Deciphering historical substitution ciphers is a challenging problem. Example
problems that have been previously studied include detecting cipher type,
detecting plaintext language, and acquiring the substitution key for segmented
ciphers. However, attacking unsegmented, space-free ciphers is still a
challenging task. Segmentation (i.e. finding substitution units) is the first
step towards cracking those ciphers. In this work, we propose the first
automatic methods to segment those ciphers using Byte Pair Encoding (BPE) and
unigram language models. Our methods achieve an average segmentation error of
2\% on 100 randomly-generated monoalphabetic ciphers and 27\% on 3 real
homophonic ciphers. We also propose a method for solving non-deterministic
ciphers with existing keys using a lattice and a pretrained language model. Our
method leads to the full solution of the IA cipher; a real historical cipher
that has not been fully solved until this work.

本文提出了一种使用字节对编码 (BPE) 和单元语言模型来自动分段非分段 (无空格) 密码的方法，对 100 个随机生成的单字母替代密码平均分段错误率为 2％，对 3 个现实中的同音替代密码平均分段错误率为 27％，并提出了一种通过创建一个晶格和使用先前经过训练的语言模型来解决已知替代密钥的非确定性密码的方法，成功解密了一个历史密码 IA，这是一个之前一直未被完全解开的密码。