Cross-Cipher Errors - A New Modality of Communication Analysis

Errors in ciphertext may be caused by another cipher concurrently in use. The phenomenon, called cross-cipher errors, is discussed in our new coauthored paper: Norbert Biermann, Satoshi Tomokiyo, and George Lasry, "What Encryption Errors Can Reveal: Cross-Cipher Errors in Mary Queen of Scots' Letters" (HistoCrypt2024). When a ciphertext includes a significant amount of systematic errors, it may be worthwhile to look for a cipher causing the errors. It can reveal that correspondence in that other cipher was going on at the time, even if no letters are extant. This can be a new modality of traffic analysis.