, it is generally recommended to use TLS to secure the channel or to deploy more modern SASL mechanisms such as SCRAM-SHA-1 or SCRAM-SHA-256.

History and status: CRAM-MD5 was specified in RFC 2195 (1997). It saw broad use in older email deployments but has declined in favor of more robust mechanisms. Some systems still support it for compatibility.