When I designed my spreadsheet cipher, I examined several classical cryptographic systems and decided I wanted to have many of their strengths and very few of their weaknesses while still leaving it easy for a single person to encrypt and decrypt.
What my method does and does not have are the following.
1. No cipher block limit. The cylindrical cryptograph of Etienne Bazeries had twenty discs with twenty-five letters on the outer rim of the discs. Thomas Jefferson's wheel cypher had thirty-six discs with twenty-six letters on rims. The strip cipher of Parker Hitt had twenty-five discs and letters, though the non-cylindrical version could be enlarged with as many additional lines of ciphertexts whenever desired and the position of each ciphertext was not fixed like on the cylinder version.
A common flaw in all these methods is the overreliance on the imposed limits. For example, the Hitt cipher could only decrypt twenty-five characters of any ciphertext before you had to use another ciphertext for the next segment. The other systems mentioned all had similar weaknesses.
I, therefore, got rid of the limit, the message can be of any length and the cipher key for that message will be applicable for the entire length of what that message is keyed to be solved with, This should prevent using knowledge of the length of each ciphertext being used to create frequency tables for cracking the cipher.
2. Ease of encryption while making cryptanalysis difficult.
Another challenge I had when creating my method was that I needed to balance the ease of use by the intended cipher solver to the third party who I would NOT want to solve it via cryptanalysis.
The Vigenere system of Blaise de Vigenere is anything but secure. The Confederacy of Northern America found to their sorrow their trust in that system was to their detriment, and I did not want to use it's known alphabet table sequence since it's a very easily solved one by anyone with knowledge of how the table is created.
I, therefore, made it so the alphabet table rows and columns can be randomized so long as each row or column (whichever is used for plaintext and ciphertext encryption and decryption) contained twenty-six letters and each part of the row or column uses each letter once.
This allows for cipher keys to be quickly created and changed on the fly so different portions of the enciphered text can switch between different sections of the alphabet tables for different encryption alphabets while keeping things just random enough a cryptanalyst would be flying completely blind so long as both the keys and alphabet table are kept secure, though only one at minimum need to remain completely secret at the very least.
3. I created this cipher on a computer, and I wanted it to be easily editable and readable on all electronic devices, and since spaced digits and characters translate well to any spreadsheet program and whatever rows and columns the cipher alphabets sit on can be completely randomized, this allows anyone to modify this cipher at will for their own purposes so long as they keep both the cipher keys and the alphabet tables from both being available to those not entrusted with them. That is the only weak link in the security chain I could not avoid, but other modern-day methods of concealing documents can be combined with my method for better concealment of that information at the discretion of the cipher creator.
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