## CryptoDB

### Martijn Stam

#### Publications

Year
Venue
Title
2020
TCHES
2018
EUROCRYPT
2017
ASIACRYPT
2017
TCC
2017
JOFC
2017
TOSC
We examine how two parallel modes of operation for Authenticated Encryption (namely CTR+PMAC and OTR mode) work when evaluated in a multiparty computation engine. These two modes are selected because they suit the PRFs examined in previous works. In particular the modes are highly parallel, and do not require evaluation of the inverse of the underlying PRF. In order to use these modes one needs to convert them from their original instantiation of being defined on binary blocks of data, to working on elememts in a large prime finite field. The latter fitting the use case of many secret-sharing based MPC engines. In doing this conversion we examine the associated security proofs of PMAC and OTR, and show that they carry over to this new setting.
2017
TOSC
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: ciphertext blocks cannot depend on later plaintext blocks, limiting diffusion and hence security. We show how one can attain the best of both worlds by providing provably secure constructions, achieving full cipher security, based on applications of an online cipher around blockwise reordering layers. Explicitly, we show that with just two calls to the online cipher, prp security up to the birthday bound is both attainable and maximal. Moreover, we demonstrate that three calls to the online cipher suffice to obtain beyond birthday bound security. We provide a full proof of this for a prp construction, and, in the ±prp setting, security against adversaries who make queries of any single length. As part of our investigation, we extend an observation by Rogaway and Zhang by further highlighting the close relationship between online ciphers and tweakable blockciphers with variable-length tweaks.
2016
EUROCRYPT
2016
CRYPTO
2016
ASIACRYPT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
ASIACRYPT
2014
EPRINT
2014
EPRINT
2014
ASIACRYPT
2013
EUROCRYPT
2013
FSE
2012
TCC
2012
EUROCRYPT
2012
ASIACRYPT
2011
CRYPTO
2011
CHES
2011
ASIACRYPT
2011
ASIACRYPT
2010
EPRINT
We prove that Tandem-DM, one of the two classical'' schemes for turning a blockcipher of $2n$-bit key into a double block length hash function, has birthday-type collision resistance in the ideal cipher model. A collision resistance analysis for Tandem-DM achieving a similar birthday-type bound was already proposed by Fleischmann, Gorski and Lucks at FSE 2009. As we detail, however, the latter analysis is wrong, thus leaving the collision resistance of Tandem-DM as an open problem until now.
2010
PKC
2010
JOFC
2010
JOFC
2010
ASIACRYPT
2010
ASIACRYPT
2010
FSE
2009
EUROCRYPT
2009
FSE
2008
EPRINT
In this note we revisit the rate-1 blockcipher based hash functions as first studied by Preneel, Govaerts and Vandewalle (Crypto'93) and later extensively analysed by Black, Rogaway and Shrimpton (Crypto'02). We analyze a further generalization where any pre- and postprocessing is considered. By introducing a new tweak to earlier proof methods, we obtain a simpler proof that is both more general and more tight than existing results. As added benefit, this also leads to a clearer understanding of the current classification of rate-1 blockcipher based schemes as introduced by Preneel et al. and refined by Black et al.
2008
EPRINT
We consider a hybrid version of Damgård's ElGamal public-key encryption scheme that incorporates the use of a symmetric cipher and a hash function for key-derivation. We prove that under appropriate choice of the hash function this scheme is IND-CCA secure under the Decisional Diffie-Hellman assumption in the standard model. Our results can be generalized to universal hash proof systems where our main technical contribution can be viewed as an efficient generic transformation from 1-universal to 2-universal hash proof systems.
2008
EPRINT
We present a new approach to the design of IND-CCA2 secure hybrid encryption schemes in the standard model. Our approach provides an efficient generic transformation from 1-universal to 2-universal hash proof systems. The transformation involves a randomness extractor based on a 4-wise independent hash function as the key derivation function. Our methodology can be instantiated with efficient schemes based on standard intractability assumptions such as DDH, QR and Paillier. Interestingly, our framework also allows to prove IND-CCA2 security of a hybrid version of 1991s Damgaards ElGamal public-key encryption scheme under the DDH assumption.
2008
CRYPTO
2007
TCC
2007
EPRINT
We consider how to build an efficient compression function from a small number of random, non-compressing primitives (fixed-key blockciphers were our original motivation). Our main goal is to achieve a level of collision resistance as close as possible to the optimal birthday bound. We present a $2n$-to-$n$ bit compression function based on three independent $n$-to-$n$ bit random functions, each called only once. We show that if the three random functions are treated as black boxes (i.e., modelled as random oracles), finding collisions requires $\Theta(2^{n/2}/n^c)$ queries for $c\approx 1$. We also give a heuristic, backed by experimental results, suggesting that the security loss is at most four bits for block sizes up to 256 bits. We believe this is the best result to date on the matter of building a collision resistant compression function from non-compressing functions. It also relates to an open question from Black et al. (Eurocrypt'05), who showed that compression functions that invoke a single non-compressing random function cannot suffice.
