Block Cipher Modes of Operation from a Hardware Implementation Perspective

Abstract

Block ciphers are one of the most important primitives in cryptology. They are based on well-understood mathematical and cryptographic principles. Due to their inherent efficiency, these ciphers are used in many kinds of applications which require bulk encryption at high speed.Generally speaking, a block cipher consists of at least two closely related algorithms: block encryption and block decryption. Block encryption takes as an input a fixed-length block (known as the plaintext) and transform it into another block of the same length (known as the ciphertext) under the action of a fixed secret key that may or may not have the same length of the plaintext. A block cipher must be invertible in the sense that by using the block decryption algorithm it should be always possible to recover the original plaintext from the ciphertext and the secret key. Figure 1.1 shows schematically the situation just described. We stress that once that the plaintext has been encrypted using a given key then, a successful decryption can only be performed by knowing that key. Due to this feature, block ciphers are classified as a secret or symmetric key primitives.Formally a block cipher is considered to be secure if it behaves like a strong pseudorandom permutation, i.e., a block cipher is secure if an adversary cannot distinguish its output from a randomly chosen permutation. This definition of security for block ciphers is very strong, it implies that for any possible input, a secure block cipher should produce random outputs. It is unfortunate that there exists no formal security model for assessing whether a block cipher is or not secure or rather, how secure a cipher is. Hence, we rely on a block cipher by the fact that no one has been able to find an attack on it.Assuming that an adversary has managed to obtain a plaintext and the corresponding cipher text, then she can always try to obtain the n-bit secret key of a given symmetric block cipher by trying all possible keys, a procedure traditionally termed brute-force attack. We say that a block cipher has a security strength of n bits if the 2 Debrup Chakraborty and Francisco Rodríguez-Henríquez best known attack against it is not computationally cheaper than the brute force attack. Modern block ciphers typically use a block and key length ranging from 64 bits up to 256 bits. At the present state of the technology, block/key sizes of 128 bits are generally considered adequate in terms of both, security and efficiency.Block ciphers have been around for civilian/commercial use since 1971, when a team leaded by H. Feistel and his colleagues at IBM designed a family of ciphers known as Lucifer [2,53]. Early versions of Lucifer operated on 24-bit long plaintext blocks. The strongest variant which was released in 1973, operated on 128-bit blocks and 128-bit secret keys. A revised version of that Lucifer variant, known as the Data Encryption Standard (DES), was adopted as a US FIPS standard in 1974 [16, 42]. Across the years, DES became the most influential block cipher ever inspiring...

Reconfigurable Hardware Implementations of Tweakable Enciphering Schemes

“…The obtained place-and-route simulation results are summarized in Table 9. 6 Let us recall that Tables 6, 7, and 8 present the Virtex 4 implementation results achieved by the TES modes when using a full pipelined, sequential, and encryption-only AES core designs, respectively. The first three portions in Table 9 show the performances obtained by those three designs when implemented on the Virtex 5 device xc5vlx110-3ff1153.…”

“…There exist numerous kinds of block cipher modes of operations designed for different functionalities, such as privacy-only modes, authenticated encryption, authenticated encryption with associated data, etc. [6]. None of these modes satisfy the requirements of low-level disk encryption.…”

Tweakable parallel OFB mode of operation with delayed thread synchronization

“…Apart from transforming block algorithm into a streaming algorithm, CFB, OFB, and CTR modes of operation have another common feature; both encryption and decryption operations do not require any inverse block cipher calls . That characteristic can also be exploited in the parallelisation of some cryptographic algorithm through its minor modifications.…”

“…As mentioned previously, any block algorithm operates as a key stream when used with CFB, OFB, and CTR modes of operation. Block algorithms with CFB, OFB, and CTR modes of operation do not involve inverse block cipher function as this function is provided through the mode construction itself .…”

“…In this paper, we examine the performance of the proposed parallel mode of operation, which utilizes tweakable parallel encryption using XEX and XE constructions in output feedback (OFB) like mode. New design is based on the fact that in OFB mode of operation block cipher decryption operation is not needed and that OFB mode is very similar to a stream cipher . In realization of its design, we use construction similar to XTS‐AES tweakable block cipher and combine it with OFB mode of operation.…”