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RIPEMD-160 (RACE Integrity Primitives Evaluation Message Digest) is a 160-bit message digest algorithm (and cryptographic hash function) developed in Leuven (Belgium) by Hans Dobbertin, Antoon Bosselaers and Bart Preneel at the COSIC research group at the Katholieke Universiteit Leuven, and first published in 1996. It is an improved version of RIPEMD, which in turn was based upon the design principles used in MD4, and is similar in performance to the more popular SHA-1.
There also exist 128, 256 and 320-bit versions of this algorithm, called RIPEMD-128, RIPEMD-256, and RIPEMD-320, respectively. The 128-bit version was intended only as a drop-in replacement for the original RIPEMD, which was also 128-bit, and which had been found to have questionable security. The 256 and 320-bit versions diminish only the chance of accidental collision, and don't have higher levels of security as compared to, respectively, RIPEMD-128 and RIPEMD-160.
RIPEMD-160 was designed in the open academic community, in contrast to, e.g., the NSA-designed family of algorithms, SHA (SHA1, SHA224, SHA256, SHA384, and SHA512). On the other hand, RIPEMD-160 appears to be used somewhat less frequently than SHA-1, which may have caused it to be less scrutinised than SHA-1.
RIPEMD-160 is not known to be constrained by any patents.