[Resource Topic] 2004/128: Elastic Block Ciphers

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Elastic Block Ciphers

Authors: Debra L. Cook, Moti Yung, Angelos D. Keromytis


We introduce the new concept of elastic block ciphers, symmetric-key encryption algorithms that (1) for a variable-size input do not expand the plaintext (i.e., do not require plaintext padding) and (2) adjust their computational load proportionally to the size increase. Contrary to stream ciphers, elastic block ciphers maintain the diffusion property and non-synchronicity of traditional block ciphers. Elastic block ciphers are ideal (when combined with encryption modes) for applications where length-preserving encryption is most beneficial, such as protecting variable-length database fields or network packets.

We present a general algorithm for converting a traditional block
cipher, such as AES, to its elastic version, and analyze the security
of the resulting cipher against key recovery attacks. Our approach
allows us to stretch'' the supported block size of a block cipher up to twice the original length, while increasing the computational load proportionally to the expanded block size. Our approach does not allow us to use the original cipher as a black box’’ (i.e., as an
ideal cipher or a pseudorandom permutation as is used in constructing
modes of encryption). Nevertheless, under some reasonable conditions
on the cipher’s structure and its key schedule, we reduce certain key
recovery attacks of the elastic version to such attacks on the fixed-size block cipher. This schema and the security reduction enable us to capitalize on secure ciphers and their already established security properties in developing elastic designs. We note that we are not aware of previous reduction type'' proofs of security in the area of concrete (i.e., non black-box’') block cipher design. Our work puts forth the notion of elasticity in block cipher design.

ePrint: https://eprint.iacr.org/2004/128

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