Complete digest

In this stage, you assemble all building blocks into the complete SHA-256 implementation: padding, initial state vectors $H_0 \dots H_7$, schedule expansion, and 64 compression rounds per block.

Mental Model: The SHA-256 Pipeline

1. Initial Vector: First 32 bits of the fractional parts of square roots of first 8 primes ($H_0 \dots H_7$).
2. Padding: Pad input UTF-8 bytes to multiple of 512 bits.
3. Block Processing:
   • Expand 16 words to 64.
   • Run 64 rounds with constants $K_0 \dots K_{63}$.
   • Additive update to $H_0 \dots H_7$.
4. Final Digest: 64-character lowercase hexadecimal string.

Your Goal in this Stage

Read a JSON object from stdin with op: "sha256" and text. Return the 64-character hex digest under "hex".

Input Format

{
  "op": "sha256",
  "text": "abc"
}

Output Format

{
  "hex": "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
}

Step-by-Step Implementation Guide

1. Encode to UTF-8 bytes: Convert input text.
2. Apply padding: Align to 512-bit blocks.
3. Execute compression rounds: Process blocks through rounds.
4. Format hex digest: Join $H_0 \dots H_7$ zero-padded to 8 hex digits each.
5. Emit output: Print JSON object.

Acceptance Criteria

• ✅ Matches NIST standard test vectors.
• ✅ Produces 64-character lowercase hex digest.

Prepare unsigned 32-bit rotation, logical shift, Choice, and Majority. Distinguish the small schedule sigmas from the capital compression sigmas. Save each block’s initial state for feed-forward after 64 rounds; all additions wrap modulo 2^32. Do not derive constants using floating point.

Use FIPS 180-4: §4.1.2 defines functions, §4.2.2 constants, §5.3.3 initialization, and §6.2.2 the update order. Trace one round before processing multiple blocks.

Multiline examples are formatted for readability. Emit compact JSON without optional spaces, with properties in the order shown by the output examples, and a final newline. Keep logs off stdout. Tests compare exact text. Invalid inputs use stderr containing error, empty stdout, and exit 1. Valid inputs exit 0. Serialize integer results as JSON integers.