This is Crypt::CBC, a Perl-only implementation of the cryptographic cipher block chaining mode (CBC). In combination with a block cipher such as Crypt::DES or Crypt::IDEA, you can encrypt and decrypt messages of arbitrarily long length. The encrypted messages are compatible with the encryption format used by SSLeay. In addition to this module you will need to install the MD5 module, and one or more of the Crypt::DES, Crypt::IDEA, or Crypt::Blowfish modules.