Hash functions

One of the fundamental primitives in modern cryptography is the cryptographic hash function, often informally called a one-way hash function. A simplified definition for the present discussion follows.

1.54 Definition A hash function is a computationally efficient function mapping binary strings of arbitrary length to binary strings of some fixed length, called hash-values.

For a hash function which outputs n-bit hash-values (e.g., n = 128 or 160) and has desirable properties, the probability that a randomly chosen string gets mapped to a particular n-bit hash-value (image) is 2-n. The basic idea is that a hash-value serves as a compact representative of an input string. To be of cryptographic use, a hash function h is typically chosen such that it is computationally infeasible to find two distinct inputs which hash to a common value (i.e., two colliding inputs x and у such that h(x) = h{y)), and that given a specific hash-value y, it is computationally infeasible to find an input (pre-image) x such that h(x) = y.

The most common cryptographic uses of hash functions are with digital signatures and for data integrity. With digital signatures, a long message is usually hashed (using a publicly available hash function) and only the hash-value is signed. The party receiving the message then hashes the received message, and verifies that the received signature is correct for this hash-value. This saves both time and space compared to signing the message directly, which would typically involve splitting the message into appropriate-sized blocks and signing each block individually. Note here that the inability to find two messages with the same hash-value is a security requirement, since otherwise, the signature on one message hash-value would be the same as that on another, allowing a signer to sign one message and at a later point in time claim to have signed another.

Hash functions may be used for data integrity as follows. The hash-value corresponding to a particular input is computed at some point in tune. The integrity of this hash-value is protected in some manner. At a subsequent point in time, to verify that the input data has not been altered, the hash-value is recomputed using the input at hand, and compared for equality with the original hash-value. Specific applications include virus protection and software distribution.

A third application of hash functions is their use in protocols involving a priori commitments, including some digital signature schemes and identification protocols (e.g., see Chapter 10).

Hash functions as discussed above are typically publicly known and involve no secret keys. When used to detect whether the message input has been altered, they are called modification detection codes (MDCs). Related to these are hash functions which involve a secret key, and provide data origin authentication (§9.76) as well as data integrity; these are called message authentication codes (MACs).

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