Exercise 2: Why a "Kiss" Compromises Security — Possible Solution
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A "kiss" hands the codebreaker something normally very hard to get:
GENUINE, CONFIRMED plaintext for a specific piece of Enigma
ciphertext, without any guessing involved at all.

Here's the chain of reasoning:

1. The weaker cipher is broken separately, unrelated to Enigma's own
   key. Once broken, its ciphertext yields the exact plaintext that
   was sent.

2. If that SAME plaintext content was also sent via Enigma (say, the
   same status report transmitted through both systems, a real
   historical occurrence), the codebreaker now has a crib that isn't
   a guess at all -- it's a certainty.

3. This eliminates the biggest source of uncertainty in ordinary
   crib-dragging (Chapter 4's main technique): normally a crib is only
   a GUESS at what plaintext might appear, and a wrong guess wastes
   time testing offsets that were never going to work. A confirmed
   crib from a kiss can be dragged against the Enigma ciphertext with
   total confidence that it really does appear somewhere in the
   message.

4. The fact that the two ciphers used completely unrelated keys
   doesn't protect Enigma here, because the kiss never attacks
   Enigma's KEY directly -- it attacks the assumption that Enigma's
   PLAINTEXT is unknown. Once the plaintext is known by an entirely
   separate route, Enigma's key can be worked out via ordinary
   crib-dragging and Bombe testing against that now-certain crib,
   exactly as described earlier in the chapter.

WHY THIS WORKS AS AN ANSWER
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This generalizes the chapter's own point about predictable message
content (the "Cribs" section) to its logical extreme: a kiss isn't
just a probable guess based on typical phrasing, it's a 100%-confirmed
crib obtained through an entirely different, independently broken
system. It reinforces this chapter's closing lesson directly -- the
weakness has nothing to do with Enigma's underlying cipher strength;
it comes from an operational practice (reusing the same content across
systems of very different strength) that undermines even a
mathematically sound cipher.