Question

For a payment provider, I need to calculate a hash-based message authentication code, using HMAC-SHA256. That is causing me quite a bit of trouble.

The payment provider gives two examples of orrectly calculated authentication code in pseudo-code. All keys are in hex.

Method 1

key = 57617b5d2349434b34734345635073433835777e2d244c31715535255a366773755a4d70532a5879793238235f707c4f7865753f3f446e633a21575643303f66
message = "amount=100&currency=EUR"
MAC = HMAC-SHA256( hexDecode(key), message )
result = b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905

Method 2

message = "amount=100&currency=EUR"
Ki = 61574d6b157f757d02457573556645750e0341481b127a07476303136c005145436c7b46651c6e4f4f040e1569464a794e534309097258550c17616075060950
Ko = 0b3d27017f151f17682f1f193f0c2f1f64692b227178106d2d096979066a3b2f2906112c0f760425256e647f032c2013243929636318323f667d0b0a1f6c633a
MAC = SHA256( hexDecode(Ko) + SHA256( hexDecode(Ki) + message ) )
result = b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905

I tried to write the code to do this, after doing some research, but I keep coming up with different results.

private static void Main(string[] args)
    {
        var key = "57617b5d2349434b34734345635073433835777e2d244c31715535255a366773755a4d70532a5879793238235f707c4f7865753f3f446e633a21575643303f66";
        var ki = "61574d6b157f757d02457573556645750e0341481b127a07476303136c005145436c7b46651c6e4f4f040e1569464a794e534309097258550c17616075060950";
        var ko = "0b3d27017f151f17682f1f193f0c2f1f64692b227178106d2d096979066a3b2f2906112c0f760425256e647f032c2013243929636318323f667d0b0a1f6c633a";
        var mm = "amount=100&currency=EUR";

        var result1 = CalcHMACSHA256Hash(HexDecode(key), mm);

        var result2 = CalcSha256Hash(string.Format("{0}{1}", HexDecode(ko), CalcSha256Hash(HexDecode(ki) + mm)));

        Console.WriteLine("Expected: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905");
        Console.WriteLine("Actual 1: " + result1);
        Console.WriteLine("Actual 2: " + result2);

        Console.WriteLine("------------------------------");
        Console.ReadKey();

    }

    private static string HexDecode(string hex)
    {
        var sb = new StringBuilder();
        for (int i = 0; i <= hex.Length - 2; i += 2)
        {
            sb.Append(Convert.ToString(Convert.ToChar(Int32.Parse(hex.Substring(i, 2), System.Globalization.NumberStyles.HexNumber))));
        }
        return sb.ToString();
    }

    private static string CalcHMACSHA256Hash(string plaintext, string salt)
    {
        string result = "";
        var enc = Encoding.Default;
        byte[]
        baText2BeHashed = enc.GetBytes(plaintext),
        baSalt = enc.GetBytes(salt);
        System.Security.Cryptography.HMACSHA256 hasher = new HMACSHA256(baSalt);
        byte[] baHashedText = hasher.ComputeHash(baText2BeHashed);
        result = string.Join("", baHashedText.ToList().Select(b => b.ToString("x2")).ToArray());
        return result;
    }


    public static string CalcSha256Hash(string input)
    {
        SHA256 sha256 = new SHA256Managed();
        byte[] sha256Bytes = Encoding.Default.GetBytes(input);
        byte[] cryString = sha256.ComputeHash(sha256Bytes);
        string sha256Str = string.Empty;
        for (int i = 0; i < cryString.Length; i++)
        {
            sha256Str += cryString[i].ToString("x2");
        }
        return sha256Str;
    }

And this is the result I get:

Expected: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905
Actual 1: 421ce16f2036bb9f2a3770c16f01e9220f0232d45580584ca41768fd16c15fe6
Actual 2: 290f14398bf8c0959dfc963e2fd9c377534c6fec1983025d2ab192382f132b92

So with none of the two methods, I can get the result the provider example wants.

What am I missing here? Is it encoding? Is my hexDecode screwed up?

Test tool from payment provider: http://tech.dibs.dk/dibs_api/other_features/hmac_tool/

PHP sample code: http://tech.dibspayment.com/dibs_api/other_features/mac_calculation/

Answer 1

Edit: You likely are looking for a quick and simple way to do HMAC-SHA256 and not get into the finer details. The original question asks of those finer details which are explained further below.

I want to perform a HMAC-SHA256 on a byte[] message input

using System.Security.Cryptography;
...
private static byte[] HashHMAC(byte[] key, byte[] message)
{
    var hash = new HMACSHA256(key);
    return hash.ComputeHash(message);
}

I want to perform HMAC-SHA256 but I have a hex string input

In .NET 5 and above, use System.Convert.FromHexString like so, (thanks @proximab). If you’re on pre-.NET 5, scroll to "Helper functions" which has alternative solutions.

using System;
using System.Security.Cryptography;
...
private static byte[] HashHMACHex(string keyHex, string messageHex)
{
    var key = Convert.FromHexString(hexKey);
    var message = Convert.FromHexString(messageHex);
    var hash = new HMACSHA256(key);
    return hash.ComputeHash(message);
}

I’m using a strange API service that sort of does HMAC, but it’s something custom

Continue reading. You likely want to use "Method 2" below as a reference point and adjust it to however your service wants you to implement HMAC for message anti-tampering.

---

How HMAC-SHA256 Works (should you need to know how…)

Here we will compute an HMAC-SHA256 manually (this answers "Method 2" from the original question).

