Ethereum: Why does Bitcoin Core output SHA256 (uint256) hash bytes in reverse order?

Understanding Ethereum’s SHA-256 Hashes and Debugging Challenges

As a developer working with Bitcoin Core, one of the most common issues encountered is the printing of SHA-256 hashes in reverse order. This phenomenon has led to frustration among developers, particularly during functional testing. In this article, we will delve into why Bitcoin Core prints SHA-256 hashes in reverse order and explore ways to resolve this issue.

The Role of SHA-256

SHA-256 (Secure Hash Algorithm 256) is a cryptographic hash function designed to produce fixed-size, unique digital fingerprints for input data. These fingerprints are generated by taking a message as input, applying the SHA-256 algorithm, and producing a 256-bit (64-byte) output. In Bitcoin Core, the ethash command uses this hash function to generate the blockchain’s Merkle tree and other cryptographic structures.

Why SHA-256 Hashes Print in Reverse Order

When converting uint256 hashes to strings using the printf format specifier in Bitcoin Core, the bytes are reversed due to the way the string is printed. Specifically:

• The %u format specifier uses a two-digit unsigned integer to represent the value.

• When printing an unsigned integer using %u, the most significant byte (MSB) comes first, followed by the least significant byte (LSB).

• In uint256, each byte represents a 8-bit unsigned integer.

• By default, when converting uint256 values to strings in Bitcoin Core, the output is reversed because of this order.

Functional Testing Challenges

The reversed output can lead to confusion during functional testing. For instance:

• When comparing two hashes using ethash compare, the comparison may fail if the hash values are not printed in the same order.

• In certain tests, the reversal could affect the expected behavior of the cryptographic functions used by the blockchain.

Resolving the Issue

To resolve this issue, you can use a few workarounds:

• Print the hashes as hexadecimal strings: Use printf with %x instead of %u to print the hashes as hexadecimal strings. This will ensure that the output is not reversed.

echo "0x1234567890abcdef" | printf "%x"

• Use a custom formatting function: Write a custom format string using printf with the desired output format. For example, you can use %08x to print hashes as 8-digit hexadecimal strings.

echo "0x1234567890abcdef" | printf "%08x"

• Use the printf command with a specific separator

  : You can specify a custom separator between values using the -t option of printf. For example:

ethash print -t, --stdout "0x1234567890abcdef" | ...

By implementing these workarounds or customizations, you should be able to resolve the issue with printing SHA-256 hashes in reverse order in Bitcoin Core and ensure that your functional tests are accurate.