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22:17 [Pub][ePrint] Meet in the Middle Attacks on Reduced Round Kuznyechik, by Riham AlTawy and Amr M. Youssef

  Kuznyechik is an SPN block cipher that has been recently chosen to be standardized by the Russian federation as a new GOST cipher. The algorithm updates a 128-bit state for nine rounds using a 256-bit key. In this paper, we present meet-in-the-middle attacks on the 4 and 5 round reduced cipher. Our attacks are based on the differential enumeration approach, where we propose a distinguisher for the middle rounds and match a sequence of state differences at its output. However, the application of the exact approach is not successful on Kuznyechik due to its optimal round diffusion. Accordingly, we adopt an equivalent representation for the last round where we can efficiently filter ciphertext pairs and launch the attack in the chosen ciphertext setting. We also utilize partial sequence matching which further reduces the memory and time complexities through relaxing the error probability. The adopted partial sequence matching approach enables successful key recovery by matching parts of the generated sequence instead of the full sequence matching used in the traditional settings of this attack. For the 4 and 5 round reduced cipher, the 256-bit master key is recovered with a time complexity of $2^{139.6}$ and $2^{140.3}$, and a memory complexity of $2^{24.6}$ and $2^{153.3}$, respectively. Both attacks have similar data complexity of $2^{113}$

22:17 [Pub][ePrint] Surreptitiously Weakening Cryptographic Systems, by Bruce Schneier and Matthew Fredrikson and Tadayoshi Kohno and Thomas Ristenpart

  Revelations over the past couple of years highlight the importance of understanding malicious and surreptitious weakening of cryptographic systems. We provide an overview of this domain, using a number of historical examples to drive development of a weaknesses taxonomy. This allows comparing different approaches to sabotage. We categorize a broader set of potential avenues for weakening systems using this taxonomy, and discuss what future research is needed to provide sabotage-resilient cryptography.

22:17 [Pub][ePrint] Adaptive-ID Secure Revocable Identity-Based Encryption from Lattices via Subset Difference Method, by Shantian Cheng and Juanyang Zhang

  In view of the expiration or reveal of user\'s private credential (or private key) in a realistic scenario, identity-based encryption (IBE) schemes with an efficient key revocation mechanism, or for short, revocable identity-based encryption (RIBE) schemes, become prominently significant. In this paper, we present an RIBE scheme from lattices by combining two Agrawal et al.\'s IBE schemes with the subset difference (SD) method. Our scheme is secure against adaptive identity-time attacks in the standard model under the learning with errors (LWE) assumption. In particular, our scheme serves as one solution to the challenge posed by Chen et al.(ACISP \'12).

22:17 [Pub][ePrint] Universally Composable Firewall Architectures using Trusted Hardware, by Dirk Achenbach and Jörn Müller-Quade and Jochen Rill

  Network firewalls are a standard security measure in computer networks that connect to the Internet. Often, ready-to-use firewall appliances are trusted to protect the network from malicious Internet traffic. However, because of their black-box nature, no one can be sure of their exact functionality. We address the possibility of actively compromised firewalls. That is, we consider the possibility that a network firewall might collaborate with an outside adversary to attack the network. To alleviate this threat, we suggest composing multiple firewalls from different suppliers to obtain a secure firewall architecture. We rigorously treat the composition of potentially malicious network firewalls in a formal model based on the Universal Composability framework. Our security assumption is trusted hardware. We show that a serial concatenation of firewalls is insecure even when trusted hardware ensures that no new packages are generated by the compromised firewall. Further, we show that the parallel composition of two firewalls is only secure when the order of packets is not considered. We prove that the parallel composition of three firewalls is insecure, unless a modified trusted hardware is used.

