Salt (cryptography) - Unix Implementations

Unix Implementations

Earlier versions of Unix used a password file (/etc/passwd) to store the hashes of salted passwords (passwords prefixed with two-character random salts). Note that in these older versions of Unix, the salt was also stored in the passwd file (as cleartext) together with the hash of the salted password. The password file was publicly readable for all users of the system. It must be readable so that user-privileged software tools can find user names and other information. The security of passwords is therefore protected only by the one-way functions (enciphering or hashing) used for the purpose.

Early Unix implementations limited passwords to 8 characters and used a 12-bit salt, which allowed for 4,096 possible salt values. While 12 bits was sufficient for the 1970s, by 2005 disk storage had become inexpensive, so much so, that an attacker could pre-compute the hashes of millions of common passwords, including all 4,096 possible salt variations for each password, and store the precomputed values on a single hard drive. An attacker with a larger budget could build a disk farm with all 6-character passwords and the most common 7- and 8-character passwords stored in hashed form for all 4,096 possible salt values.