encrypt message with one digest 0-9 instead of 01-26 How much will the security of one-time pad decrease if I use one digit from 0-9 instead of 1-26?

for example plaintext = "blue" and key = "5 9 4 6":

 b  l  u  e  -  plaintext
02 12 21 05  -  plaintext in numeric form
 5  9  4  6  -  key
07 21 25 11  -  ciphertext in numeric form
 g  u  y  k  -  ciphertext


so blue + 5946 = guyk

Is that as secure as if I use numbers from 1-26?

And will blue + 5946 be the unbreakable one time-pad?
Firstly, I presume this is not something you are going to use for protecting data in any kind of real life scenario, but are only asking out of curiosity.

Secondly, just to get the terminology straight and avoid confusion, what gives an OTP cryptographic scheme information theoretic security is that it meets both of the following two criteria:


The key space is exactly as large as the total message space across all messages encrypted. Hence the "One Time" part of the term. The criterion is not that the key is as "long" as each message you encrypt, but both that it is as long as all messages you encrypt combined, and that there are, at each position in your message, at least as many possible key values as there are possible plain text values. Also, when the key is selected, it must be in such way that at each position, no key value at that position is any more or any less probable than any other key value.
When, at each position in your message, you combine a plain text value with a key value to produce a cipher text value, this has to be done using an operation that does not make any cipher text value any more or any less probable, than any other cipher text value. It just happens that a couple of simple operations, such as modular addition or exclusive-or, have this property, provided that the first criterion is already met. Hence, it is not the use of those operations for combining the key with the plain text that makes an OTP into what it is. These operations only happen to be used because they are conveniently simple, provided you already got a key that meets the first criterion.


Consequently, the problem with your proposal is that it doesn't meet the first criterion, that the key space must be as large as the message space. Going with your example, if you are only going to encrypt a four letter string (and only going to do this once), you will get a key space of only  $10^4 = 10000$ keys, but a message space of $26^4 = 456976$ possible plain texts.