Cipher methods Symbols
Maritime Signals (ICS)
Maritime flags are one of the few ciphers still in daily use on CipherChronicle: the system is codified in the International Code of Signals (ICS), adopted by the United Kingdom in 1857 and revised up to the modern 1969 version maintained by the International Maritime Organization (IMO). It is the official standard for visual communication between ships at sea and between ship and shore.
Each Latin letter is associated with a flag of distinctive colours and patterns (Alpha = white/blue, Bravo = pointed red, Charlie = blue/white/red stripes, etc.) and each digit 0-9 with a numbered triangular pennant. Flags can be hoisted alone — each then carries a standardised message (e.g. Charlie = “affirmative”, Lima = “stop your vessel instantly”) — or combined to spell a word or form a three-letter code from the ICS dictionary.
How does the alphabet work?
When used to spell a message (rather than for its standardised messages), the maritime code behaves like a monoalphabetic substitution: each cleartext letter is replaced by the corresponding flag. Same mechanic as the Caesar cipher (~50 BC) — a 1↔1 table — but standardised internationally by the IMO.
The table covers 26 letters + 10 digits (numerical pennants 0 to 9). The flags and pennants are designed to be distinguishable at great distance: vivid contrasting colours (red, navy, yellow, white, black), simple patterns (stripes, crosses, squares).
Cryptographic strength: none. It’s not a secret cipher, it’s a universal communication language. Any trained sailor reads the flags at sight. The interest on CipherChronicle is pedagogical: showcasing an alphabet still in service, seen in regattas, military exercises and port signalling.
Historical and modern usage
- Merchant and military navies — ship-to-ship communication.
- Regattas — race signalling (start, halt, ranking).
- Ports — the Quebec flag hoisted by a ship returning from abroad.
- Maritime pedagogy — naval school curricula.
Related variants
- Flag semaphore — see our entry, communication with two hand-held flags.
- Chappe semaphore — see our entry, French optical telegraph.
- Morse code — see our entry, electric telegraph.
What are the weaknesses?
- No confidentiality — every flag has a public meaning.
- Reduced visibility in bad weather — rain, fog, night.
- Limited to 36 signs — no punctuation or accents.
The 36 flags
