hashcrack is a fast (as fast as OpenSSL allows) hash cracker. It features an additive hash checking to speed up the process. For example, to check for the hashes of abc and abd, hashcrack only computes the hash of ab, then computes one round each for c and d totaling 4 rounds (two for ab, one for c and one for d). This eliminates a good number of rounds if we calculate hashes from the beginning (6 in this case). The longer the key, the more the saving.
A draw back to this feature is it does not work with non-additive hash algorithms. Luckily, popular algorithms, such as SHA-1, RIPEMD160, MD5, are additive.
Compared to vshark (another hash cracker by rd), hashcrack is so much faster. An unscientific benchmark to RIPEMD160-scan the whole 6-character a-zA-Z0-9 space ended in about 6 minutes with hashcrack, and more than 2 hours with vshark. To be fair to vshark, there was another version of hashcrack written in pure Python. It was 6 times slower than vshark.