Computer Security and Cryptography

18.1 UNIX PASSWORD ENCIPHERMENT

A Log-In to a UNIX system requires the user to provide a password Pass(User_ID), which is hashed to Hass[Pass(User_ID)] and compared with the entries in the UNIX password file consisting of

where the number [n] is the length in characters (bytes). A cryptographic salt consists of approximately 4096 = 2¹² randomly chosen bits, which are used to further “mix up” the Hash(UserPass_ID). The UNIX crypt(3) command chooses the salt to be a pair of the characters

The original idea is due to George Purdy [1987], who proposed using a one-way function to

1. Recalculate the Pass(User_ID) → Hash[Pass(User_ID)] during a log-in, and
2. Compare the result in (1) with a stored value in the file /etc/passwd., which contains (User_Name, Salt(User_ID), Hash[Pass(User_ID)], User_ID)¹ or if a shadow password implementation is being used (User_Name, Salt(User_ID), *, User_ID), signalling that Hash[Pass(User_ID)] is stored (encrypted) in another file.

Figure 18.1 crypt(3) Computation of Hash[Pass(User_ID)].

Purdy suggested using polynomials over a prime modulus to construct the one-way function. There have been various implementations of the hash function. The crypt(3) has several ...