Can we use numbers as a pad in the Vernam cipher - why or why not? I was playing with the Vernam cipher on some online converter.

But when I tried to encrypt my message string with numbers, it remained unchanged.

Moreover, it was ignoring numbers and was encrypting as some alphabet appears in the key. 



So, can we use numbers as a pad in the Vernam cipher - why or why not?
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.


  


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


The reason you could not insert numbers into the online encrypt/decrypt tool you found is because that tool was only designed for the modulo 26 addition of letters.

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