Caesar cipher

Explicit alphabet cycle

Recognition requires a finite ordered alphabet with distinct symbols and a defined indexing convention.

1. Which distinct symbols form the alphabet, and in what exact order? definition
2. Is that order supplied by the configuration, established by a source, or assumed during analysis? provenance

Uniform modular displacement

Every supported substitution must have the same modular displacement; variation by position or symbol contradicts the declared Caesar rule.

1. Does every observed supported substitution satisfy (ciphertext index − plaintext index) mod N = k for the same k? measurement
2. Does any observed mapping require an arbitrary permutation, a changing shift or a rearrangement of positions? boundary

Canonical shift

Represent encryption as addition of k modulo N and reduce equivalent signed or oversized shifts to the same canonical value.

1. What value of k represents encryption under the declared positive alphabet direction? definition
2. What canonical value results when the supplied shift is reduced modulo N? measurement

Inverse shift behaviour

Decoding uses −k modulo N; k = 0 is the identity, and a rotation is self-inverse when 2k is divisible by N.

1. Which inverse shift restores supported input symbols after encryption? action
2. Does this configuration produce an identity mapping or a self-inverse rotation? measurement

Case and normalization policy

The agent needs explicit rules for case and normalization rather than assuming that all visually similar text denotes identical cipher symbols.

1. What constitutes one cipher symbol, and how are uppercase, lowercase and accented forms handled? definition
2. If case uses separate alphabets, do they apply the same shift under aligned alphabet orders? boundary

Unsupported symbols and round-trip recovery

Preservation, rejection or removal of unsupported symbols must be recorded, together with any effect on recovering the original input.

1. Are spaces, punctuation, digits and other unsupported symbols preserved, rejected or removed? definition
2. Does decoding an encoded sample reproduce the original input exactly or only its normalized form? measurement

Sample consistency

An aligned supported symbol pair determines a candidate offset for a fixed alphabet; additional pairs test whether that offset remains consistent.

1. Where did the paired samples come from, and is their alignment established? provenance
2. Which pairs support the candidate shift, and which contradict it after the declared preprocessing? measurement

Candidate set and ambiguity

A fixed alphabet permits enumeration of N distinct shift mappings, including the identity; language expectations can rank outputs without guaranteeing a unique answer.

1. Which shifts remain plausible after enumeration, and what evidence supports their ranking? measurement
2. What additional known text, language information or external clue would distinguish the remaining candidates? action

Pattern preservation

Within each shifted alphabet, the substitution preserves symbol-frequency counts up to relabelling and repeated-symbol patterns; preserved separators also expose text structure.

1. Which repeated-symbol patterns, frequency counts and separators remain observable after transformation? measurement
2. How many distinct shift mappings must be tried for the declared alphabet? measurement

Concealment and confidentiality requirements

A Caesar cipher can serve teaching, puzzles and reversible obfuscation, but its small enumerable shift space makes it unsuitable for modern confidentiality requirements.

1. Is the requested outcome demonstration, puzzle solving, casual concealment or protection against an adversary? boundary
2. Should the agent perform the declared transformation or route a confidentiality requirement to a suitable encryption model? action

Kinds and varieties

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Classical Caesar (k = 3 on the Latin alphabet, the form Suetonius attributes to Julius Caesar)
• Augustan shift (k = 1, the form Suetonius attributes to Augustus)
• General shift/additive cipher (arbitrary k, usually 1-25 on a 26-letter alphabet)
• ROT13 (k = 13 on 26 letters; an involution, widely used as a distinct named convention)
• ROT5 / ROT18 / ROT47 (digit-only, alphanumeric, and printable-ASCII shift variants used in puzzles and light obfuscation)
• Non-Latin modular Caesar (same additive rule on other cyclic alphabets, e.g. 32- or 33-letter Cyrillic)

1. Which of these kinds and varieties hold for the sense of Caesar cipher this model covers, and on what evidence? provenance

Identifiers and schemes

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Wikidata item - Q190234 - Item for the Caesar cipher as a cipher type.
• Wikidata item - Q541169 - Item for ROT13, the k = 13 special case treated as a named convention of its own.

