Idea Ripper // ArchiveCards: 1462 [Live] | Updated: 2026-10-14

La Cryptographie militaire

by Auguste Kerckhoffs

15 cards

Military cryptography — the 1883 paper that founded modern cryptanalysis: six desiderata for ciphers, the repeated-fragment key-length attack, and the maxim that a cipher must baffle even its inventor.

Card #c918Synthesis

The enemy knows the system

Il faut qu'il n'exige pas le secret, et qu'il puisse sans inconvénient tomber entre les mains de l'ennemi. The system must not require secrecy, and must be able to fall into the enemy's hands without inconvenience.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When you are evaluating any security system, protocol, or anyone's secret sauce.

[Why it matters]

Security that depends on the method staying hidden dies the day it is captured, reverse-engineered, or leaked. Design as if the enemy already has the blueprint — because eventually they will.

Card #c919Synthesis

A break doesn't expire

Une fois qu'un cryptogramme intercepté a pu être déchiffré, toute nouvelle dépêche, écrite avec la même clef et qui subit le même sort, peut être lue instantanément. Once one intercepted cryptogram is broken, every later dispatch written with the same key can be read instantly.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When you are tempted to reuse a key, password, or credential because this one use is low-stakes.

[Why it matters]

Against the claim that war messages only need three or four hours of secrecy: a break compounds. It doesn't expire with the message — it retroactively opens everything ever sent under that key.

Card #c920Synthesis

Secrecy doesn't scale

J'entends par secret, non la clef proprement dite, mais ce qui constitue la partie matérielle du système : tableaux, dictionnaires ou appareils mécaniques. By secrecy I mean not the key itself, but the material part of the system: tables, dictionaries, mechanical devices.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When deciding what in your operation must stay secret versus what must survive exposure.

[Why it matters]

If the machinery must stay secret, every new user is a new capture risk — so the system can never be issued beyond a handful of generals. A secret you can't distribute is a system you can't deploy.

Card #c921Synthesis

Destroy the weapon by publishing it

Ce n'était pas un des moindres services à rendre à la patrie que d'anéantir par la publicité l'arme la plus dangereuse des ennemis secrets. In 1793 Dlandol published the royalists' cipher keys — annihilating by publicity the most dangerous weapon of the Republic's secret enemies.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When you need to kill a bad standard, a compromised credential, or a practice that survives on obscurity.

[Why it matters]

Publicity as an offensive move. A broken secret, widely known to be broken, stops being a weapon — nobody will trust a cipher the newspapers have already printed the key to.

Card #c922Synthesis

Every break is two jobs

Le déchiffrement d'un cryptogramme dont on n'a pas la clef comporte deux opérations bien distinctes : un calcul de probabilité et un travail de tâtonnement. Breaking a cryptogram without the key is two distinct jobs: a probability calculation and groping trial-and-error.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When sizing up any adversarial problem — split it into the math and the groping.

[Why it matters]

For polyalphabetic systems the two unknowns are the number of alphabets and their arrangement. Decomposing the attacker's job into countable subproblems is also how the defender prices each layer of defense.

Card #c923Synthesis

Repetition is the leak

Dans tout texte chiffré, deux polygrammes semblables sont le produit de deux groupes de lettres semblables, cryptographiés avec les mêmes alphabets ; le nombre de chiffres compris dans l'intervalle des deux polygrammes est un multiple du nombre des lettres de la clef. Two identical ciphertext fragments come from two identical plaintext groups under the same alphabets — and the distance between them is a multiple of the key length.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When hunting hidden periodicity in any data — ciphertext, logs, signals.

[Why it matters]

Take the greatest common divisor of the intervals between repeats and you have the key length. Redundancy betrays structure: the key is hiding in the gaps between repeats.

Card #c924Synthesis

The whole cipher is one letter: E

La valeur d'un chiffre se mesure aux garanties qu'il offre contre la découverte du signe correspondant à cette lettre. The value of a cipher is measured by the guarantees it offers against discovery of the sign for the letter E.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When attacking any substitution-like structure — find the anchor first.

[Why it matters]

In French, E is one letter in five. Find the most frequent symbol and the rest of the system unravels by trial. Every substitution-like structure has a load-bearing letter — the defense lives or dies on hiding it.

Card #c925Synthesis

The traitor words

Dans les cas un peu difficiles ce sont généralement les deux ou trois premiers mots qui viennent jouer le rôle de traîtres. In the hard cases it is generally the first two or three words that play the traitors.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When guessing the predictable parts of any structured message or protocol.

