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Is it possible that if a plaintext is encrypted first with a key, $n_1$, and then this ciphertext is again encrypted using the same algorithm but a different key, $n_2$ that the resulting ciphertext could be converted back to the original plaintext with a single key?

For example: keys $n_1$, $n_2$, and $n_3$

$p_1$ = "plaintext"

$p_2 = C(n_1, p_1)$

$p_3 = C(n_2, p_2)$

Is this possible: $Dec(n_3, p_3) = p_1$? (where $C$ is an encryption algorithm taking the key and plaintext)

An example I can think of is the Caesar Cipher (the key can be seen as the shift). Where first a shift of 4 is used, and then a shift of 6. A shift of 16 would reveal the plaintext in a single operation (a single key).

Obviously this is a trivial example. Are there any cases where this could happen with a stronger algorithm, such as AES?

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    $\begingroup$ Hi, Joseph. The title of your question asks about decrypting with a third key, but then you wrote $C(n_3, p_3) = p_1$. Do you want the encryption algorithm to work as a decryption when used with the third key $n_3$ or that equation should be something like $Dec(c_3, p_3) = p_1$? $\endgroup$ – Hilder Vítor Lima Pereira Jul 31 at 12:59
  • $\begingroup$ Hilder Vítor Lima Pereira You're right, I'll change that. Thanks. $\endgroup$ – Joseph Jul 31 at 13:03
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    $\begingroup$ The expression describing this is commutative encryption, see this question. It does not work for today's symmetric ciphers. $\endgroup$ – tylo Jul 31 at 15:38
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    $\begingroup$ @tylo a cipher is commutative if $Enc_{k_1}(Enc_{k_2}(m)) = Enc_{k_2}(Enc_{k_1}(m))$? If yes, then this is not what the question is asking... The searched property here is $Dec_{k_3}(Enc_{k_2}(Enc_{k_1}(m))) = m$, which implies only $Enc_{k_2}(Enc_{k_1}(m)) = Enc_{k_3}(m)$ $\endgroup$ – Hilder Vítor Lima Pereira Jul 31 at 17:40
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    $\begingroup$ @tylo, no, this is asking if encryption under some cipher forms a group, and commutativity is neither necessary nor sufficient for something to be a group. $\endgroup$ – Mark Jul 31 at 22:48
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It is possible with the old Pohlig-Hellman cipher.

Here's how it works:

  • The global parameter is a prime $p$

  • A secret key is a value $k$ which is relatively prime to $p-1$

  • To encrypt a message $M$, you compute $C = M^k \bmod p$, and that's you

  • To decrypt a ciphertext $C$, you compute $k^{-1} \bmod p-1$, and then compute $M = C^{k^{-1}} \bmod p-1$

With this system, if you encrypt with $k_1$, and then with $k_2$, the resulting ciphertext is $C = (M^{k_1} \bmod p)^{k_2} \bmod p = M^{k_1k_2} \bmod p$; hence, this is identical to encrypting with the key $k_1k_2 \bmod p-1$, and so can be decrypted by that key.

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  1. You cannot understand encryption as a Caesar Cipher. Caesar cipher is just a simple kind of linear transformation. Two linear operations (e.g., + - * / shift bits, etc) can be combined into one single operation easily.

  2. AES definitely cannot. AES consists of 10 rounds of calculations. Each round consists of both linear & non-linear steps. There is no relations between different keys, in your case they are n1, n2, n3.

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