22:17 [Pub][ePrint] Influence of Electrical Circuits of ECC Designs on Shape of Electromagnetic Traces measured on FPGA, by Christian Wittke and Zoya Dyka and Peter Langendoerfer

  Side channel attacks take advantage from the fact that the behavior of crypto implementations can be observed and provides hints that simplify revealing keys. The energy consumption of the chip that performs a cryptographic operation depends on its inputs, on the used cryptographic key and on the circuit that realizes the cryptographic algorithm. An attacker can experiment with different inputs and key candidates: he studies the influence of these parameters on the shape of measured traces with the goal to extract the key. The main assumption is here that the circuit of the attacked devices is constant. In this paper we investigated the influence of variable circuits on the shape of electromagnetic traces. We changed only a part of the cryptographic designs i.e. the partial multiplier of our ECC designs. This part calculates always the same function in a single clock cycle. The rest of the design was kept unchanged. So, we obtained designs with significantly different circuits: in our experiments the number of used FPGAs LUTs differs up to 15%. These differences in the circuits caused a big difference in the shape of electromagnetic traces even when the same data and the same key are processed. Our experiments show that the influence of different circuits on the shape of traces is comparable with the influence of different inputs. We assume that this fact can be used as a protection means against side channel attacks, especially if the cryptographic circuit can be changed before the cryptographic operation is executed or dynamically, i.e. while the cryptographic operation is processed.

22:17 [Pub][ePrint] Multi-Key Security: The Even-Mansour Construction Revisited, by Nicky Mouha and Atul Luykx

  At ASIACRYPT 1991, Even and Mansour introduced a block cipher construction based on a single permutation. Their construction has since been lauded for its simplicity, yet also criticized for not providing the same security as other block ciphers against generic attacks. In this paper, we prove that if a small number of plaintexts are encrypted under multiple independent keys, the Even-Mansour construction surprisingly offers similar security as an ideal block cipher with the same block and key size. Note that this multi-key setting is of high practical relevance, as real-world implementations often allow frequent rekeying. We hope that the results in this paper will further encourage the use of the Even-Mansour construction, especially when the secure and efficient implementation of a key schedule would result in a significant overhead.

22:17 [Pub][ePrint] GCM-SIV: Full Nonce Misuse-Resistant Authenticated Encryption at Under One Cycle per Byte, by Shay Gueron and Yehuda Lindell

  Authenticated encryption schemes guarantee both privacy and integrity, and have become the default level of encryption in modern protocols. One of the most popular authenticated encryption schemes today is AES-GCM due to its impressive speed. The current CAESAR competition is considering new modes for authenticated encryption that will improve on existing methods. One property of importance that is being considered more today is due to the fact that the nonce or IV repeats, then this can have disastrous effects on security. A (full) nonce misuse-resistant authenticated encryption scheme has the property that if the \\emph{same} nonce is used to encrypt the \\emph{same} message twice, then the same ciphertext is obtained and so the fact that the same message was encrypted is detected. Otherwise, full security is obtained -- even if the same nonce is used for different messages.

In this paper, we present a new fully nonce misuse-resistant authenticated encryption scheme that is based on carefully combining the GCM building blocks into the SIV paradigm of Rogaway and Shrimpton. We provide a full proof of security of our scheme, and an optimized implementation using the AES-NI and PCLMULQDQ instruction sets. We compare our performance to the highly optimized OpenSSL 1.0.2 implementation of GCM and show that our \\emph{nonce misuse-resistant} scheme is only 14\\% slower on Haswell architecture and 19\\% slower on Broadwell architecture. On Broadwell, GCM-SIV encryption takes only {\\em 0.92 cycles per byte}, and GCM-SIV decryption is exactly the same as GCM decryption taking only 0.77 cycles per byte. Beyond being very fast, our new mode of operation uses the same building blocks as GCM and so existing hardware and software can be utilized to easily deploy GCM-SIV. We conclude that GCM-SIV is a viable alternative to GCM, providing full nonce misuse-resistance at little cost.

22:17 [Pub][ePrint] Mergeable Functional Encryption, by Vincenzo Iovino and Karol Zebrowski

