March 7, 2017

We examined the Solitaire Cipher of Bruce Schneier in a previous exercise. In an article on his blog, Ted Unangst complains that the cipher is too complicated for manual use, and proposes instead RC40, a variant of RC4 that uses only 40 characters instead of 256. Unangst’s alphabet is the 26 lower-case letters, the ten digits 0 through 9, the characters period, question mark, and space, and a “shift” character that gives an upper-case version of those 39 characters; two consecutive shift characters enter or leave caps-lock mode. Otherwise, RC40 is exactly the same as RC4, except that all references to 256 are changed to 40.

Your task is to write a program that enciphers and deciphers strings based on the RC40 cipher; use that program to decipher the string “5cxaxlfrfhy6kh38fbplm0mDko58xs.l9Hkz8” with key “tedunangst”. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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