Schneier on Security

Anura • April 26, 2015 2:24 PM

@Wael

For verifying, if he signs a text-only version, you could just manually copy the text into your computer and verify it that way.

“Hmm… Verification failed, let me check… Aha! He had a typo on page 173 that I didn’t copy correctly. Now it passed signature verification!”

Anura • April 28, 2015 1:37 PM

Okay, you asked for it, a pencil and paper hash function:

Get a code wheel, randomize the inner disk while leaving the outer in order, and initialize a 36 character hash state to 0,1,..,9,A,B,..,Z. Now, take the first and last characters of the state and align those characters the inner and outer wheels, respectively. Now, take the first character from the message and find it on the outer disk, the new first character of the state is the corresponding character on the inner disk. Repeat using the FIRST character of the message for every character in the state, lining up the previous character of the state on the inner disk with the current character of the state on the outer disk. When all 36 characters of the state have been set, do the same for the next character in the message and then repeat until you have consumed the entire message. Then append an underscore followed by the base ten representation of the length of the message.

Once you are done, reduce the state to an 18 character hash using the following method: Line up the zero on the inner wheel with the first character of the state on the outer wheel, then find the nineteenth character on the outer wheel. The first character of the hash is the corresponding character on the inner wheel. Repeat with the second character and the twentieth character, the third and the twenty-first, all the way through the eighteenth and the thirty-sixth character.

Now, you can simply use a hash-based signature scheme to sign it.