mcrypt is an encryption program which replaces old unix crypt program. The libmcrypt is the library file of mcrypt.
CentOS version 6 repolist does not maintain mcrypt and libmcrypt, if you really must use mcrypt or libmcrypt use the Fedoraproject EPEL repolist:
You should see the libmcrypt and other mcrypt programs.
UPDATE: It seems the above link is not available, try http://mirror.nus.edu.sg/Fedora/epel/6/i386/epel-release-6-7.noarch.rpm for epel 6.