1. Cornell University, English letter frequency, http://pi.math.cornell.edu/~mec/2003-2004/cryptography/subs/frequencies.html
2. Project Gutenberg, The Gutenberg Project, https://www.gutenberg.org
3. R. W. Hamming, Coding and Information Theory, 2nd edn. (Prentice-Hall, Hoboken, 1986), p. 103ff
4. Natural Language Tool Kit, Brown Corpus, http://www.nltk.org
5. The Crypto Museum, The Vernam cipher, https://www.cryptomuseum.com/crypto/vernam.htm. Accessed 24 Jan 2020