encrypt.m

NSData *msg = [@"This is a message" dataUsingEncoding:NSUTF8StringEncoding];
NSData *key = [@"the password" dataUsingEncoding:NSUTF8StringEncoding];

NSLog(@"Unencrypted message: [%@]", msg);


uint8_t iv[kCCBlockSizeAES128];
if (SecRandomCopyBytes(kSecRandomDefault, sizeof(iv), iv))
{
    // ERROR
    NSLog(@"Can't generate IV");
}

size_t retSize = 0;
CCCryptorStatus result = CCCrypt(kCCEncrypt,
                                 kCCAlgorithmAES,
                                 kCCOptionPKCS7Padding,
                                 key.bytes,
                                 key.length,
                                 iv,
                                 msg.bytes,
                                 msg.length,
                                 0,
                                 0,
                                 &retSize);
if (result != kCCBufferTooSmall)
{
    // ERROR
    NSLog(@"Can't get encrypted size");
}
NSMutableData *data = [NSMutableData.alloc initWithLength:retSize + sizeof(iv)];
result = CCCrypt(kCCEncrypt,
                 kCCAlgorithmAES,
                 kCCOptionPKCS7Padding,
                 key.bytes,
                 key.length,
                 iv,
                 msg.bytes,
                 msg.length,
                 (uint8_t *)data.mutableBytes + sizeof(iv),
                 retSize,
                 &retSize);
if (result != kCCSuccess)
{
    // ERROR
    NSLog(@"Can't encrypt!");
}

// -------
if (data.length < kCCBlockSizeAES128)
{
    // ERROR
    NSLog(@"Bad data");
}
size_t decryptedSize = 0;
CCCryptorStatus decryptionResult = CCCrypt(kCCDecrypt,
                                           kCCAlgorithmAES,
                                           kCCOptionPKCS7Padding,
                                           key.bytes,
                                           key.length,
                                           data.bytes,
                                           (uint8_t *)data.bytes + kCCBlockSizeAES128,
                                           data.length - kCCBlockSizeAES128,
                                           0,
                                           0,
                                           &decryptedSize);
if (decryptionResult != kCCBufferTooSmall)
{
    // ERROR
    NSLog(@"Can't get decrypted size");
}
NSMutableData *decryptedData = [NSMutableData.alloc initWithLength:decryptedSize];
decryptionResult = CCCrypt(kCCDecrypt,
                           kCCAlgorithmAES,
                           kCCOptionPKCS7Padding,
                           key.bytes,
                           key.length,
                           data.bytes,
                           (uint8_t *)data.bytes + kCCBlockSizeAES128,
                           data.length - kCCBlockSizeAES128,
                           decryptedData.mutableBytes,
                           decryptedSize,
                           &decryptedSize);
if (decryptionResult != kCCSuccess)
{
    // ERROR
    NSLog(@"Can't decrypt");
}

NSLog(@"Decrypted message: [%@]", decryptedData);

// Booh:
// 2015-02-26 16:29:24.244 Foo[58451:6269388] Unencrypted message: [<54686973 20697320 61206d65 73736167 65>]
// 2015-02-26 16:29:24.435 Foo[58451:6269388] Decrypted message: [<00000000 00000000 00000000 00000000 00a04f5d ff7f0000 0c521705 01000000>]