How to translate Python HMAC Request into Swift - swift

Question

I've been at this about 10 hours now and no matter what HMAC combination I use for swift I can not get it to match the key generated by python.
Python Code:
signature = hmac.new(secret.decode('hex'), msg=datastring, digestmod=hashlib.sha256).hexdigest()
Swift Code:
let key = SymmetricKey(data: self.secret.data(using: .utf8)!)
let hexData = HMAC<SHA256>.authenticationCode(for: datastring.data(using: .utf8)!, using: key)
let signature = Data(hexData).map { String(format: "%02hhx", $0) }.joined()
Any help with what I'm doing wrong (or missing) in Swift would be greatly appreciated.

Answer 1

Based on the assumption that self.secret is a String containing the hex representation of the secret key, the difference between the two comes down to your use of:
self.secret.data(using: .utf8)!
which will just perform a straight conversion to the underlying bytes instead of converting each character pair into the corresponding byte, as:
secret.decode('hex')
does in Python 2.
From what I can tell, there isn't a function to do this conversion in the Swift standard library, but you could do it with something like:
func bytes(fromHex input: String) -> Data {
var result = Data()
var byte: UInt8 = 0
for (index, character) in input.enumerated() {
let codeUnit = character.utf8[character.utf8.startIndex]
var nibble: UInt8 = 0
switch codeUnit {
case 0x30..<0x3a:
nibble = codeUnit - 0x30
case 0x61..<0x67:
nibble = codeUnit - 0x57
default:
break
}
if index % 2 == 0 {
byte |= (nibble << 4)
} else {
byte |= nibble
result.append(contentsOf: [byte])
byte = 0
}
}
return result
}
and then your code would become:
let key = SymmetricKey(data: bytes(fromHex: self.secret))
let hexData = HMAC<SHA256>.authenticationCode(for: datastring.data(using: .utf8)!, using: key)
let signature = Data(hexData).map { String(format: "%02hhx", $0) }.joined()