These instructions build OpenSSL which provides libcrypto.
./Configure aix64-cc shared # Note: it is an upper case C .make (as root) make install
GPG on AIX requires the setuid bit to be set. This can be done via chmod u+s /path/to/gpg. Please see http://www.gnupg.org/documentation/faqs.en.html#q6.1
./configure CC="xlc -q64" --disable-asm ($OBJECT_MODE=64) or CC=cc CFLAGS=-q64