1. Which of these identifiers and schemes hold for the sense of Caesar cipher this model covers, and on what evidence? provenance

Standards and regulation

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Not an approved cryptographic algorithm under NIST FIPS 140-3 or in the NSA Commercial National Security Algorithm (CNSA) suite; those regimes require modern, specified primitives rather than classical shifts.
• Taught as an insecure classical cipher in several national secondary-computing specifications (notably UK GCSE Computer Science papers such as OCR and AQA), which is curricular description, not an operational crypto standard.
• ROT13 is a community convention of Netnews/Usenet and later forums for hiding spoilers, not a standardised encryption algorithm in the IETF sense.

1. Which of these standards and regulation hold for the sense of Caesar cipher this model covers, and on what evidence? provenance

Real-world use

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Classroom and textbook introduction to substitution, modular arithmetic, and why key space and frequency statistics matter.
• ROT13 (and ROT47) to hide spoilers, punchlines, and puzzle solutions on Usenet, mailing lists, and forums.
• Puzzle hunts, escape rooms, geocaching listings, and CTF warm-up challenges.
• Toy or legacy obfuscation in source comments and config values; not relied on for actual secrecy.
• Historical reconstruction of Roman military correspondence as reported by Suetonius, not as a living field cipher.

1. Which of these real-world use hold for the sense of Caesar cipher this model covers, and on what evidence? provenance

Typical measurements

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Shift key k - 0-25 (often 1-25; 0 is the identity) - alphabet positions
• Alphabet size n - 26 for English Latin; 27 if Ñ is included; ~32-33 for Russian Cyrillic - letters
• Keyspace size - n distinct shifts, n−1 non-trivial - keys
• Nominal key entropy (English 26-letter case) - about 4.7 - bits
• Trials for exhaustive search - 25 for a 26-letter alphabet - decryptions

1. Which of these typical measurements hold for the sense of Caesar cipher this model covers, and on what evidence? provenance

Failure modes and hazards

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Exhaustive search over 25 keys recovers English plaintext immediately.
• Letter-frequency analysis identifies k because each plaintext letter has a single ciphertext image.
• A single known plaintext-ciphertext letter pair determines k.
• Using it as if it provided confidentiality (passwords, exam answers, 'encrypted' backups) yields a false sense of security.
• Implementations disagree on case, diacritics, digits, and whether non-letters are passed through, so interoperable decrypt often fails.
• Ciphertext is malleably additive: an attacker who knows the scheme can apply a further shift without the key.

1. Which of these failure modes and hazards hold for the sense of Caesar cipher this model covers, and on what evidence? provenance

Regional variation

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• English usage splits between 'Caesar cipher' for any constant shift and a stricter use that reserves 'Caesar' for k = 3 and calls the rest a shift cipher.
• French chiffre de César, German Cäsar-Chiffre / Cäsar-Verschlüsselung, and Russian шифр Цезаря are the ordinary textbook names.
• ROT13 is a named everyday practice mainly in English-language internet culture; it is less often treated as a separate object in non-English school syllabi.
• Alphabet size and wrap-around follow the local orthographic alphabet, so a 'Caesar' tool built for A-Z mis-handles Ñ, Ä/Ö/Ü, or Cyrillic unless n is changed.

1. Which of these regional variation hold for the sense of Caesar cipher this model covers, and on what evidence? provenance

Neighbouring kinds and how to tell them apart

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Vigenère cipher - Vigenère applies a repeating sequence of Caesar shifts keyed by a word, so the same plaintext letter can map to different ciphertext letters; a true Caesar map is one shift for the whole text.
• Affine cipher - Affine encryption is c ≡ ap + b (mod n) with gcd(a, n) = 1; Caesar is the special case a = 1. Presence of a multiplier other than 1 separates them.
• Atbash - Atbash reflects the alphabet (A↔Z, B↔Y, …), which is not a rotation; the mapping is an involution whose pairs sit at complementary positions, not a constant offset.
• General monoalphabetic substitution - A mixed-alphabet cipher uses an arbitrary permutation of the alphabet. Caesar is only the n rotations; if the permutation is not a cyclic shift, it is not Caesar.
• Transposition ciphers (e.g. rail fence, columnar) - Those rearrange letter order and leave the letters themselves unchanged; Caesar changes identities and leaves order unchanged.

1. Which of these neighbouring kinds and how to tell them apart hold for the sense of Caesar cipher this model covers, and on what evidence? provenance