Why Do Java and Golang HOTP (RFC-4226) Implementations Produce Different Results?

HOTP (RFC-4226) Implementation in Java vs. Golang

When implementing HOTP (RFC-4226) in Golang, discrepancies may arise in generating valid HOTPs compared to Java. To address this issue, it is essential to understand the nuanced differences in byte handling between Java and Golang.

Byte Representation

In Java, the byte type is signed, ranging from -128 to 127. However, in Golang, the byte type is an alias for uint8, resulting in a range of 0 to 255. This distinction is crucial in understanding the byte array comparison.

Java Implementation

The Java code snippet uses ByteBuffer.allocate(8).putLong(counter).array() to generate a byte array for the counter value. Specifically, the putLong method converts the counter to an 8-byte long integer and stores it in the byte array.

Golang Implementation

In Golang, the code uses binary.BigEndian.PutUint64(bs, counter) to generate a byte array for the counter value. The PutUint64 method converts the counter to a 64-bit unsigned integer and stores it in the byte array in big-endian order.

Discrepancy in Byte Values

The difference between these two methods stems from the signed vs. unsigned nature of the byte type. When the Java code converts a negative counter value to a byte array, the resulting bytes are interpreted as signed values. In contrast, Golang's conversion preserves the unsigned nature of the counter value.

Solution

To compensate for the differing byte interpretations, it is necessary to shift negative Java byte values by 256 to obtain unsigned values.

Base64 vs. Hex Encoding

Another disparity between the Java and Golang implementations lies in the encoding scheme used. The Java implementation returns a hex-encoded string, while the Golang implementation returns a base64-encoded string. For the HOTP results to match, one must ensure that both implementations use the same encoding scheme.

Additional Tips

• For visual clarity, display byte values in hex or unsigned form when comparing results.
• To display signed Golang bytes, convert them to int8 prior to printing.

By considering the byte representation nuances and encoding differences, you can resolve the discrepancies between Java and Golang implementations of HOTP (RFC-4226).

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