Crypt (Unix)

In Unix computing, crypt is a utility program used for encryption. Due to the ease of breaking it, it is considered to be obsolete.

Robert Morris wrote crypt, which first appeared in Version 3 Unix, to encourage codebreaking experiments; Morris himself broke crypt by hand. Dennis M. Ritchie automated decryption with a method by James Reeds, and an improved version appeared in Version 7 which Reeds and Peter J. Weinberger also broke.

There is also a Unix password hash function with the same name, crypt. Though both are used for encrypting data in some sense, they are otherwise essentially unrelated. 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 password hash function as crypt(3), because its documentation is in manual section 3.

crypt(1) is a simple command to encrypt or decrypt data. Usually this is used as a filter, and it has traditionally been implemented using an algorithm based on the Enigma machine. It is considered to be far too cryptographically weak to provide any security against brute force attacks by modern, commodity personal computers.

Some versions of Unix shipped with an even weaker version of the crypt(1) command in order to comply with contemporaneous laws and regulations, which limited the exportation of cryptographic software (for example by classifying them as munitions). Some of these were simply implementations of the Caesar cipher (effectively no more secure than ROT13, which is implemented as a Caesar cipher with a well known key).

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