Encryption library for Objective-C

Other than OpenPGP, are there any other libraries to encrypt text?

You could use OpenSSL, which is built into both Mac OS X and iOS. You just need to add Security.framework, then you can #include <ssl.h>.

For simple encryption using AES, TripleDES, you can just use built-in CommonCrypto library,

http://developer.apple.com/mac/library/documentation/Darwin/Reference/ManPages/man3/CCCryptor.3cc.html#//apple_ref/doc/man/3cc/CCCryptor