Question

I am attempting to use System.Security.Cryptography.AesManaged to encrypt a file in my .net application. It needs to be decrypted in an embedded Linux enviroment, so the .net libraries will not be available to me.

The code I have at the moment looks something like this:

string encPassword = "ABCDABCDABCDABCDABCDABCDABCDABCD";
string sourceFile = "myFile.txt";
string targetFile = "myFile.encrypted.txt";
FileStream fsInput = = new FileStream(sourceFile, FileMode.Open, FileAccess.Read);
FileStream fsOutput = new FileStream(targetFile, FileMode.OpenOrCreate, FileAccess.Write);

CryptoStream cryptoStream = null;

try
{
    byte[] key = Encoding.ASCII.GetBytes(encPasswd);
    byte[] IV = new byte[16];
    Array.Copy(key, 0, IV, 0, 16);

    AesManaged aes = new AesManaged();
    aes.Key = key;
    aes.IV = IV;
    aes.BlockSize = 128;
    aes.KeySize = 256;
    aes.Mode = CipherMode.CBC;

    ICryptoTransform encryptor = aes.CreateEncryptor();

    cryptoStream = new CryptoStream(fsOutput, encryptor, CryptoStreamMode.Write);

    byte[] buffer = new byte[BUFFER_LENGTH];
    long bytesProcessed = 0;
    long fileLength = fsInput.Length;
    int bytesInCurrentBlock;

    do
    {
        bytesInCurrentBlock = fsInput.Read(buffer, 0, BUFFER_LENGTH);
        cryptoStream.Write(buffer, 0, bytesInCurrentBlock);
        bytesProcessed = bytesProcessed + bytesInCurrentBlock;
    }
    while (bytesProcessed < fileLength);

    return true;
}
// ...

This encrypts the file okay. Now I am trying to decrypt the file using a 3rd-party utility on Windows that is also supported in Linux, to give me confidence that the Linux developer will be able to decrypt it.

A quick search on SourceForge let me to Enqrypt. However, if I use Enqrypt on the encrypted file like this:

enqrypt.exe -d -aes -256 -cbc -k ABCDABCDABCDABCDABCDABCDABCDABCD myFile.encrypted.txt

where -d indicates decrypt, -256 indicates the key size, -cbc the mode, and -k preceding the key.

it doesn’t give me the original file.

I have tried this with a few 3rd party utilities but I can’t seem to decrypt it.

Are there any obvious errors with how I am attempting to encrypt and decrypt this file?

Update

In response to recommendations from @Paŭlo, I now have the following test code (don’t worry, I plan to change the key and IV to be different):

byte[] key = { 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88 };
byte[] IV =  { 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88 };

The block size is still 128 and the key size is still 256 in code.

I now try to decrypt the file using openssl like so:

openssl enc -d -aes-256-cbc -in c:\encrypted.txt -out c:\decrypted.txt -K 11223344556677881122334455667788 -iv 11223344556677881122334455667788

This results in the following error:

bad decrypt 11452:error:06065064:digital envelope routines:EVP_DecryptFinal:bad decrypt:evp_enc.c:450:

Any idea what I am doing wrong?

Answer 1

I found the solution to my problem with decrypting using openssl (in the Update section of the question).

Firstly, my key length was wrong (as suggested by @Paŭlo Ebermann) – it should have been 256 bits.

But the final problem was that I was setting the key size after the key:

AesManaged aes = new AesManaged();
aes.Key = key;
aes.IV = IV;
aes.BlockSize = 128;
aes.KeySize = 256;
aes.Mode = CipherMode.CBC;

If I changed the above code to the following, I could decrypt it using openssl:

AesManaged aes = new AesManaged();
aes.BlockSize = 128;
aes.KeySize = 256;
aes.Key = key;
aes.IV = IV;
aes.Mode = CipherMode.CBC;

Thanks to this answer which led me in the right direction, and thanks to everyone else for their answers!