4 ms·

> Multiple ciphertexts using the same key, a fatal flaw for a cryptosystem even today. To avoid confusing readers, one should mention that modern ciphers separ

by taejo 6y ago

> Multiple ciphertexts using the same key, a fatal flaw for a cryptosystem even today. To avoid confusing readers, one should mention that modern ciphers separate the key, which can be reused but must be kept secret, from the initialization vector (IV), which must not be reused but can be transmitted in the clear. These correspond roughly to the Enigma's daily settings and per-message rotor start positions, but AIUI the procedure for selecting and transmitting the latter changed over the course of the war, and at times insecure even if done according to the procedure.

stan_rogers 6y ago
It was actually far worse for the Lorenz (Tunny) traffic. The wheel patterns - the equivalent of the key in, say, AES - were changed quite infrequently early on. The five χ-wheels (in Bletchley parlance) were only changed monthly, while the five ψ-wheels were changed quarterly. While the two μ-wheel patterns were changed daily, there was little consideration given to what those patterns actually were. The χ-wheels generated the first of two sets of bit patterns to be XORed with the plain text; the ψ-wheels generated the second set. That would have been fine, except that the two μ-wheels together determined whether or not the ψ-wheels advanced - μ1 advanced on every keystroke along with the χ-wheels, μ2 would advance only if μ1 was a 1, and the ψ-wheels advanced only if μ2 was a 1 - and bad patterns on the ψ-wheels would mean that the ψ-wheels would be essentially frozen for extended runs of the message. What was meant to introduce a sort of dither to make it harder to determine how the key was being generated became the Achilles heel that highlighted the mechanism. (Not to take anything away from Bill Tutte and the rest of the crew at all. There was still a lot of cleverness and tedious attention to detail involved.) With a month of life on the χ patterns and three months on ψ, depths and near-depths on the message settings (the nonce or IV) were absolute killers. Later on, all of the wheel patterns were changed daily, and the μ-wheel patterns were verified for short stall lengths, but by then the machine's mechanism was well understood, pattern-derivation methods had been devised, and Colossus had largely eliminated the χ component from the puzzle.
- stan_rogers 6y ago
  The phrase "and bad patterns on the ψ-wheels" should have read "and bad patterns on the μ-wheels". Sorry if there was any confusion; I caught that too late for editing.
tialaramex 6y ago
We're discussing (in this sub-thread) Lorenz rather than Enigma. But yes, the per-message rotor positions are what would be repeated resulting in "depth".
- taejo 6y ago
  Yeah, I got side-tracked in the middle of writing my comment, and it didn't seem worth fixing since I only noticed after the edit window closed and my main point holds either way.

by taejo 6y ago

stan_rogers 6y ago

It was actually far worse for the Lorenz (Tunny) traffic. The wheel patterns - the equivalent of the key in, say, AES - were changed quite infrequently early on. The five χ-wheels (in Bletchley parlance) were only changed monthly, while the five ψ-wheels were changed quarterly. While the two μ-wheel patterns were changed daily, there was little consideration given to what those patterns actually were. The χ-wheels generated the first of two sets of bit patterns to be XORed with the plain text; the ψ-wheels generated the second set. That would have been fine, except that the two μ-wheels together determined whether or not the ψ-wheels advanced - μ1 advanced on every keystroke along with the χ-wheels, μ2 would advance only if μ1 was a 1, and the ψ-wheels advanced only if μ2 was a 1 - and bad patterns on the ψ-wheels would mean that the ψ-wheels would be essentially frozen for extended runs of the message. What was meant to introduce a sort of dither to make it harder to determine how the key was being generated became the Achilles heel that highlighted the mechanism. (Not to take anything away from Bill Tutte and the rest of the crew at all. There was still a lot of cleverness and tedious attention to detail involved.) With a month of life on the χ patterns and three months on ψ, depths and near-depths on the message settings (the nonce or IV) were absolute killers. Later on, all of the wheel patterns were changed daily, and the μ-wheel patterns were verified for short stall lengths, but by then the machine's mechanism was well understood, pattern-derivation methods had been devised, and Colossus had largely eliminated the χ component from the puzzle.

stan_rogers 6y ago
The phrase "and bad patterns on the ψ-wheels" should have read "and bad patterns on the μ-wheels". Sorry if there was any confusion; I caught that too late for editing.

tialaramex 6y ago

We're discussing (in this sub-thread) Lorenz rather than Enigma. But yes, the per-message rotor positions are what would be repeated resulting in "depth".

taejo 6y ago
Yeah, I got side-tracked in the middle of writing my comment, and it didn't seem worth fixing since I only noticed after the edit window closed and my main point holds either way.