Not exactly base-64 related but Apple does provide a limited set of crypto functions via a CommonCrypto API in 10.5+; start with the CCCryptor man pages. The advantage to this approach is the CommonCrypto routines are provided in the libSystem library not from OpenSSL (which reduces overall code size). A quick google search didn't find any frameworks or extensions built around this API (probably because it'd be trivial to implement).
Also semi-related, there is a Keychain framework which provides wrappers around the Common Data Security Architecture (CDSA) library in OS X.