Assume outerKey, innerKey, and message are already byte arrays, we perform the following:

Notation: Assume A + B concatenates byte array A and B. You may
alternatively see A || B notation used in more academic settings.

HMAC = SHA256( outerKey + SHA256( innerKey + message  )   )
              .          .       `------------------´ .  .
               \          \           `innerData`    /  /
                \          `------------------------´  /   
                 \               `innerHash`          /
                  `----------------------------------´
                               `data`

So the code can be broken down into these steps (using the above as a guide):

1. Create an empty buffer byte[] innerData the length of innerKey.Length + message.Length (again assuming byte arrays)
2. Copy the innerKey and the message into the byte[] innerData
3. Compute SHA256 of innerData and store it in byte[] innerHash
4. Create an empty buffer byte[] data the length of outerKey.Length + innerHash.Length
5. Copy the outerKey and innerHash (from step #3)
6. Compute the final hash of data and store it in byte[] result and return it.

To do the byte copying I’m using the Buffer.BlockCopy() function since it apparently faster than some other ways (source).

n.b. There is likely (read: most certainly) a better way to do this using the the new ReadOnlySpan<T> API.

We can translate those steps into the following:

using System;
using System.Security.Cryptography;
...
private static byte[] HashSHA(byte[] innerKey, byte[] outerKey, byte[] message)
{
    var hash = new SHA256Managed();

    // Compute the hash for the inner data first
    byte[] innerData = new byte[innerKey.Length + message.Length];
    Buffer.BlockCopy(innerKey, 0, innerData, 0, innerKey.Length);
    Buffer.BlockCopy(message, 0, innerData, innerKey.Length, message.Length);
    byte[] innerHash = hash.ComputeHash(innerData);

    // Compute the entire hash
    byte[] data = new byte[outerKey.Length + innerHash.Length];
    Buffer.BlockCopy(outerKey, 0, data, 0, outerKey.Length);
    Buffer.BlockCopy(innerHash, 0, data, outerKey.Length, innerHash.Length);
    byte[] result = hash.ComputeHash(data);

    return result;
}

Helper functions

string -> byte[]

You have plain ASCII or UTF8 text, but need it to be a byte[].
Use ASCIIEncoding or UTF8Encoding or whichever exotic encoding you’re using.

private static byte[] StringEncode(string text)
{
    var encoding = new System.Text.ASCIIEncoding();
    return encoding.GetBytes(text);
}

byte[] -> hex string

You have a byte[], but you need it to be a hex string.

private static string HashEncode(byte[] hash)
{
    return BitConverter.ToString(hash).Replace("-", "").ToLower();
}

hex string -> byte[]

You have a hex string, but you need it to be a byte[]`.

.NET 5 and above

private static byte[] HexDecode(string hex) =>
    System.Convert.FromHexString(hex);

Before .NET 5 (thanks @bobince)

private static byte[] HexDecode(string hex)
{
    var bytes = new byte[hex.Length / 2];
    for (int i = 0; i < bytes.Length; i++)
    {
        bytes[i] = byte.Parse(hex.Substring(i * 2, 2), NumberStyles.HexNumber);
    }
    return bytes;
}

n.b. If you need a performance tuned version on .NET Framework 4.x, you can alternatively backport the .NET 5+ version (by replacing ReadOnlySpan<byte> with byte[]). It uses proper lookup tables and conscious about hot-code paths. You can reference the .NET 5 (MIT licensed) System.Convert code on Github.

---

For completeness, here are the final methods answering the question using both "Method 1" and "Method 2"

"Method 1" (using .NET libraries)

private static string HashHMACHex(string keyHex, string message)
{
    byte[] hash = HashHMAC(HexDecode(keyHex), StringEncode(message));
    return HashEncode(hash);
}

"Method 2" (manually computed)

private static string HashSHAHex(string innerKeyHex, string outerKeyHex, string message)
{
    byte[] hash = HashSHA(HexDecode(innerKeyHex), HexDecode(outerKeyHex), StringEncode(message));
    return HashEncode(hash);
}

---

We can perform a quick sanity check with a console app:

static void Main(string[] args)
{
    string message = "amount=100&currency=EUR";
    string expectedHex = "b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905";
    Console.WriteLine("Expected: " + expectedHex);

    // Test out the HMAC hash method
    string key = "57617b5d2349434b34734345635073433835777e2d244c31715535255a366773755a4d70532a5879793238235f707c4f7865753f3f446e633a21575643303f66";
    string hashHMACHex = HashHMACHex(key, message);
    Console.WriteLine("Method 1: " + hashHMACHex);

    // Test out the SHA hash method
    string innerKey = "61574d6b157f757d02457573556645750e0341481b127a07476303136c005145436c7b46651c6e4f4f040e1569464a794e534309097258550c17616075060950";
    string outerKey = "0b3d27017f151f17682f1f193f0c2f1f64692b227178106d2d096979066a3b2f2906112c0f760425256e647f032c2013243929636318323f667d0b0a1f6c633a";
    string hashSHAHex = HashSHAHex(innerKey, outerKey, message);
    Console.WriteLine("Method 2: " + hashSHAHex);
}

You should have all the hashes line up correctly:

Expected: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905
Method 1: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905
Method 2: b436e3e86cb3800b3864aeecc8d06c126f005e7645803461717a8e4b2de3a905

The original code for this answer can be accessed at:
http://pastebin.com/xAAuZrJX