2006
EPRINT
An obfuscation O of a function F should satisfy two requirements: firstly, using O it should be possible to evaluate F; secondly, O should not reveal anything about F that cannot be learnt from oracle access to F. Several definitions for obfuscation exist. However, most of them are either too weak for or incompatible with cryptographic applications, or have been shown impossible to achieve, or both. We give a new definition of obfuscation and argue for its reasonability and usefulness. In particular, we show that it is strong enough for cryptographic applications, yet we show that it has the potential for interesting positive results. We illustrate this with the following two results: - If the encryption algorithm of a secure secret-key encryption scheme can be obfuscated according to our definition, then the result is a secure public-key encryption scheme. - A uniformly random point function can be easily obfuscated according to our definition, by simply applying a one-way permutation. Previous obfuscators for point functions, under varying notions of security, are either probabilistic or in the random oracle model (but work for arbitrary distributions on the point function). On the negative side, we show that - Following Hada and Wee, any family of deterministic functions that can be obfuscated according to our definition must already be approximately learnable.'' Thus, many deterministic functions cannot be obfuscated. However, a probabilistic functionality such as a probabilistic secret-key encryption scheme can potentially be obfuscated. In particular, this is possible for a public-key encryption scheme when viewed as a secret-key scheme. - There exists a secure probabilistic secret-key encryption scheme that cannot be obfuscated according to our definition. Thus, we cannot hope for a general-purpose cryptographic obfuscator for encryption schemes.
2005
CRYPTO
2005
EUROCRYPT
2004
EPRINT
The output of the Tate pairing on an elliptic curve over a finite field may be viewed as an element of an algebraic torus. Using this simple observation, we transfer techniques recently developed for torus-based cryptography to pairing-based cryptography, resulting in more efficient computations, and lower bandwidth requirements. To illustrate the efficacy of this approach, we apply the method to pairings on supersingular elliptic curves in characteristic three.
2004
EPRINT
Although identity based cryptography offers a number of functional advantages over conventional public key methods, the computational costs are significantly greater. The dominant part of this cost is the Tate pairing which, in characteristic three, is best computed using the algorithm of Duursma and Lee. However, in hardware and constrained environments this algorithm is unattractive since it requires online computation of cube roots or enough storage space to pre-compute required results. We examine the use of normal basis arithmetic in characteristic three in an attempt to get the best of both worlds: an efficient method for computing the Tate pairing that requires no pre-computation and that may also be implemented in hardware to accelerate devices such as smart-cards. Since normal basis arithmetic in characteristic three has not received much attention before, we also discuss the construction of suitable bases and associated curve parameterisations.
2004
EPRINT
At Crypto 2004, van Dijk and Woodruff introduced a new way of using the algebraic tori T_n in cryptography, and obtained an asymptotically optimal n/phi(n) savings in bandwidth and storage for a number of cryptographic applications. However, the computational requirements of compression and decompression in their scheme were impractical, and it was left open to reduce them to a practical level. We give a new method that compresses orders of magnitude faster than the original, while also speeding up the decompression and improving on the compression factor (by a constant term). Further, we give the first efficient implementation that uses T_30, compare its performance to XTR, CEILIDH, and ECC, and present new applications. Our methods achieve better compression than XTR and CEILIDH for the compression of as few as two group elements. This allows us to apply our results to ElGamal encryption with a small message domain to obtain ciphertexts that are 10% smaller than in previous schemes.
2003
PKC
2002
CHES
2001
ASIACRYPT

CHES 2019
Eurocrypt 2019
Crypto 2018
FSE 2018
FSE 2017
Eurocrypt 2017
Eurocrypt 2015
FSE 2015
FSE 2014
Crypto 2013
Eurocrypt 2012
Eurocrypt 2011
FSE 2011
PKC 2010
PKC 2008