Setup additional hash algorithm

Question

I'm really new with C# and I'm looking at code that my predecessor generated. Here's the code:

public static string ComputeHash(string plainText,
                                     string hashAlgorithm, byte[] saltBytes)
    {
        if (saltBytes == null)
            saltBytes = CreateSalt(8);

        // Convert plain text into a byte array.
        byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);

        // Allocate array, which will hold plain text and salt.
        byte[] plainTextWithSaltBytes =
                new byte[plainTextBytes.Length + saltBytes.Length];

        // Copy plain text bytes into resulting array.
        for (int i = 0; i < plainTextBytes.Length; i++)
            plainTextWithSaltBytes[i] = plainTextBytes[i];

        // Append salt bytes to the resulting array.
        for (int i = 0; i < saltBytes.Length; i++)
            plainTextWithSaltBytes[plainTextBytes.Length + i] = saltBytes[i];

        // Because we support multiple hashing algorithms, we must define
        // hash object as a common (abstract) base class. We will specify the
        // actual hashing algorithm class later during object creation.
        HashAlgorithm hash;

        // Make sure hashing algorithm name is specified.
        if (hashAlgorithm == null)
            hashAlgorithm = "";

        // Initialize appropriate hashing algorithm class.
        switch (hashAlgorithm.ToUpper())
        {
            case "SHA1":
                hash = new SHA1Managed();
                break;

            case "SHA256":
                hash = new SHA256Managed();
                break;

            case "SHA384":
                hash = new SHA384Managed();
                break;

            case "SHA512":
                hash = new SHA512Managed();
                break;

            default:
                hash = new MD5CryptoServiceProvider();
                break;
        }

        // Compute hash value of our plain text with appended salt.
        byte[] hashBytes = hash.ComputeHash(plainTextWithSaltBytes);

        // Create array which will hold hash and original salt bytes.
        byte[] hashWithSaltBytes = new byte[hashBytes.Length +
                                            saltBytes.Length];

        // Copy hash bytes into resulting array.
        for (int i = 0; i < hashBytes.Length; i++)
            hashWithSaltBytes[i] = hashBytes[i];

        // Append salt bytes to the result.
        for (int i = 0; i < saltBytes.Length; i++)
            hashWithSaltBytes[hashBytes.Length + i] = saltBytes[i];

        // Convert result into a base64-encoded string.
        string hashValue = Convert.ToBase64String(hashWithSaltBytes);

        // Return the result.
        return hashValue;
    }

    public static bool VerifyHash(string plainText,
                                  string hashAlgorithm,
                                  string hashValue)
    {
        // Convert base64-encoded hash value into a byte array.
        byte[] hashWithSaltBytes = Convert.FromBase64String(hashValue);

        // We must know size of hash (without salt).
        int hashSizeInBits, hashSizeInBytes;

        // Make sure that hashing algorithm name is specified.
        if (hashAlgorithm == null)
            hashAlgorithm = "";

        // Size of hash is based on the specified algorithm.
        switch (hashAlgorithm.ToUpper())
        {
            case "SHA1":
                hashSizeInBits = 160;
                break;

            case "SHA256":
                hashSizeInBits = 256;
                break;

            case "SHA384":
                hashSizeInBits = 384;
                break;

            case "SHA512":
                hashSizeInBits = 512;
                break;

            default: // Must be MD5
                hashSizeInBits = 128;
                break;
        }

        // Convert size of hash from bits to bytes.
        hashSizeInBytes = hashSizeInBits / 8;

        // Make sure that the specified hash value is long enough.
        if (hashWithSaltBytes.Length < hashSizeInBytes)
            return false;

        // Allocate array to hold original salt bytes retrieved from hash.
        byte[] saltBytes = new byte[hashWithSaltBytes.Length -
                                    hashSizeInBytes];

        // Copy salt from the end of the hash to the new array.
        for (int i = 0; i < saltBytes.Length; i++)
            saltBytes[i] = hashWithSaltBytes[hashSizeInBytes + i];

        // Compute a new hash string.
        string expectedHashString =
                    ComputeHash(plainText, hashAlgorithm, saltBytes);

        // If the computed hash matches the specified hash,
        // the plain text value must be correct.
        return (hashValue == expectedHashString);
    }

The company has upgraded their security standards and requires secure hashing algorithms such as SHA-1, 3DES (triple DES) or AES MAC. I have no idea where to include them. Would someone please help?

Answer 1

The company has upgraded their security standards and requires secure hashing algorithms such as SHA-1, 3DES (triple DES) or AES MAC.

First of all, you already have SHA-1, and that hash algorithm, although slightly weaker than SHA-256/512, is still pretty good. Sticking to SHA-512 will keep you very safe, unless you are dealing with villains which are willing to spend 10 years breaking your messages using a supercomputer.

As for the other two algorithms, 3DES is a symmetric cypher and therefore unsuitable for hashing, while MACs are created using hashing algorithms such as SHA-2 (the difference is that you hash you a "secret key" along your message (something like "fixed salt") to ensure its authenticity. AES is also a symmetric cypher, and therefore also unsuitable for hashing.

Tell the guys from your company to check this page and settle for one of these hashing functions (in other words: don't change anything). If you are not experienced with cryptography, chances are you might make the system insecure regardless of your hashing choice.

Answer 2

I assume by "AES MAC" you are referring to Poly1305-AES. Poly1305-AES isn't a simple hash, it needs a AES key, and a 128 bit nonce, which is used when you communicate between two entities.

3DES (Triple DES) is an encryption cipher, it doesn't ensure authentication or integrity of a message whatsoever. The only function of 3DES is to ensure confidentiality of a message.

In respect to SHA-1, you should no longer use that hash as it has been broken since 2005.

I would suggest you get a formal description of what these new security standards are. 2 of the things you listed aren't even hash algorithms, and the third is a bad choice. Of the hash algorithms listed in your current implementation SHA-256 and above should be fine (aka SHA-2 category). These don't have any published vulnerabilities I am aware of at the moment.

Side note: You probably want to use arraycopy instead of looping over the bytes.