Maintenance edition. Take a look at 6.1 instead. Repository is now only merging 6.0 maintenance updates into stable/6.0 since the cryptolib 6.1 takes over completely.
Crypto 6.1 contains crypto only! The IO library has been separated into its own repository. If you are building something, do not use master!
The CRYPTO Library in the TorneLIB suite, is a library that contains a bunch of methods that handles I/O data (transforming from and to different data formats). It also handles - as described in the name - string/data cryptos. In the prior versions of MODULE_CRYPTO, the module utilized mcrypt, but since mcrypt is deprecated and completely removed from PHP 7.2, I feel that we should not force people to again install mcrypt (by PECL). Especially since mcrypt has not been developed since 2016. Instead MODULE_CRYPTO now trusts openssl cryptos, which opens for more variants of encryption. The functions that primary has been used mcrypt will in the newer versions of the crypto library skips mcrypt (as long as it is not forced to be used, as a backward compatibility support) for the sake of openssl.
Of course, the crypto library also requires either mcrypt or openssl to function. However the I/O library is independent even if it is included in the CRYPTO library.