A simple substitution cipher vs. one-time pad software

I'm not entirely sure I understand your question, so let's look at what's meant by a simple substitution cipher and then what's meant by a one-time pad or one-time pad software.

In encryption, a substitution cipher replaces units of plaintext with ciphertext according to a regular system. The recipient of the ciphertext can decipher it by performing an inverse substitution. The unit can be anything from a single letter, letters or a mixture of both. Although the plaintext units themselves are altered, they remain in the same sequence in the ciphertext. (This contrasts with a transposition cipher where the units are left unchanged, but their order is rearranged.)

A simple substitution cipher operates on single letters. Using the example below, we can turn the word BADGE into WQRUT in ciphertext:

Using the example above, you take the first letter in the plaintext message and add it to the first random letter from the one-time pad. This number is then converted to the corresponding letter of the alphabet, with the alphabet wrapping around to the beginning if the addition results in a number beyond 26. Using this one-time pad, the word BADGE becomes RWHXX. Because each one-time pad has a different key, the ciphertext of the word BADGE in this case will be different every time. In the above example, you can also see that frequency analysis is impossible as X occurs for both the letter G and E. With a simple substitution cipher, the word BADGE will always become WQRUT. The drawbacks with the one-time pad are: If these problems are not overcome, particularly the randomness of the key, the one-time pad is no longer unbreakable. Even if it is theoretically secure, it may be insecure in practice.