Create a keyed-hash HMAC in hex-encoded format from hex-encoded input.
public static string HexFromHex( string messageHex, string keyHex, HashAlgorithm hashAlg )
Public Shared Function HexFromHex ( messageHex As String, keyHex As String, hashAlg As HashAlgorithm ) As String
VB6/C equivalent: HMAC_HexFromHex