Can One Use Numbers as a One-Time-Pad Key? (a question at Cryptography Stack Exchange)

Yes, you can use numbers as a one-time-pad key. In fact, the CIA used to do it all the time, as did many.

When you use numbers the plaintext becomes letters by referring to a conversion table such as the venerable “Tapir” used by the STASI.

enter image description here

Here the addition will be modulo 10. Vernam Cipher, or the one-time pad (OTP), can also be done modulo 2:

SENDING


message: 0 0 1 0 1 1 0 1 0 1 1 1 … pad: 1 0 0 1 1 1 0 0 1 0 1 1 … XOR ————————— cipher: 1 0 1 1 0 0 0 1 1 1 0 0 …

RECEIVING


cipher: 1 0 1 1 0 0 0 1 1 1 0 0 … pad: 1 0 0 1 1 1 0 0 1 0 1 1 … XOR ————————— message: 0 0 1 0 1 1 0 1 0 1 1 1 …

Or it can be done modulo 26 (with English letters, for example):

Plaintext: DARLING THE NIST CURVES HAVE BEEN COMPROMISED AND MY RANDOM NUMBER GENERATOR HAS A KLEPTOGRAPHIC BACKDOOR I FEEL SAD

Key: NLQVT ZBOFW MFAVS RJMDE PGNEX GGQMU VOFNE PBWXT ICDWK VEEYL EGVWS ZRDKD IDJGO HWKFF MBEGA KEUNQ BEYDO


Ciphertext: QLHGB MHHMA ZNSOU LAHHW WGIIY KKDOI HDWBQ XTAAT VFPUB VRHMX RAHXW QXHXH ZDCUF OWCFP XFTZO QVUCX JGZDQ MUYBN VQUZE RBR

Here is one example of what a CIA one-time-pad key looked like during the Cold War:

enter image description here

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