A Statistical Attack on the Running Key Cipher

Solving the Running Key Cipher with the Viterbi Algorithm

“…. This is a direct comparison between the results of the Viterbi algorithm (solid line) and the results reported in [1] (dashed line). Both methods used n-gram statistics to solve a running key cipher made by combining the first 1000 characters of Dracula [12] with the first 1000 characters of The Cask of Amontillado (in [9]).…”

“…The first experiment was a comparison with the approach in [1], so it used the same training set and attempted to solve the same ciphertext using the same n-gram sizes. The second experiment attempted a more general evaluation by using many texts from Project Gutenberg [6].…”

“…The algorithm was tested on 100 English (message, key) pairs, and usually the message and key were mostly recovered. This method is shown to be an improvement on an earlier approach [1].…”

“…As in [1], the first 1000 letters of Dracula were enciphered using the first 1000 letters of The Cask of Amontillado as the running key [12,9]. This ciphertext was solved by the Viterbi algorithm using n-gram sizes between 1 and 6, where the conditional probabilities for the Markov chain were calculated using maximum likelihood estimation, i.e., without smoothing, using the union of [12] and [9] as the training text.…”

“…Bauer and Tate [1] enciphered the first 1000 letters of Dracula using the first 1000 letters of The Cask of Amontillado as the running key [12,9]. The resulting ciphertext was partitioned into disjoint n-letter blocks that were each solved separately.…”