SHA-256 padding

Append the one bit, zeroes, and the 64-bit length to complete 512-bit blocks. This stage adds one observable capability to the project; keep every operation from earlier stages working.

Model and contract

The harness reads one JSON request from stdin. Bytes are represented as UTF-8 text or hexadecimal, and all binary output uses lowercase hexadecimal.

Representative input:

{ "op": "pad", "hex": "616263" }

Exact output:

{
  "hex": "61626380000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000018"
}

The output ends with a newline. For an invalid request, leave stdout empty, write a diagnostic containing error to stderr, and exit with a non-zero status.

How to approach it

Keep input parsing, core logic, and output serialization separate. First write down the invariants behind “sha-256 padding”, walk through the example by hand, and exercise boundaries before optimizing. Do not replace the mechanism taught by this stage with a library function that solves it completely.

Pay particular attention to empty inputs, index or length boundaries, and malformed data. The result must be deterministic: preserve the ordering required by the request and emit compact JSON.

Acceptance criteria

• The representative input produces exactly the output shown.
• Invalid input follows the stderr and exit-status contract.
• Capabilities from earlier stages keep working.