On Modes of Operations of a Block Cipher for Authentication and Authenticated Encryption

This work deals with the various requirements of encryption and authentication in cryptographic applications. The approach is to construct suitable modes of operations of a block cipher to achieve the relevant goals. A variety of schemes suitable for specific applications are presented. While none of the schemes are built completely from scratch, there is a common unifying framework which connects them. All the schemes described have been implemented and the implementation details are publicly available. Performance figures are presented when the block cipher is the AES and the Intel AES-NI instructions are used. These figures suggest that the constructions presented here compare well with previous works such as the famous OCB mode of operation. In terms of features, the constructions provide several new offerings which are not present in earlier works. This work significantly widens the range of choices of an actual designer of cryptographic system

On the Security of TrCBC

TrCBC is a variant of CBC-MAC which appeared in Information Processing Letters, 112(7):302-307, 2012. The authors claimed TrCBC to be a secure message authentication code (MAC) with some interesting properties. If TrCBC is instantiated with a block cipher with block length n, then it requires ⌈λ/n⌉ block cipher calls for authenticating a λ-bit message and requires a single key, which is the block cipher key. The authors state that TrCBC can have tag lengths of size less than n/2. We show that with high probability, an adversary can forge TrCBC with tag length n/2 − 1 with just three queries. The attack that we show can be applied to forge a large class of messages. The authors proved TrCBC to be a pseudorandom function (PRF). A scrutiny of the claimed PRF bound shows that for some recommended values of tag lengths, the bound turns out to be quite large. Thus, the security theorem does not imply security of TrCBC for all recommended tag lengths

Double Ciphertext Mode : A Proposal for Secure Backup

Security of data stored in bulk storage devices like the hard disk has gained a lot of importance in the current days. Among the variety of paradigms which are available for disk encryption, low level disk encryption is well accepted because of the high security guarantees it provides. In this paper we view the problem of disk encryption from a different direction. We explore the possibility of how one can maintain secure backups of the data, such that loss of a physical device will mean neither loss of the data nor the fact that the data gets revealed to the adversary. We propose an efficient solution to this problem through a new cryptographic scheme which we call as the double ciphertext mode (DCM). In this paper we describe the syntax of DCM, define security for it and give some efficient constructions. Moreover we argue regarding the suitability of DCM for the secure backup application and also explore other application areas where a DCM can be useful

On Securing Communication From Profilers

A profiling adversary is an adversary which aims to classify messages into pre-defined profiles and thus gain useful information regarding the sender or receiver of such messages. Usual chosen-plaintext secure encryption schemes are capable of securing information from profilers, but these schemes provide more security than required for this purpose. In this paper we study the requirements for an encryption algorithm to be secure only against profilers and finally give a precise notion of security for such schemes. We also present a full protocol for secure (against profiling adversaries) communication, which neither requires a key exchange nor a public key infrastructure. Our protocol guarantees security against non-human profilers and is constructed using CAPTCHAs and secret sharing schemes

Another Look at XCB

XCB is a tweakable enciphering scheme (TES) which was first proposed in 2004. The scheme was modified in 2007. We call these two versions of XCB as XCBv1 and XCBv2 respectively. XCBv2 was later proposed as a standard for encryption of sector oriented storage media in IEEE-std 1619.2 2010. There is no known proof of security for XCBv1 but the authors provided a concrete security bound for XCBv2 and a ``proof\u27\u27 for justifying the bound. In this paper we show that XCBv2 is not secure as a TES by showing an easy distinguishing attack on it. For XCBv2 to be secure, the message space should contain only messages whose lengths are multiples of the block length of the block cipher. For such restricted message spaces also, the bound that the authors claim is not justified. We show this by pointing out some errors in the proof. For XCBv2 on full block messages, we provide a new security analysis. The resulting bound that can be proved is much worse than what has been claimed by the authors. Further, we provide the first concrete security bound for XCBv1, which holds for all message lengths. In terms of known security bounds, both XCBv1 and XCBv2 are worse compared to existing alternative TES

A Fast Single-Key Two-Level Universal Hash Function

Universal hash functions based on univariate polynomials are well known, e.g. Poly1305 and GHASH. Using Horner’s rule to evaluate such hash functionsrequire l − 1 field multiplications for hashing a message consisting of l blocks where each block is one field element. A faster method is based on the class of Bernstein-Rabin-Winograd (BRW) polynomials which require ⌊l/2⌋ multiplications and ⌊lgl⌋ squarings for l≥3 blocks. Though this is significantly smaller than Horner’s rule based hashing, implementation of BRW polynomials for variable length messages present significant difficulties. In this work, we propose a two-level hash function where BRW polynomial based hashing is done at the lower level and Horner’s rule based hashing is done at the higher level. The BRW polynomial based hashing is applied to a fixed number of blocks and hence the difficulties in handling variable length messages is avoided. Even though the hash function has two levels, we show that it is sufficient to use a single field element as the hash key. The basic idea is instantiated to propose two new hash functions, one which hashes a single binary string and the other can hash a vector of binary strings. We describe two actual implementations, one over F2128 and the other over F2256 both using the pclmulqdq instruction available in modern Intel processors. On both the Haswell and Skylake processors, the implementation over F2128 is faster than both an implementation of GHASH by Gueron; and a highly optimised implementation, also by Gueron, of another polynomial based hash function called POLYVAL. We further show that the Fast Fourier Transform based field multiplication over F2256 proposed by Bernstein and Chou can be used to evaluate the new hash function at a cost of about at most 46 bit operations per bit of digest, but, unlike the Bernstein-Chou analysis, there is no hidden cost of generating the hash key. More generally, the new idea of building a two-level hash function having a single field element as the hash key can be applied to other finite fields to build new hash functions

Disk Encryption: Do We Need to Preserve Length?

In the last one-and-a-half decade there has been a lot of activity towards development of cryptographic techniques for disk encryption. It has been almost canonised that an encryption scheme suitable for the application of disk encryption must be length preserving, i.e., it rules out the use of schemes like authenticated encryption where an authentication tag is also produced as a part of the ciphertext resulting in ciphertexts being longer than the corresponding plaintexts. The notion of a tweakable enciphering scheme (TES) has been formalised as the appropriate primitive for disk encryption and it has been argued that they provide the maximum security possible for a tag-less scheme. On the other hand, TESs are less efficient than some existing authenticated encryption schemes. Also TES cannot provide true authentication as they do not have authentication tags. In this paper, we analyze the possibility of the use of encryption schemes where length expansion is produced for the purpose of disk encryption. On the negative side, we argue that nonce based authenticated encryption schemes are not appropriate for this application. On the positive side, we demonstrate that deterministic authenticated encryption (DAE) schemes may have more advantages than disadvantages compared to a TES when used for disk encryption. Finally, we propose a new deterministic authenticated encryption scheme called BCTR which is suitable for this purpose. We provide the full specification of BCTR, prove its security and also report an efficient implementation in reconfigurable hardware. Our experiments suggests that BCTR performs significantly better than existing TESs and existing DAE schemes