Superencryption refers to a situation where an encrypted message is then encrypted again using either the same encryption system or a different system. This is also called cascading encryption.

Superciphers built this way are also called cascade ciphers.

Security of Superencrypted Messages

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Suppose that Alice wants to send Bob an encrypted message, and Turing wants to listen in on the secret. If Alice uses the German Enigma Machine to encrypt her message, Turing can run the message through his Bombes and decode rather easily because it is easy to figure out if each Enigma key gives intelligible results or not. If Alice uses a Vigenère Cipher for the first encryption and then uses the Enigma Machine on that, Turing's Bombes will never find intelligible results from simple Enigma decrypts, and Turing will have to build a new machine, thus ensuring that Alice and Bob's message is safe for quite a while.

Independent keys

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If you choose any couple of ciphers, and if you use the same key for both ciphers, the second cipher could undo (a part of) the work of the first cipher. This is obviously the case in ciphers where the decryption process is exactly the same as the encryption process, where the second cipher would completely undo the first.

To prevent that risk, you can use keys that are statistically independents for each layer (e.g. independent RNGs).

References

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• A "way to combine multiple block algorithms" so that "a cryptanalyst must break both algorithms" in §15.8 of Applied Cryptography, Second Edition: Protocols, Algorithms, and Source Code in C by Bruce Schneier. Wiley Computer Publishing, John Wiley & Sons, Inc.
• S. Even and O. Goldreich, On the power of cascade ciphers, ACM Transactions on Computer Systems, vol. 3, pp. 108–116, 1985.
• M. Maurer and J. L. Massey, Cascade ciphers: The importance of being first, Journal of Cryptology, vol. 6, no. 1, pp. 55–61, 1993.

Cryptography