Question

I am a newbie to this encryption. I am creating an application for both android and iOS in which i have to encrypt(using AES Encryprtion) a file at server side and decrypt at client side in both iOS and Android App. I got code in internet to perform AES encryption and decryption for both Android and iOS, they are working fine. Server side they are using java. But the problem is Java Encrypted File cant be decrypted by iOS program, even I got the same filesize, But the file is not in correct format. I posted the code below…

Java Encryption and Decryption:

public static byte[] encrypt(byte[] data, byte[] key, byte[] ivs) {
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");

            SecretKeySpec secretKeySpec = new SecretKeySpec(key, "AES");
            byte[] finalIvs = new byte[16];
            int len = ivs.length > 16 ? 16 : ivs.length;
            System.arraycopy(ivs, 0, finalIvs, 0, len);
            IvParameterSpec ivps = new IvParameterSpec(finalIvs);
            cipher.init(Cipher.ENCRYPT_MODE, secretKeySpec, ivps);
            return cipher.doFinal(data);
        } catch (Exception e) {
            e.printStackTrace();
        }

        return null;
    }

    public static byte[] decrypt(byte[] data, byte[] key, byte[] ivs) {
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            SecretKeySpec secretKeySpec = new SecretKeySpec(key, "AES");
            byte[] finalIvs = new byte[16];
            int len = ivs.length > 16 ? 16 : ivs.length;
            System.arraycopy(ivs, 0, finalIvs, 0, len);
            IvParameterSpec ivps = new IvParameterSpec(finalIvs);
            cipher.init(Cipher.DECRYPT_MODE, secretKeySpec, ivps);
            return cipher.doFinal(data);
        } catch (Exception e) {
            e.printStackTrace();
        }

        return null;
    }

iOS Code for Decryption:

- (NSData *) decryptFile:(NSString *)key withData:(NSData *)fileData {

    char keyPtr[kCCKeySizeAES128+1]; 
    bzero(keyPtr, sizeof(keyPtr)); 

    NSString* iv = @"12345678";

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

    NSUInteger dataLength = [fileData length];

    size_t bufferSize = dataLength + kCCBlockSizeAES128;
    void *buffer = malloc(bufferSize);

    size_t numBytesDecrypted = 0;

    CCCryptorStatus cryptStatus = CCCrypt(kCCDecrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,keyPtr, kCCKeySizeAES128,
                                          iv /* initialization vector (optional) */,
                                          [fileData 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;
}

Give me any solution or suggestion for this problem

Answer 1

The issue is with the iv parameter.

1) You are passing a NSString* as iv. You probably want to pass in the actual bytes.

2) The length of iv should be 16 (in this case) as per the api docs of CCCrypt. See link below:

http://www.opensource.apple.com/source/CommonCrypto/CommonCrypto-36064/CommonCrypto/CommonCryptor.h