Source: https://guthlabs.ai/docs/instruments/caesar-substitution-cipher-tool/

instrument / verified-clean

# Caesar / Substitution Cipher Tool

The current ledger marks this instrument verified clean within the source-bound verification corpus.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Inspectable records

[01**Overview**Purpose and lifecycle](<https://guthlabs.ai/docs/instruments/caesar-substitution-cipher-tool/>)[02**Mechanism**Method and assumptions](<https://guthlabs.ai/docs/instruments/caesar-substitution-cipher-tool/mechanism/>)[03**Implementation**Binding and validation](<https://guthlabs.ai/docs/instruments/caesar-substitution-cipher-tool/implementation/>)[04**Qualification**Evidence and errata](<https://guthlabs.ai/docs/instruments/caesar-substitution-cipher-tool/qualification/>)

## Ledger state

The current ledger marks this instrument verified clean within the source-bound verification corpus.

**Area**: Computing & Information

**Tranche**: tranche-5

**Evidence ID**: t5-099

**Ledger rows**: 1

No ledger note is attached.

## Review findings

The core Caesar and positional substitution concepts are standard, but the specification needs explicit alphabet, case, nonletter, key-validation, inverse-mapping, negative-shift, and brute-force-direction rules before implementation can be verified as unambiguous.

### Formula and scope findings

- The Caesar formula C=(P+k) mod 26 is correct for encryption when letters are indexed A=0 through Z=25 and k is an integer shift; decryption must use P=(C-k) mod 26 or an equivalent inverse shift.

- Direct substitution is correct only when the substitution alphabet is a valid 26-letter permutation with a clearly defined direction, such as plaintext alphabet A-Z mapped positionally to the supplied key alphabet. Decryption requires the inverse mapping.

- The specification does not define whether shifts are normalized modulo 26, how negative shifts are handled, whether case is preserved, or whether nonalphabetic characters are left unchanged.

- The specification does not define whether the all-26-shifts table shows encryption or decryption, nor the ordering and labels of the shifts.

### Unit assumptions

- Alphabetic characters use the standard English alphabet with A=0, B=1, ..., Z=25.

- The Caesar shift k is a dimensionless integer and is normalized modulo 26; for example, k=27 is equivalent to k=1 and k=-1 is equivalent to k=25.

- Spaces, punctuation, and digits are assumed to remain unchanged and do not consume shift positions.

- The examples assume uppercase output for uppercase input; case-preservation behavior for lowercase input is unspecified.

- A substitution key is assumed to contain exactly 26 unique alphabetic characters, with the character at position i replacing plaintext letter A+i.

## Boundary hardening

**BREAK_CLAIMED** · SELF_FLAGGED

An incomplete 25-character substitution alphabet causes a lookup outside the defined mapping: encrypting Z produces an undefined result. The tool should reject this input instead of returning malformed ciphertext.

## Errata and corrections

No applied erratum is attached to this unique-name record.

## Inspectable example

4 review vectors are recorded. One source-bound sample is shown below.

````
{
  "inputs": {
    "mode": "caesar_encrypt",
    "text": "MEET ME AT 9",
    "shift": 3
  },
  "expected_outputs": {
    "encoded_text": "PHHW PH DW 9"
  },
  "working": "M=12: (12+3) mod 26=15=P; E=4: (4+3) mod 26=7=H; E=4: (4+3) mod 26=7=H; T=19: (19+3) mod 26=22=W. The second MEET becomes PHHW. M=12 -> P=15; E=4 -> H=7. A=0: (0+3) mod 26=3=D; T=19 -> W=22. Spaces and 9 are unchanged. Result: PHHW PH DW 9."
}
````

## Related workflows

No source-defined multi-instrument workflow membership is recorded.

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json · VERIFIED_CLEAN`

- `evidence-tranche5/tranche5-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`