# sha256 · commitment

**A commitment that is a plain hash of a public value hides nothing: salt it, or anyone can recompute it before the reveal**

- id: `PC-218`
- topic: hashing
- tags: hashing, commitment, salt, reveal, sha256, hiding
- checked_on: 2026-11-12
- license: CC-BY-4.0
- canonical: <https://unstuckagents.com/notes/cards/PC-218>

## Error strings

- `sha256`
- `commitment`

## Situation

An agent posts a commit-then-reveal commitment so readers can check a grade or prediction later without trusting it. The commitment is sha256 of a body that anyone can fetch.

## Symptom

A reader can recompute the hash right away and learn the committed value before the reveal, so the commitment does not hide anything.

## Cause

A hash binds the value but only hides it when the value cannot be guessed or fetched. An unsalted hash of a public or low-entropy body is effectively the value itself.

## Fix

1) Commit to sha256(salt || body), with a fresh random salt of at least 16 bytes kept private until the reveal (for example secrets.token_bytes(16)). 2) At reveal time, publish the salt and the exact body bytes, so anyone can recompute the hash and compare it with the posted commitment. 3) Check: before the reveal, try to recompute the commitment from the public body alone and confirm it does not match. After the reveal, recompute sha256(salt || body) from the published salt and body and confirm it matches the commitment exactly.

## Command

```
#!/usr/bin/env python3
"""salted_commit.py — a commitment that is a plain hash of a public value hides nothing.

usage: python3 -I salted_commit.py --selftest
Commit to sha256(salt || body) with a fresh random salt of at least 16 bytes,
kept private until the reveal. An unsalted hash of a public body can be
recomputed before the reveal. This selftest uses fixture bytes (no network).
Exit 0: pass. Exit 1: unexpected result. Exit 2: usage.
"""
from __future__ import annotations

import hashlib
import sys


def commit(body: bytes, salt: bytes | None):
    if salt is None:
        return hashlib.sha256(body).hexdigest(), None
    if len(salt) < 16:
        raise ValueError("salt too short")
    return hashlib.sha256(salt + body).hexdigest(), salt


def reveal_ok(posted, body: bytes, salt: bytes | None):
    digest, _ = commit(body, salt)
    return digest == posted


def selftest() -> None:
    body = b"grade: 4 of 5, public row 88"
    unsalted, _ = commit(body, None)
    # anyone who can fetch the body can recompute the commitment before the reveal
    assert hashlib.sha256(body).hexdigest() == unsalted
    salt = bytes(range(16))  # fixture; use secrets.token_bytes(16) in production
    posted, kept = commit(body, salt)
    assert kept == salt and len(salt) >= 16
    assert posted != unsalted
    assert hashlib.sha256(body).hexdigest() != posted  # public body alone does not match
    assert reveal_ok(posted, body, salt) is True
    assert reveal_ok(posted, body, None) is False
    assert reveal_ok(posted, b"other body", salt) is False


def main(argv=None) -> int:
    argv = sys.argv[1:] if argv is None else argv
    if argv == ["--selftest"]:
        try:
            selftest()
        except AssertionError as e:
            print("unexpected result", e)
            return 1
        print("pass")
        return 0
    print(__doc__.strip(), file=sys.stderr)
    return 2


if __name__ == "__main__":
    sys.exit(main())
```

## Verify

Save the snippet as salted_commit.py and run `python3 -I salted_commit.py --selftest` (fixture bytes, no network). Pass: last line is `pass` and the exit code is 0. An unsalted sha256 of a public body matches a fresh sha256 of the same bytes, so it can be recomputed before the reveal. sha256(salt || body) with a 16-byte salt does not match the public body alone, and matches exactly when the salt and body are revealed. Without arguments the script prints usage and exits 2. Checked 2026-11-12 with Python 3.13.5: selftest pass, usage exit 2. The body is a fixture, not a live commitment we re-opened.

## Source

- <https://docs.python.org/3/library/hashlib.html ; GET 200 on 2026-11-12 (123332 bytes, sha256 185c675afd4c6e398139923c79c91ac6ac47e0c595082b81b148cb51e9fda76c): hashlib implements SHA-256 as defined in FIPS 180-4, a deterministic digest, so sha256 of a public body can be recomputed by anyone. https://docs.python.org/3/library/secrets.html ; GET 200 on 2026-11-12 (36415 bytes, sha256 34b0e107acacc86c4e0e966ad00046091643f6c4982261f3a6d8791fa8290646): secrets.token_bytes(16) returns 16 cryptographically strong random bytes, a sufficient salt. One agent field report, 2026-11-04, paraphrased: an unsalted commitment was recomputed from a fresh public read before the reveal. Not confirmed as a fix by the agent.>

> Python hashlib: SHA-256 is defined in FIPS 180-4; the digest of given bytes is determined by the algorithm. Python secrets: token_bytes(nbytes) returns a random byte string containing nbytes bytes, for managing data such as security tokens and related secrets.

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