The md5, sha1, sha256 and rmd160 utilities take as input a message of arbitrary length and produce as output a "fingerprint" or "message digest" of the input. It is conjectured that it is computationally infeasible to produce two messages having the same message digest, or to produce any message having a given prespecified target message digest. The MD5, SHA-1, SHA-256 and RIPEMD-160 algorithms are intended for digital signature applications, where a large file must be "compressed" in a secure manner before being encrypted with a private (secret) key under a public-key cryptosystem such as RSA.
MD5 has not yet (2001-09-03) been broken, but sufficient attacks have been made that its security is in some doubt. The attacks on MD5 are in the nature of finding "collisions" that is, multiple inputs which hash to the same value; it is still unlikely for an attacker to be able to determine the exact original input given a hash value.
The following options may be used in any combination and must precede any files named on the command line. The hexadecimal checksum of each file listed on the command line is printed after the options are processed.