# Is there a practical security difference between OTP with letters and OTP with numbers?

Is there a practical security difference between “OTP with letters” and “OTP with numbers”?

If, for example, I encrypt a letter message using Tabula Recta with a random letter key, I would get:

THISI SATES TXAAA - message
MOSSK VFXNN EJRQW - OTP key
FVAKS NFQRF XGRQW - cipher

Would such encryption be less secure when it comes to frequency analysis, compared to (let's say) Dirk Rijmenant's CT-46 OTP where the most frequent letters (AEINOR) are associated with a single digit instead of two digits, like the rest of the less frequent letters in the alphabet?

Example:

THISI SATES TXAAA
83753 82390 82183 28290 83871 11000 - message
08733 64840 36236 58407 58482 94160 - OTP key
85020 28550 56957 70893 35499 27940 - cipher
• No, OTP has perfect secrecy regardless of what encoding your message uses. – otus Aug 29 '15 at 12:10
• Hint: Can you find an OTP key(part) that maps an arbitrary cipher text to an arbitrary plain text. Or formulated differently: Can every cipher text letter be decrypted to every plain text letter if an appropriate OTP is used? – SEJPM Aug 29 '15 at 12:11

There is no security difference. Of course, purely random characters with entropy rate $\log M$ where $M$ is the size of the alphabet should be independently generated and used for the OTP, whatever the size $M$ of the alphabet.

Letters only allow you to use letters and nothing else.

So you would only be able to send a message like this: