A simple substitution cipher vs. one-time pad software
Both a simple subsitution cipher and one-time pad software have data encryption benefits despite their differences.
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:
| A | B | C | D | E | F | G | H | I | J | . . . |
| Q | W | E | R | T | Y | U | I | O | P | . . . |
The disadvantage of this method is that with any message of reasonable length, fifty letters or more, frequency...
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analysis can be used to deduce the meaning of the most common symbols, allowing a cryptanalyst to build partial words and progressively break the message. Now, a one-time pad is similar to a substitution cipher, but the plaintext letters are combined not substituted, and it has been proven to be mathematically unbreakable. The recipient of the ciphertext requires a copy of the one-time pad to reverse the process. There are many different ways to apply one-time pads. Here's an example using letters for the one-time pad key:
| Plaintext | B | A | D | G | E |
| 1 | 0 | 3 | 6 | 4 | |
| OTP Key | Q | W | E | R | T |
| 16 | 22 | 4 | 17 | 19 | |
| Result | 17 | 22 | 7 | 23 | 23 |
| Ciphertext | R | W | H | X | X |
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:
- The key has to be as long as the plaintext, thus leaking some information about the message.
- The key has to be genuinely random, which is hard to achieve for large keys.
- The key can only be used once and must be kept entirely secret from all except the sender and receiver, creating a distribution problem.
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.
