There's a plot point in my current piece that involves some encrypted radio messages to/from a hidden lab and corporate headquarters, some time in the 1910s. In-universe, the corporation's board of directors have settled on a combination of a book cipher with the Vigenère cipher, where the key is chosen using a dictionary and changes at regular intervals. The process can be summed up as follows:
- Check the date to see what page, column and line number are mapped to this time period (e.g. page 30, column 1 and line 1).
- Open the dictionary and find the matching word (e.g. bugler)
- Use that word as the cipher key.
- The key must be changed every two weeks. The Board of Directors have been advised that this is Best Practice, and therefore shall not be questioned!
I know that the Vigenère cipher was formally broken by the 1860s, and that part has not changed. However, would combining it with the rest of this actually make it harder to crack, given the state of the art about a century ago?
- In our universe, an attempt to fix the Vigenère around the tail end of WW1 led to the one-time pad (which is proven to be unbreakable, if used properly). For this question, assume that this does not exist or is not widely known.
- The company's threat model considers the government (any major government) to be the primary threat here, followed distantly by any rival company.