Question

Please excuse my inexperience with Objective-C. I’ve only been playing with it for a couple weeks.

I am trying to test out Apple’s methods for encrypting and decrypting data (in this case an NSString). The end goal is to have the user type something in a text area, and then encrypt it.

I am using a basic single-view application in Xcode and added in these two files (From here ):

NSDataEncryption.h

#import <Foundation/Foundation.h>

@interface NSData (AES256) 
- (NSData *)AES256EncryptWithKey:(NSString *)key;
- (NSData *)AES256DecryptWithKey:(NSString *)key;
@end

and NSDataEncryption.m

#import "NSDataEncryption.h"

#import <CommonCrypto/CommonCryptor.h>

@implementation NSData (AES256)

- (NSData *)AES256EncryptWithKey:(NSString *)key {
    // 'key' should be 32 bytes for AES256, will be null-padded otherwise
    char keyPtr[kCCKeySizeAES256+1]; // room for terminator (unused)
    bzero(keyPtr, sizeof(keyPtr)); // fill with zeroes (for padding)

    // fetch key data
    [key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];

    NSUInteger dataLength = [self length];

    //See the doc: For block ciphers, the output size will always be less than or 
    //equal to the input size plus the size of one block.
    //That's why we need to add the size of one block here
    size_t bufferSize = dataLength + kCCBlockSizeAES128;
    void *buffer = malloc(bufferSize);

    size_t numBytesEncrypted = 0;
    CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,
                                          keyPtr, kCCKeySizeAES256,
                                          NULL /* initialization vector (optional) */,
                                          [self bytes], dataLength, /* input */
                                          buffer, bufferSize, /* output */
                                          &numBytesEncrypted);
    if (cryptStatus == kCCSuccess) {
        //the returned NSData takes ownership of the buffer and will free it on deallocation
        return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
    }

    free(buffer); //free the buffer;
    return nil;
}

- (NSData *)AES256DecryptWithKey:(NSString *)key {
    // 'key' should be 32 bytes for AES256, will be null-padded otherwise
    char keyPtr[kCCKeySizeAES256+1]; // room for terminator (unused)
    bzero(keyPtr, sizeof(keyPtr)); // fill with zeroes (for padding)

    // fetch key data
    [key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];

    NSUInteger dataLength = [self length];

    //See the doc: For block ciphers, the output size will always be less than or 
    //equal to the input size plus the size of one block.
    //That's why we need to add the size of one block here
    size_t bufferSize = dataLength + kCCBlockSizeAES128;
    void *buffer = malloc(bufferSize);

    size_t numBytesDecrypted = 0;
    CCCryptorStatus cryptStatus = CCCrypt(kCCDecrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,
                                          keyPtr, kCCKeySizeAES256,
                                          NULL /* initialization vector (optional) */,
                                          [self bytes], dataLength, /* input */
                                          buffer, bufferSize, /* output */
                                          &numBytesDecrypted);

    if (cryptStatus == kCCSuccess) {
        //the returned NSData takes ownership of the buffer and will free it on deallocation
        return [NSData dataWithBytesNoCopy:buffer length:numBytesDecrypted];
    }

    free(buffer); //free the buffer;
    return nil;
}

@end

Based on this question, I am calling the method like this:

#include "NSDataEncryption.h"

//...

NSString * key = @"ThisIsAKey";
    NSDataEncryption *encryptionClass = [[NSDataEncryption alloc] init]; //Errors: "Use of undeclared identfier 'encryptionClass'" and "Use of undeclared identifer 'NSDataEncryption'"
    NSData * newData = [encryptionClass AES256EncryptionWithKey:key]; //Errors: "Use of undeclared identfier 'encryptionClass'" and "Use of undeclared identifer 'NSDataEncryption'"

I’ve tried putting this inside main() and inside a new function in another class (ViewController):

- (IBAction)someFunctionName { code here }

The Big Question: Why isn’t Xcode accepting NSDataEncryption as a class, nor letting me call its function, AES256EnryptionWithKey? Should I be performing the encryption somewhere else in the App?

Thanks!

Answer 1

NSDataEncryption is not a class. It’s a category on the standard NSData class. This means it ‘extends’ the NSData class with two methods: - (NSData *)AES256EncryptWithKey:(NSString *)key; and - (NSData *)AES256DecryptWithKey:(NSString *)key;. These both return NSData and take one NSString as parameter.

You call them like this:

NSData *dataToBeEncrypted = [NSData data]; //Put your data here
NSString *key = @"ThisISAKEy";
NSData *newData = [dataToBeEncrypted AES256EncryptionWithKey:key];

You can use these methods for encrypting/decrypting a string:

- (NSData*) encryptString:(NSString*)plaintext withKey:(NSString*)key {
    return [[plaintext dataUsingEncoding:NSUTF8StringEncoding] AES256EncryptWithKey:key];

}

- (NSString*) decryptData:(NSData*)ciphertext withKey:(NSString*)key {
    return [[[NSString alloc] initWithData:[ciphertext AES256DecryptWithKey:key]
                                  encoding:NSUTF8StringEncoding] autorelease];
}