If you want to use Encryption on iOS you can use what is known as Common Crypto. This is what people normally use with Keychain as a storage mechanism for persistent data. Keychain has other features such as synchronization and uses very competent encryption methods.
If you want to use 3rd party local libraries, the most commonly known is libsodium. It has a cross-platform fork by the name of NaCl. The source code of libsodium is available on github. Just glancing at the source code, it appears to use C99 standard C based source code.
If you want ease of use for iOS, then there is NaOH which uses Swift binding. It is available on Gitlab and mirrored on Github as source code or precompiled archives, i.e. software binaries. This was written by Drew Crawford and intruced during a tech talk given at a Cocoacoders meetup group in Roundrock, Texas on Feb 25, 2016.