Key Stretching - Hash Based Key Stretching

Hash Based Key Stretching

A collision prone simple key stretching method:

key = hash(password) for 1 to 65536 do key = hash(key)

A better simple key stretching method. ("+" denotes the operation of concatenation):

key = "" for 1 to 65536 do key = hash(key + password)

Even better method with a salt:

key = "" for 1 to 65536 do key = hash(key + password + salt)

Many libraries provide functions which perform key stretching as part of their function; see crypt(3) for an example. Note that PBKDF2 is for generating an encryption key from a password, and not necessarily for password authentication. PBKDF2 can be used for both if the number of output bits is less than or equal to the internal hashing algorithm used in PBKDF2 which is usually SHA-1 (160 bits) or used as an encryption key to encrypt static data.

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