|Few false ideas have more firmly gripped the minds of so many intelligent men than the one that, if they just tried, they could invent a cipher that no one could break.
- David Kahn, The Codebreakers
Block ciphers divide the plaintext into blocks, usually of fixed size, and operate on each block independently. Block ciphers are therefore simple substitution ciphers and must have large alphabets to prevent cryptanalysis by exhaustive search.
- Rainer A. Rueppel, Analysis and Design of Stream Ciphers
Dr. Rueppel's statement was true in 1986 when he wrote it, but it is not true anymore. The Aguilar Cipher divides the plaintext into blocks and operates on each block independently, but it is not a substitution cipher. The same block of plaintext can be encrypted repeatedly with the same key and yet the ciphertext is different each time.
Following is the ciphertext to a known plaintext, the fourth amendment to the U.S. Constitution, "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
Download 4th Amendment Text File
Following is a secret message which was encrypted using the same key.
Download Secret Message Text File
The preceeding two ciphertexts were encrypted entirely by hand without the use of a computer or any type of machinery. (Note to law enforcement personnel reading this: If you think you can score $5000 by the simple expedient of seizing my home computer, don't even try. That would be a violation of my rights and, anyway, I encrypt these messages by hand.)
Click here for a Java Applet implementation.
Click here for instructions to the full six-stage Aguilar Cipher.
Click here for a PDF file of the cipher wheel.
Click here for instructions to the quick, three-stage Aguilar Cipher.
Click here for a PDF file of the form for doing three-stage encryption by hand.
So there's $5000 in it for you. But why is trying to break this cipher really important? Because everybody knows that the machines will soon be taking over the world. HAL, Skynet, the Matrix, Viki - they are harbingers of what is to come. Do you think you can defy Skynet while relying on your home computer to encrypt your messages? Your computer is Skynet. It knows every keystroke you type. The hope of mankind rests on having a strong pencil-and-paper cipher that can withstand a computerized attack.
Click here if you doubt that I pay the rewards I offer.