Java vs. Golang HOTP Implementation: How to Resolve Discrepancies?

Java vs. Golang for HOTP (rfc-4226)

When attempting to implement HOTP in Golang, you may encounter discrepancies between the output produced by Java and Golang implementations. This can be attributed to subtle differences in how the two languages handle byte arrays, particularly when dealing with signed and unsigned values.

Java vs. Golang Byte Types

Java's byte type is signed, meaning it ranges from -128 to 127, while Golang's byte type is an alias of uint8, which ranges from 0 to 255. This difference leads to variations in the byte arrays generated by the respective languages.

Conversion between Signed and Unsigned Values

To compare the byte arrays generated by Java and Golang, it is necessary to convert the signed Java byte values to their corresponding unsigned values. This can be achieved by adding 256 to negative values.

Alternatively, you can display Java byte values in an unsigned format using bitwise operations:

Byte Order Differences

Another difference between Java and Golang is the byte ordering of long integers. Java follows big-endian byte ordering, while Golang uses little-endian byte ordering. This means that the order of bytes in the byte array will differ between the two languages.

Big-Endian to Little-Endian Conversion

To ensure consistent byte ordering between Java and Golang, it is necessary to convert Java's big-endian byte arrays to Golang's little-endian format. This can be achieved using external libraries or by manually swapping the byte order.

Encoding Differences

In the provided code snippets, the Java implementation returns a hex-encoded result, while the Golang implementation returns a base64-encoded result. To match the Java output, you can use the hex.EncodeToString function in Golang.

Signed vs. Unsigned Display

To display Go's byte values in a signed fashion, you can convert them to int8, which is a signed type.

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