Ritam Bhaumik

École Polytechnique Fédérale de Lausanne | EPFL · LASEC

The sum of two n-bit pseudorandom permutations is known to behave like a pseudorandom function with n bits of security. A recent line of research has investigated the security of two public n-bit permutations and its degree of indifferentiability. Mandal et al. (INDOCRYPT 2010) proved 2n/3-bit security, Mennink and Preneel (ACNS 2015) pointed out a...

A nonce-respecting tweakable blockcipher is the building-block for the OCB authenticated encryption mode. An XEX-based TBC is used to process each block in OCB. However, XEX can provide at most birthday bound privacy security, whereas in Asiacrypt 2017, beyond-birthday-bound (BBB) forging security of OCB3 was shown in [14]. In this paper we study h...

It was long thought that symmetric cryptography was only mildly affected by quantum attacks, and that doubling the key length was sufficient to restore security. However, recent works have shown that Simon’s quantum period finding algorithm breaks a large number of MAC and authenticated encryption algorithms when the adversary can query the MAC/enc...

Coron et al. in TCC 2010 proposed a 2n-bit queryPlease check and confirm the corresponding author is identified correctly.permutation by using an n-bit ideal cipher with an n-bit key in a Feistel type structure for three rounds (in: Micciancio (ed) Theory of cryptography, Springer, Berlin, Heidelberg, 2010). The authors showed n/2-bit queryPlease c...

OCB3 is the current version of the OCB authenticated encryption mode which is selected for the third round in CAESAR. So far the integrity analysis has been limited to an adversary making a single forging attempt. A simple extension for the best known bound establishes integrity security as long as the total number of query blocks (including encryp...

At CRYPTO 2015, Minaud and Seurin introduced and studied the iterated random permutation problem, which is to distinguish the r-th iterate of a random permutation from a random permutation. In this paper, we study the closely related iterated random function problem, and prove the first almost-tight bound in the adaptive setting. More specifically,...

CAESAR has caused a heated discussion regarding the merits of one-pass encryption and online ciphers. The latter is a keyed, length preserving function which outputs ciphertext blocks as soon as the respective plaintext block is available as input. The immediacy of an online cipher affords a clear performance advantage, but it comes at a price: cip...

Online ciphers, in spite of being insecure against an sprp adversary, can be desirable at places because of their ease of implementation and speed. Here we propose a single-keyed inverse-free construction that achieves online sprp security with an optimal number of blockcipher calls. We also include a partial block construction, without requiring a...

In CRYPTO 2003, Halevi and Rogaway proposed CMC, a tweakable enciphering scheme (TES) based on a blockcipher. It requires two blockcipher keys and it is not inverse-free (i.e., the decryption algorithm uses the inverse (decryption) of the underlying blockcipher). We present here a new inverse-free, single-keyed TES. Our construction is a tweakable...