How decrypt string in c# was encrypted in iOS using Rijndael

Question

I'm trying to encrypt and decrypt the string using objective c and C#. both are working fine in native code, but when I was try to decrypt string in c# was encrypted in iOS. I get some error.

This was the code I used in the objective c

- (NSData *)AES256EncryptWithKey:(NSString *)key  Data: (NSData *) data
{
    char keyPtr[kCCKeySizeAES256+1]; // room for terminator (unused)

    bzero(keyPtr, sizeof(keyPtr)); // fill with zeroes (for padding)

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

    NSUInteger dataLength = [data length];

    NSData *iv =  [@"abcdefghijklmnopqrstuvwxyz123456" dataUsingEncoding:NSUTF8StringEncoding];

    size_t bufferSize = dataLength + kCCBlockSizeAES128;

    void *buffer = malloc(bufferSize);

    size_t numBytesEncrypted = 0;

    CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,
                                          keyPtr, kCCKeySizeAES256,
                                          [iv bytes] /* initialization vector (optional) */,
                                          [data bytes], dataLength, /* input */
                                          buffer, bufferSize, /* output */
                                          &numBytesEncrypted);

    if (cryptStatus == kCCSuccess)
    {
        return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
    }

    free(buffer); //free the buffer;

    return nil;
}

In want to know how to decrypt in C#, I give blocksize is 256, ivsize to 32 and used "RijndaelManaged()". I'm not using salt & password. Error: something like "Padding is invalid and cannot be removed." I tried to set padding too like PKCS7, none, zero but nothing help to decrypt.

can any one help this?

Edit: My C# code here

public string DecryptString(string encrypted)
{
    string result = null;
    _encoder = new UTF8Encoding();
    if (!string.IsNullOrWhiteSpace(encrypted) && (encrypted.Length >= 32))
    {
        var messageBytes = Convert.FromBase64String(encrypted);
        using (var rm = new RijndaelManaged())
        { 
            rm.BlockSize = _blockSize;
            rm.Key = _encoder.GetBytes("mykey_here");
            rm.IV = _encoder.GetBytes("abcdefghijklmnopqrstuvwxyz123456"); ;
            rm.Padding = PaddingMode.Zeros;
            var decryptor = rm.CreateDecryptor(rm.Key, messageBytes.Take(_ivSize).ToArray());
            result = _encoder.GetString(Transform(messageBytes.Skip(_ivSize).ToArray(), decryptor));
        }
    }

    return result;
}

protected byte[] Transform(byte[] buffer, ICryptoTransform transform)
{
    byte[] result;
    using (var stream = new MemoryStream())
    using (var cs = new CryptoStream(stream, transform, CryptoStreamMode.Write))
    {
        cs.Write(buffer, 0, buffer.Length);
        cs.FlushFinalBlock();
        result = stream.ToArray();
    }

    return result;
}

Did you try kCCOptionECBMode | kCCOptionPKCS7Padding as third param into CCCrypt ? — Doro
Yes, I have tried but no use. If I try this kCCOptionECBMode I cant be encrypt in ios itself — Rajesh
@Doro one does not use an iv and ECB mode. Also in general ECB mode should not be used because it is less secure than CBC mode. — zaph
@zaph, understood. i've asked it, cause in C# default mode is ECB, as i read from documentation — Doro
No, the default mode for RijndaelManaged is CBC. The docs (see link in my answer): "Property Value Type: System.Security.Cryptography.PaddingMode The padding mode used in the symmetric algorithm. The default is PaddingMode.PKCS7." — zaph

zaph zaph · Accepted Answer · 2015-06-26T10:59:18

iOS (Common Crypto) explicitly specifies all encryption parameters, the C# code implicitly determines many parameters. These implicit parameters while simplifying usage are problematic when trying to achieve interoperability.

The C# class RijndaelManaged allows explicitly specifying parameter, change your code to use these, in particular BlockSize (128), KeySize (128), Mode (CipherMode.CBC) and Padding (PaddingMode.PKCS7). The defaults for mode and Padding are OK. See RijndaelManaged Documentation

AES and Rijndael are not the same, in particular AES uses only a block size of 128 bits (16 bytes) and Rijndael allows several block sizes. So one needs to specify a block size of 128 bits for Rijndael. Thus the iv is also 128 bits (16 bytes). Both support encryption keys of 128, 192 and 256 bytes.

You would probably be better off using the AESManaged class than the RijndaelManaged class. See AesManaged Documentation

The C# side expects the data to be Base64 encoded, the iOS side does not show that encoding operation, make sure that is being done on the iOS side.

Since you are using an iv make sure you are using CBC mode on both sides. In Common Crypto CBC mode is the default, make sure CBC mode is being used on the C# side.

Make sure the C# side is using PKCS#7 or PKCS#5 padding, they are equivalent. It appears that PKCS#7 is the default on the C# side so this should be OK.

It is best to use a key of exactly the size specified and not rely on default padding. In Common Crypto the key size is explicitly specified and null padded if the supplied key is to short. The C# looks like it is determining the key size by the supplied key, in this case the key is 10 bytes so the decryption key probably defaults to 128 bits and the key is being internally padded with nulls. On iOS you are explicitly specifying a key size of 256 bits. This is a mis-match that needs to be fixed. Supply a key that is the exact size specified on the iOS side.

Finally there is the iv, the C# code expects the iv to be prepended to the encrypted data but the iOS code is not providing that. The solution is to change the iOS code to prepend the iv to the encrypted code. Change the iv to be 16 bytes, the AES block size.

Finally provide hex dumps of the test data in, data out, iv and key just prior to and after the encryption call if you need more help.

How decrypt string in c# was encrypted in iOS using Rijndael

1 Answers