RIPEMD is based on the Message Digest-4

RIPEMD is based on the Message Digest-4 (MD-4) algorithm and was primarily developed by the academic community instead of cryptological institutions as was the case with the SHA-1 algorithm. Initially created for the RACE Integrity Primitives Evaluation Message Digest European Project (1988-1992), the original RIPEMD algorithm was essentially two MD-4 algorithms running parallel and hot-swapping data on the fly. The algorithm did not receive widespread attention because SHA-1 dominated as the de-facto standard of the time and was advocated by standardisation bodies such as NIST.
As time progressed and computers became more powerful, NIST expressed concern over the relatively small output of RIPEMD and its reliance on the security of MD-4. As a result, the RIPEMD algorithm transitioned to a family of algorithms following the RIPEMD-X format, where X represents the number of bits in the algorithm’s output. The new family attempted to rectify the issues of MD-4 and tried to become more robust with regards to its security analysis.
Although the new algorithms were deemed relatively safe by security analysts, the comparably slow speed of the algorithm and the presence of already robust hashing algorithms in the SHA family contributed to its seldom usage in commercial applications. It is generally used in niche applications as the bit-size output is considered small by today’s standards and the newest iteration in the RIPEMD family, the RIPEMD-256 algorithm, is a relatively new and obscure design that has not yet been analysed sufficiently by the scientific community.