Message digest
A message digest, also sometimes called a digital fingerprint, is a number which is calculated from all the information in a message through the use of a cryptographic hash function, and which can be used to verify the data integrity of the message. Any change to the message, even of a single bit, typically results in a dramatically different message digest. A message digest algorithm is considered "secure" if it is not computationally feasible to determine the content of a message from its message digest, nor to find "collisions," wherein two different messages have the same message digest.Determination of whether or not any changes have been made to a file, for example, can be accomplished by comparing message digests calculated before and after transmission (or any other event). The message digest can also serve as a means of reliably identifying a file.
In order to increase speed, most digital signature algorithms specify that only the digest of the message is "signed." Message digests can also be used in the generation of pseudorandom bits.
SHA-1, MD5, and RIPEMD-160 are among the most commonly-used message digest algorithms.
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