[Why it matters]

Military dispatches open predictably — le général, vous, ne — so the breaker keeps a classified list of likely openings indexed by where E falls. Every message has a predictable edge; the attacker catalogs it like factory-default passwords.

Card #c926Synthesis

Symmetry of position

Du moment que les alphabets sont disposés en nombre carré, et que la signification d'un chiffre commun a été trouvée dans deux ou plusieurs alphabets, on peut, au moyen d'une simple addition, déterminer la place que doit occuper dans ces différents alphabets tout nouveau chiffre. In square-arranged alphabets the letters keep their relative order — solve one letter in two alphabets and every other letter transfers by offset.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When exploiting any regular arrangement — one solved cell positions the whole grid.

[Why it matters]

Structure leaks sideways. One solved position determines all parallel positions by simple addition — Kerckhoffs notes no cryptography book had ever pointed this out. Regularity is a side channel.

Card #c927Synthesis

One key per message

Quelle que soit la disposition adoptée des alphabets, les correspondants sont toujours tenus d'écrire chacune de leurs dépêches avec une clef différente. Whatever the alphabets, correspondents must write each dispatch with a different key.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When reusing any secret across sessions — each reuse feeds the same attack.

[Why it matters]

Stack a dozen short cryptograms under one key, align them in columns, and the frequency math works across all of them at once. Reusing a key pools many small unbreakable texts into one big breakable one.

Card #c928Verbatim excerpt

Ars ipsi secreta magistro

Un chiffre n'est bon qu'autant qu'il reste indéchiffrable pour le maître lui-même qui l'a inventé : Ars ipsi secreta magistro. A cipher is only good insofar as it remains indecipherable to the very master who invented it.

— Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When testing your own system, argument, or lock — try to break it with the home-field advantage.

[Why it matters]

Borrowed from Du Carlet (1644) as the closing line of the whole paper. The ultimate test isn't whether strangers fail to break your system — it's whether you, holding every advantage, cannot read it either.

Card #c929Synthesis

Teach your officers to break it

Ce ne sera que lorsque nos officiers auront étudié les principes de la cryptographie et appris l'art de déchiffrer, qu'ils seront en état d'éviter les nombreuses bévues qui compromettent la clef des meilleurs chiffres. Only when our officers have studied cryptography and learned the art of breaking will they avoid the blunders that compromise the key of the best ciphers.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When rolling out any security practice to non-experts — teach the break first.

[Why it matters]

The Germans drilled every officer in cryptanalysis; the French issued ciphers nobody understood. Users who can't attack the system will misuse it into insecurity — the weakest link is the operator, so train the operator as an attacker.

Card #c930Synthesis

Beaufort unmasked

Tous ces appareils ne sont en réalité autre chose qu'un simple tableau de Vigenère. The English marveled at Admiral Beaufort's supposedly indecipherable system — it is Vigenère with a reversed alphabet, nothing more.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When someone pitches you a revolutionary method — strip the costume first.

[Why it matters]

Most celebrated new systems are old systems in costume. Before awe, check for isomorphism: relabel the parts and see if the machine you already know is hiding inside.

Card #c931Synthesis

Krohn's 3,200 alphabets

Ce n'est pas la possibilité de se servir d'un grand nombre d'alphabets différents qui donne de la valeur à un système, mais plutôt la difficulté plus ou moins grande de déterminer le nombre des alphabets employés. Krohn built a dictionary of 3,200 alphabets — c'est à la fois trop et trop peu: too much and too little.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When someone dazzles you with big numbers — ask which number the attacker actually needs.

[Why it matters]

What matters isn't how many alphabets you have, it's how hard it is to determine how many you used. Complexity theater: the metric being maximized is not the metric that matters.

Card #c932Synthesis

One wrong digit, wrong war

Qu'un seul chiffre dans un groupe soit fautif, que le télégraphe mette un 3 où il faut un 5, et le sens sera complètement changé. One wrong digit in a code group and the meaning is completely changed — an Italian cable once turned an embassy attaché into Count Andrassy.

— In our words · Source: La Cryptographie militaire (Auguste Kerckhoffs)

[Use when]

When designing anything where a typo changes the order — make errors loud, not plausible.

[Why it matters]

Codebooks fail silently and totally: a garbled group doesn't look garbled, it looks like a different order. In 1870 half the army's dispatches arrived partly illegible. A system whose failure mode is confident misreading will misread confidently.