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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Famous quotes containing the words based, key and/or stretching:
“Any reductionist program has to be based on an analysis of what is to be reduced. If the analysis leaves something out, the problem will be falsely posed.”
—Thomas Nagel (b. 1938)
“The word “forbearance” is the key to a happy home.”
—Chinese proverb.
“In the continual enterprise of trying to guide appropriately, renegotiate with, listen to and just generally coexist with our teenage children, we ourselves are changed. We learn even more clearly what our base-line virtues are. We listen to our teenagers and change our minds about some things, stretching our own limits. We learn our own capacity for flexibility, firmness and endurance.”
—Jean Jacobs Speizer. Ourselves and Our Children, by Boston Women’s Health Collective, ch. 4 (1978)