apop

APOP stands for Authenticated Post Office Protocol, a niche extension of the POP3 email retrieval protocol. It provides a challenge-response authentication mechanism intended to avoid sending the user’s password in clear text over the network. APOP is part of the POP3 family and was designed to improve security on connections that did not yet use TLS, though it is now largely superseded by encrypted transport and more modern authentication methods.

In operation, when a POP3 session starts, the server greeting includes a unique timestamp enclosed in angle brackets, which serves as a nonce. The client then issues the APOP command with a username and a digest. The digest is the MD5 hash of the concatenation of the timestamp and the user’s password. The server computes the same hash and verifies it, authenticating the user without transmitting the password itself.

Security considerations: APOP reduces password exposure compared with plain POP3, but its security relies on password strength. It is vulnerable to offline dictionary attacks if an attacker captures the digest and the timestamp from the greeting. With the widespread use of TLS (POP3 over TLS or STARTTLS) and modern authentication methods (SASL, OAuth), APOP usage has declined and is mostly encountered in legacy systems.

Status and usage: APOP remains a historic feature of POP3 implementations. Many contemporary mail servers and clients prefer TLS-encrypted POP3 or alternative authentication schemes. It is most relevant for interoperability with older systems that do not support newer security protocols.

Other meanings: APOP can be an acronym in other domains, but in networking and email, Authenticated Post Office Protocol is the most common reference.