One Message, Two Encryptions
A weak copy can supply the plaintext an analyst needs to attack another copy.
Why Codebreakers Welcomed a “Kiss”
At Bletchley Park, a message sent under different keys or through different cipher systems could be called a kiss. Reading one version could supply a crib for another. Bauer describes general orders repeated across Enigma networks and messages also carried by simpler systems.
The shared element is the message. This is different from the Lorenz depth, where related messages reused a keystream. Here the copies may use different keys and algorithms.
Everything below is invented. Copy A uses Caesar with a shift of 4, assumed already known to the analyst. Copy B uses repeating-key Vigenère with a known period of five. Both are broken historical cipher families; Copy B is only harder within this teaching comparison. This is not an Enigma simulation.
Test Whether the Copies Share a Plaintext
Predict: if you recover five letters of Copy A, can they determine five Vigenère key letters? Would fitting those five positions prove that the other copy says the same thing?
Spaces are removed and X is padding in these fixtures. No real intercepted message is reproduced.
1. Read the weaker copy
Apply the already known Caesar shift. In a real case, obtaining that weak-system solution would be a separate task.
2. Fit a candidate key
Assume the first five letters of A also begin B. With A = 0, B = 1, …, Z = 25, subtract plaintext from ciphertext modulo 26. Only the first five positions are used here.
| Position | B cipher | A plain | Key calculation |
|---|
3. Test the remaining positions
The first five positions fit by construction. Now decrypt B and compare positions 6–20 with the rest of A. Then select the different-message control and repeat the investigation.
What the Result Establishes
A successful check supports the shared-plaintext hypothesis within this controlled fixture. Equal lengths, a plausible phrase or a fitted key alone would not authenticate a historical message. The negative control gives you a case where a key fits the crib but fails elsewhere.
Known plaintext is an expected attacker capability for modern encryption. Sending the same plaintext under independent keys does not by itself break a sound modern scheme. The key recovery here works because repeating-key Vigenère exposes the repeated shifts when plaintext is known.
Continue with Enigma’s crib checks, known-plaintext cryptanalysis or the Evidence Workbench.
Sources and Reading
Friedrich L. Bauer, Decrypted Secrets: Methods and Maxims of Cryptology, 4th ed. (Springer, 2007), pp. 288–289 and 419–421, especially §19.5.1. The history is paraphrased; the messages, controls and exercise are original museum material.
Tony Sale’s Virtual Bletchley Park account of registration and traffic analysis explains the contemporary use of “kisses.” The exercise uses neither historical intercepts nor historical key material.