  In recent years, there has been great interest in Functional Encryption (FE), a generalization of traditional encryption where a token enables a user to learn a specific function of the encrypted data and nothing else. In this paper we put forward a new generalization of FE that we call Mergeable FE (mFE). In a mFE system, given a ciphertext $c_1$ encrypting $m_1$ and a ciphertext $c_2$ encrypting $m_2$, it is possible to produce in an oblivious way (i.e., given only the public-key and without knowledge of the messages, master secret-key or any other auxiliary information) a ciphertext encrypting the string $m_1||m_2$ under the security constraint that this new ciphertext does not leak more information about the original messages than what may be leaked from the new ciphertext using the tokens. For instance, suppose that the adversary is given the token for the function $f(\\cdot)$ defined so that for strings $x\\in\\zu^n$, $f(x)=g(x)$ for some function $g:\\zu^n\\rightarrow\\zu$ and for strings $y=(x_1||x_2)\\in\\zu^{2n}$, $f(x_1||x_2)=g(x_1) \\vee g(x_2)$. Furthermore, suppose that the adversary gets a ciphertext $c$ encrypting $(x_1||x_2)$ that is the result of ``merging`` some ciphertexts $c_1$ and $c_2$ encrypting respectively $x_1$ and $x_2$, and suppose that the token for $f$ evaluates to $1$ on $c$. Then, the security of mFE guarantees that the adversary only learns the output $f(x_1,x_2) = g(x_1) OR g(x_2)=1$ and nothing else (e.g., the adversary should not learn whether $g(x_1)=1 or g(x_2)=1$). This primitive is in some sense FE with the ``best possible`` homomorphic properties and, besides being interesting in itself, it offers wide applications. For instance, it has as special case multi-inputs FE and thus indistinguishability obfuscation (iO) and extends the latter to support more efficiently homomorphic and re-randomizable properties. We construct mFE schemes supporting a single merging operation, one from indistinguishability obfuscation for Turing machines and one for messages of unbounded length from public-coin differing-inputs obfuscation. Finally, we discuss a construction supporting unbounded merging operations from new assumptions.

22:17 [Pub][ePrint] Weakening the Isolation Assumption of Tamper-proof Hardware Tokens, by Rafael Dowsley and Jörn Müller-Quade and Tobias Nilges

  Recent results have shown the usefulness of tamper-proof hardware tokens as a setup assumption for building UC-secure two-party computation protocols, thus providing broad security guarantees and allowing the use of such protocols as buildings blocks in the modular design of complex cryptography protocols. All these works have in common that they assume the tokens to be completely isolated from their creator, but this is a strong assumption. In this work we investigate the feasibility of cryptographic protocols in the setting where the isolation of the hardware token is weakened.

We consider two cases: (1) the token can relay messages to its creator, or (2) the creator can send messages to the token after it is sent to the receiver. We provide a detailed characterization for both settings, presenting both impossibilities and information-theoretically secure solutions.

03:08 [PhD][New] Jerzy Jaworski

  Name: Jerzy Jaworski
Category: (no category)

03:08 [PhD][New] Przemyslaw Sokolowski: Contributions to cryptanalysis: design and analysis of cryptographic hash functions

  Name: Przemyslaw Sokolowski
Topic: Contributions to cryptanalysis: design and analysis of cryptographic hash functions
Category: secret-key cryptography


A cryptographic hash function is a mechanism producing a fixed-length output of a message of arbitrary length. It fulfills a collection of security requirements guaranteeing that a hash function does not introduce any weakness into the system to which it is applied. The example applications of cryptographic hash functions include digital signatures and message authentication codes. This thesis analyzes cryptographic hash functions and studies the design principles in the construction of secure cryptographic hash functions.

We investigate the problem of building hash functions from block ciphers and the security properties of different structures used to design compression functions. We show that we can build open-key differential distinguishers for Crypton, Hierocrypt-3, SAFER++ and Square. We know that our attack on SAFER++ is the first rebound attack with standard differentials. To demonstrate the efficiency of proposed distinguishers, we provide formal proof of a lower bound for finding a differential pair that follows a truncated differential in the case of a random permutation. Our analysis shows that block ciphers used as the underlying primitive should also be analyzed in the open-key model to prevent possible collision attacks.

We analyze the IDEA-based hash functions in a variety of cipher modes. We present practical complexity collision search attacks and preimage attacks, where we exploit a null weak-key and a new non-trivial property of IDEA. We prove that even if a cipher is considered secure in the secret-key model, one has to be very careful when using it as a building block in the hashing modes.

Finally, we investigate the recent rotational analysis. We show how to extend the rotational analysis to subtractions, shifts, bit-wise Boolean functions, multi additions and multi subtractions. In particular, we develop formulae for calculation of probabilities of preserving the rotation property for multiple modular additions and subtra[...]