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When we solved this stage we had not yet solved Stage 5, and we did
thus not know what type of cipher we had to deal with. However,
several properties of the ciphertext suggested that this cipher was a
Playfair cipher. For instance,
The Playfair cipher seemed connected with Great Britain so our approach
was that the plaintext should be written in English.
- The letter J was missing from the ciphertext.
- The frequency distribution of single letters was rather smooth,
while the bigram frequency distribution was skewed.
- The chronological order in which the ciphers tend to appear made
a Playfair cipher seem probable.