Multiple encryption means encrypting a message two or more times using either the same, or a different algorithm.
Multiple encryption, cascade ciphering or superencryption refers to the process of encrypting the same data several times.
A well known result by Maurer and Massey (1993) shows that, under very general assumptions, a multiple encryption scheme is at least as secure as the innermost layer, provided that the encryption keys for each layer are independent. Furthermore, if the encryption layers commute (as is the case e.g. for additive stream ciphers), this shows that the combined encryption scheme is at least as secure as the strongest layer.
- M. Maurer and J. L. Massey, "Cascade ciphers: The importance of being first", Journal of Cryptology, vol. 6, no. 1, pp. 55–61, 1993.