AES string encryption in Objective-C

This line near the top says you're adding AES functionality to NSMutableData:

@implementation NSMutableData(AES)

In Objective-C, this is called a category; categories let you extend an existing class.

This code would typically go in a file named NSMutableData-AES.m. Create a header file too, NSMutableData-AES.h. It should contain:

@interface NSMutableData(AES)
- (NSMutableData*) EncryptAES: (NSString *) key;
@end

Include (#import) that header in your main file. Add a call to the encryption function in your code:

NSData *InputData = [Input dataUsingEncoding:NSUTF8StringEncoding];
NSData *encryptedData = [InputData EncryptAES:@"myencryptionkey"];

Similarly for decryption.

Since this appears to have been ignored so far:

CCCryptorStatus result = CCCrypt( kCCDecrypt , kCCAlgorithmAES128, kCCOptionPKCS7Padding,
                                 keyPtr, kCCKeySizeAES256,
                                 **NULL**,
                                 [self mutableBytes], [self length],
                                 buffer_decrypt, bufferSize,
                                 &numBytesEncrypted );

From the header file CommonCrypto/CommonCryptor.h:

@param iv Initialization vector, optional. Used by block ciphers when Cipher Block Chaining (CBC) mode is enabled. If present, must be the same length as the selected algorithm's block size. If CBC mode is selected (by the absence of the kCCOptionECBMode bit in the options flags) and no IV is present, a NULL (all zeroes) IV will be used. This parameter is ignored if ECB mode is used or if a stream cipher algorithm is selected.

The NULL in bold corresponds to the IV. Sadly, whoever designed the API made it optional. This makes this CBC mode essentially equivalent to ECB, which is not recommended for a variety of reasons.