crypt is a POSIX C library function. It is typically used to compute the hash of user account passwords. The function outputs a text string which also encodes the salt (usually the first two characters are the salt itself and the rest is the hashed result), and identifies the hash algorithm used (defaulting to the "traditional" one explained below). This output string forms a password record, which is usually stored in a text file.

More formally, crypt provides cryptographic key derivation functions for password validation and storage on Unix systems.

There is an unrelated crypt utility in Unix, which is often confused with the C library function. To distinguish between the two, writers often refer to the utility program as crypt(1), because it is documented in section 1 of the Unix manual pages, and refer to the C library function as crypt(3), because its documentation is in manual section 3.

This same crypt function is used both to generate a new hash for storage and also to hash a proffered password with a recorded salt for comparison.

Modern Unix implementations of the crypt library routine support a variety of hash schemes. The particular hash algorithm used can be identified by a unique code prefix in the resulting hashtext, following a de facto standard called Modular Crypt Format.

The crypt() library function is also included in the Perl, PHP, Pike, Python (although it is now deprecated as of 3.11), and Ruby programming languages.

Over time various algorithms have been introduced. To enable backward compatibility, each scheme started using some convention of serializing the password hashes that was later called the Modular Crypt Format (MCF). Old crypt(3) hashes generated before the de facto MCF standard may vary from scheme to scheme. A well-defined subset of the Modular Crypt Format was created during the Password Hashing Competition. The format is defined as:

$<id>[$<param>=<value>(,<param>=<value>)*][$<salt>[$<hash>]]