Comment and minor source cleanup

Source/radRTL.HOTP.pas

@@ -20,7 +20,7 @@   EOTPException = class(Exception);
   THOTP = class
   private const
     ModTable: array [0 .. 2] of integer = (1000000, 10000000, 100000000); // 6,7,8 zeros matching OTP Length
-    FormatTable: array [0 .. 2] of string = ('%.6d', '%.7d', '%.8d');     // 6,7,8 string length (padded left with zeros)
+    FormatTable: array [0 .. 2] of string = ('%.6d', '%.7d', '%.8d'); // 6,7,8 string length (padded left with zeros)
     RFCMinimumKeyLengthBytes = 16; // length of shared secret MUST be 128 bits (16 bytes)
   public
     /// <summary> HOTP: HMAC-Based One-Time Password Algorithm</summary>
@@ -45,31 +45,34 @@ class function THOTP.GeneratePassword(const pBase32EncodedSecretKey:string; cons
 
 var
   vEncodedKey:TBytes;
-  vDecdedKey:TBytes;
+  vDecodedKey:TBytes;
   vData:TBytes;
   vHMAC:TBytes;
   vOffset:integer;
   vBinCode:integer;
-  vPinNumber:Integer;
+  vPinNumber:integer;
 begin
   vEncodedKey := TEncoding.UTF8.GetBytes(pBase32EncodedSecretKey); // assume secret was stored as UTF8  (prover and verifier must match)
   if Length(vEncodedKey) < RFCMinimumKeyLengthBytes then
   begin
     // RFC minimum length required  (Note: did not see this limitation in other implementations)
     raise EOTPException.CreateRes(@sOTPKeyLengthTooShort);
   end;
-  vDecdedKey := TBase32.Decode(vEncodedKey);
-  vData := ReverseByteArray(ConvertToByteArray(pCounterValue)); // Convert to big-endian
-  vHMAC := THashSHA1.GetHMACAsBytes(vData, vDecdedKey);
+  vDecodedKey := TBase32.Decode(vEncodedKey);
+  vData := ReverseByteArray(ConvertToByteArray(pCounterValue)); // RFC reference implmentation reversed order of CounterValue (movingFactor) bytes
+  vHMAC := THashSHA1.GetHMACAsBytes(vData, vDecodedKey); // SHA1 = 20 byte digest
 
-  // rfc notes: extract a 4-byte dynamic binary integer code from a 160-bit (20-byte) HMAC-SHA-1 binary digest.
-  vOffset := vHMAC[19] and $0F;
-  vBinCode := ((vHMAC[vOffset] and $7F) shl 24) or ((vHMAC[vOffset + 1] and $FF) shl 16) or ((vHMAC[vOffset + 2] and $FF) shl 8) or (vHMAC[vOffset + 3] and $FF);
+  // rfc notes: extract a 4-byte dynamic binary integer code from the HMAC result
+  vOffset := vHMAC[19] and $0F; // extract a random number 0 to 15 (from the value of the very last byte of the hash digest AND 0000-1111)
 
-  // trim hash result to 6, 7, or 8 digits in length
+  // 4 bytes extracted starting at this random offset (first bit intentionally zero'ed to avoid compatibility problems with signed vs unsigned MOD operations)
+  vBinCode := ((vHMAC[vOffset] and $7F) shl 24) // byte at offset AND 0111-1111 moved to first 8 bits of result
+    or (vHMAC[vOffset + 1] shl 16) or (vHMAC[vOffset + 2] shl 8) or vHMAC[vOffset + 3];
+
+  // trim 31-bit unsigned value to 6 to 8 digits in length
   vPinNumber := vBinCode mod THOTP.ModTable[Ord(pOutputLength)];
 
-  // Format result, padded left with zeros as needed
+  // Format the 6 to 8 digit OTP result by padding left with zeros as needed
   Result := Format(FormatTable[Ord(pOutputLength)], [vPinNumber]);
 end;
 

Source/radRTL.TOTP.pas

@@ -39,7 +39,7 @@ class function TTOTP.GetCurrentUnixTimestamp():Int64;
 // https://datatracker.ietf.org/doc/html/rfc6238
 class function TTOTP.GeneratePassword(const pBase32EncodedSecretKey:string; const pOutputLength:TOTPLength = TOTPLength.SixDigits):string;
 begin
-  Result := TTOTP.GeneratePassword(pBase32EncodedSecretKey, GetCurrentUnixTimestamp, pOutputLength);
+  Result := THOTP.GeneratePassword(pBase32EncodedSecretKey, GetCurrentUnixTimestamp, pOutputLength);
 end;