New answers tagged multiple-encryption
If the ciphers are different, with independent keys, you can say that it is at least as strong as the first cipher. If the ciphers commute, like with stream ciphers, you can even say that it is at least as strong as the strongest. See Cascade Ciphers: The Importance of Being First. That's really all you can say in general. In practice, the combinations you ...
In general, all you can say is it can be as weak as the weakest encryption layer, if you're lucky. Edit: It can also be even weaker, for badly chosen components that cancel out some mathematically desirable properties, as pointed out in the comment.
There is a very simple, completely generic solution, that unlike the other solution doesn't assume anything about how the two encryption schemes work internally (e.g., that they are built from block ciphers or have pseudorandom ciphertexts): given a message $m$, choose a uniformly random $m_1$ of the same length and let $m_2 = m \oplus m_1$. Then encrypt ...
Your formula: ciphertext = twofish(message) XOR aes(message) Looks more as a kind of random number generation formula. Indeed , aes output will looks like random. twofish output will looks like random. Read Bruce schneier Book 'Applied cryptograpy' and you will learn that the best way to generate a random number is to Xor together random values.
The usual method to do this is to turn the block cipher into a stream cipher. In that way the ciphertext is generated by XOR'ing the plaintext with a generated key stream. This key stream in turn is generated by the mode of operation that turns the block cipher into a key stream. There are several of these modes, but CTR mode of operation is most often used ...
If you have a uniformly random string $s$ as long as your message $m$, then $s \oplus m$ is a one-time pad (using $\oplus$ for XOR). If a cipher is secure, then its ciphertext should look like uniform randomness to anyone who does not have the key (even if they know the message). The XOR combination is then effectively a one-time pad. At least, that's the ...
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