Abstract

The Playfair cipher is a well-known manual encryption method developed in the 19th century. Until 2018, known cryptanalysis techniques, with computer assistance, could solve non-keyword-based Playfair ciphertexts if they had at least 60 letters to work with. Shorter ciphertexts were effectively impossible to solve in the absence of a crib. In this article, we show how we introduced several improvements in these cryptanalysis methods, which made it possible to do much better. This resulted in the (unofficial) world record for the shortest Playfair message broken going down from 60 via 50, 40, 32, and 28 to 26 letters. The cryptanalysis techniques used include hill climbing, simulated annealing, tabu search, and plaintext-based dictionary attacks. For readers interested in improving the current record, we also provide unsolved Playfair challenges consisting of 24 and 22 letters.

摘要

Playfair 密码是 19 世纪开发的一种著名的手动加密方法。直到 2018 年，已知的密码分析技术在计算机辅助下，如果至少有 60 个字母可以处理，就可以解决非基于关键字的 Playfair 密文。在没有婴儿床的情况下，更短的密文实际上是不可能解决的。在这篇文章中，我们展示了我们如何在这些密码分析方法中引入了一些改进，这使得我们可以做得更好。这导致（非官方）最短的 Playfair 消息世界纪录从 60 到 50、40、32 和 28 个字母下降到 26 个字母。使用的密码分析技术包括爬山、模拟退火、禁忌搜索和基于明文的字典攻击。对于有兴趣改善当前记录的读者，