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Modern cryptosystems rely on huge prime numbers for secure cyphers. Historically, secret messages could be sent by replacing one letter with another. An outsider cannot unscramble the code without knowing the secret letter. But if many messages are sent, an enemy can statistically detect the pattern of the device and the secret key. During World War II, all sides used mechanical ciphers. The U.S. military’s M-209 portable crypto machine required users to encrypt text one letter at a time. By turning a crank, the scrambled message printed out on paper tape. Gears within the M-209 have prime-number ratios to prevent repetitive positions—with over one million positions before the key repeats. The M-209, however, is not fully secure, partly because its prime numbers are short. It worked well for ephemeral battlefield messages as the enemy typically deciphered the message too late to be useful. See “Mathematics: Vintage and Modern” on display, post-security, in Terminal 2 and online at: https://bit.ly/MathAtSFO
This image was posted on March 28, 2022.