International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Praveen Gauravaram

Affiliation: Tata Consultancy Services Limited

Publications

Year
Venue
Title
2015
EPRINT
2015
EPRINT
2012
JOFC
Security Analysis of Randomize-Hash-then-Sign Digital Signatures
Praveen Gauravaram Lars R. Knudsen
At CRYPTO 2006, Halevi and Krawczyk proposed two randomized hash function modes and analyzed the security of digital signature algorithms based on these constructions. They showed that the security of signature schemes based on the two randomized hash function modes relies on properties similar to the second preimage resistance rather than on the collision resistance property of the hash functions. One of the randomized hash function modes was named the RMX hash function mode and was recommended for practical purposes. The National Institute of Standards and Technology (NIST), USA standardized a variant of the RMX hash function mode and published this standard in the Special Publication (SP) 800-106.In this article, we first discuss a generic online birthday existential forgery attack of Dang and Perlner on the RMX-hash-then-sign schemes. We show that a variant of this attack can be applied to forge the other randomize-hash-then-sign schemes. We point out practical limitations of the generic forgery attack on the RMX-hash-then-sign schemes. We then show that these limitations can be overcome for the RMX-hash-then-sign schemes if it is easy to find fixed points for the underlying compression functions, such as for the Davies-Meyer construction used in the popular hash functions such as MD5 designed by Rivest and the SHA family of hash functions designed by the National Security Agency (NSA), USA and published by NIST in the Federal Information Processing Standards (FIPS). We show an online birthday forgery attack on this class of signatures by using a variant of Dean’s method of finding fixed point expandable messages for hash functions based on the Davies-Meyer construction. This forgery attack is also applicable to signature schemes based on the variant of RMX standardized by NIST in SP 800-106. We discuss some important applications of our attacks and discuss their applicability on signature schemes based on hash functions with ‘built-in’ randomization. Finally, we compare our attacks on randomize-hash-then-sign schemes with the generic forgery attacks on the standard hash-based message authentication code (HMAC).
2009
EUROCRYPT
2009
FSE
2007
EPRINT
Cryptanalysis of a class of cryptographic hash functions
Praveen Gauravaram John Kelsey
We apply new cryptanalytical techniques to perform the generic multi-block multicollision, second preimage and herding attacks on the Damg{\aa}rd-Merkle hash functions with linear-XOR/additive checksums. The computational work required to perform these attacks on the Damg{\aa}rd-Merkle hash functions with linear-XOR/additive checksum of message blocks (GOST), intermediate states (\textbf{3C}, MAELSTROM-0, F-Hash) or both is only a little more than what is required on the Damg{\aa}rd-Merkle hash functions. Our generic attacks on GOST answers the open question of Hoch and Shamir at FSE 2006 on the security of the iterated hash functions with the linear mixing of message blocks.
2006
EPRINT
The classic Merkle-Damg{\aa}rd (\textbf{MD}) structure provides a popular way of turning a fixed-length compression function into a variable-length input cryptographic hash function. However, the multi-block collision attacks (MBCA) on the \textbf{MD}-style hash functions MD5, SHA-0 and SHA-1 demonstrate the weakness of the \textbf{MD} construction in extending the collision resistance property of a single compression function to its iterations. In this paper, we investigate a recently proposed cryptographic construction (called \textbf{3C}) devised by enhancing the \textbf{MD} construction, and prove it provides quantitatively more resistance against MBCA than does the \textbf{MD}-style. Specifically, we prove that it requires at least $2^{t/2}$ computational effort to perform any MBCA on the $t$-bit \textbf{3C} hash function when the same attack on a $t$-bit \textbf{MD} hash function (using the same compression function) requires an effort not less than $2^{t/4}$. This is the first result showing a generic construction with resistance to MBCA. We further improve the resistance of the \textbf{3C} design against MBCA and propose the new \textbf{3C+} hash function construction. We prove that \textbf{3C+} is completely \emph{immune} to MBCA since it costs at least $2^{t/2}$ effort to perform any MBCA on the \textbf{3C+} construction. This reduces the collision security of \textbf{3C+} to the collision security of the underlying compression function, hence restoring the paradigm that one only needs to design a secure compression function to obtain a secure iterated hash function. Both the \textbf{3C} and \textbf{3C+} constructions are very simple adjustments to the \textbf{MD} construction and they are immune to the straight forward extension attacks which apply to the \textbf{MD} hash functions. The second preimage attacks on $t$-bit hash functions also do not work on the constructions presented in this paper.
2005
EPRINT
3C- A Provably Secure Pseudorandom Function and Message Authentication Code.A New mode of operation for Cryptographic Hash Function
We propose a new cryptographic construction called 3C, which works as a pseudorandom function (PRF), message authentication code (MAC) and cryptographic hash function. The 3C-construction is obtained by modifying the Merkle-Damgard iterated construction used to construct iterated hash functions. We assume that the compression functions of Merkle-Damgard iterated construction realize a family of fixed-length-input pseudorandom functions (FI-PRFs). A concrete security analysis for the family of 3C- variable-length-input pseudorandom functions (VI-PRFs) is provided in a precise and quantitative manner. The 3C- VI-PRF is then used to realize the 3C- MAC construction called one-key NMAC (O-NMAC). O-NMAC is a more efficient variant of NMAC and HMAC in the applications where key changes frequently and the key cannot be cached. The 3C-construction works as a new mode of hash function operation for the hash functions based on Merkle-Damgard construction such as MD5 and SHA-1. The generic 3C- hash function is more resistant against the recent differential multi-block collision attacks than the Merkle-Damgard hash functions and the extension attacks do not work on the 3C- hash function. The 3C-X hash function is the simplest and efficient variant of the generic 3C hash function and it is the simplest modification to the Merkle-Damgard hash function that one can achieve. We provide the security analysis for the functions 3C and 3C-X against multi-block collision attacks and generic attacks on hash functions. We combine the wide-pipe hash function with the 3C hash function for even better security against some generic attacks and differential attacks. The 3C-construction has all these features at the expense of one extra iteration of the compression function over the Merkle-Damgard construction.
2005
EPRINT
Some thoughts on Collision Attacks in the Hash Functions MD5, SHA-0 and SHA-1
The design principle of Merkle-Damg{\aa}rd construction is collision resistance of the compression function implies collision resistance of the hash function. Recently multi-block collisions have been found on the hash functions MD5, SHA-0 and SHA-1 using differential cryptanalysis. These multi-block collisions raise several questions on some definitions and properties used in the hash function literature. In this report, we take a closer look at some of the literature in cryptographic hash functions and give our insights on them. We bring out some important differences between the 1989's Damg{\aa}rd's hash function and the hash functions that followed it. We conclude that these hash functions did not consider the pseudo-collision attack in their design criteria. We also doubt whether these hash functions achieve the design principle of Merkle-Damg{\aa}rd's construction. We formalise some definitions on the properties of hash functions in the literature.