The RKES (Remotely Keyed Encryption Schemes) are greatly useful in solving the vital problem of how to do bulk encryption/ decryption for high-bandwidth applications (like multimedia and video encryption) in a way that takes advantage of both the superior power of the host and the superior security of the smart card. According to this issue, we propose a novel length increasing (LI) RKES, in which, the output ciphertext length is larger than input plaintext length. In this scheme, an extra ciphertext block is used as a self validation or signature of the whole ciphertext, so an adversary can’t forge the scheme. The proposed LIRKES needs a strong pseudorandom permutation (PRP) as its basic building block, so we introduce a new symmetric-key block cipher, with variable block and key lengths, referred to as PATFC (Pseudorandom Affine Transformation based Feistel Cipher), appropriate for software and hardware implementations. PATFC adopts the 3-round Luby-Rackoff construction (a compact form of the Feistel network structures) for fusing pseudorandom functions of the plaintext partitions to obtain a pseudorandom permutation. PATFC mainly makes use of a novel keyed pseudorandom function (PRF) that is based on a pseudorandom affine transformation (constructed using a highly nonlinear pseudorandom sequence generator) followed by a data and key dependent encoding and a simple hashing scheme.Extensive statistical tests of PATFC and its underlying round function consistently demonstrated their competitive diffusion, confusion and pseudorandomness characteristics. Furthermore, PATFC is provably secure and not vulnerable to known/chosen/adaptive plaintext/ ciphertexts attacks.At the end of this paper, we show how we can apply PATFC as a strong PRP in the suggested LIRKES to be used for smart cards.
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