For example, A encrypts a message.

B encrypts that.

C encrypts that.

Then B decrypts, followed by A then C.

Are there systems that allow this? Does such a property have a name?

  • $\begingroup$ A very insecure example is the Caesar Shift cipher. $\endgroup$ – DynamoBlaze Sep 30 '17 at 9:20

Yes, systems that allow this have a name: commutative encryption.

In practice, there are two varieties:

  • If A, B, C just xor in a keystream, it all commutes. Of course, anyone seeing the intermediate results can deduce quite a lot; this may make this unacceptable for some uses.

  • Pohlig-Hellman (not related to the Pohlig-Hellman algorithm); we pick a global prime $p$, and everyone selects a secret value relatively prime to $p-1$; A picks $a$, B picks $b$, and C picks $c$. Then;

    • Alice encrypts $M$ by computing $M^a \bmod p$

    • Alice decrypts $C$ by computing $M^{a^{-1} \bmod p-1} \bmod p$

If you go through the math, you'll see that:

  • Encryption and decryption are inverses of each other, e.g. $\operatorname{Dec}_a(\operatorname{Enc}_a(M)) = M$

  • It commutes just like you asked: $\operatorname{Enc}_b(\operatorname{Enc}_a(M)) = \operatorname{Enc}_a(\operatorname{Enc}_b(M))$, etc

  • No one can deduce anything by observing intermediate results (assuming that the prime is large enough to make the DLog and Diffie-Hellman problems hard)

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