## CryptoDB

### Avijit Dutta

#### Publications

**Year**

**Venue**

**Title**

2020

TOSC

Release of Unverified Plaintext: Tight Unified Model and Application to ANYDAE
📺
Abstract

Authenticated encryption schemes are usually expected to offer confidentiality and authenticity. In case of release of unverified plaintext (RUP), an adversary gets separated access to the decryption and verification functionality, and has more power in breaking the scheme. Andreeva et al. (ASIACRYPT 2014) formalized RUP security using plaintext awareness, informally meaning that the decryption functionality gives no extra power in breaking confidentiality, and INT-RUP security, covering authenticity in case of RUP. We describe a single, unified model, called AERUP security, that ties together these notions: we prove that an authenticated encryption scheme is AERUP secure if and only if it is conventionally secure, plaintext aware, and INT-RUP secure. We next present ANYDAE, a generalization of SUNDAE of Banik et al. (ToSC 2018/3). ANYDAE is a lightweight deterministic scheme that is based on a block cipher with block size n and arbitrary mixing functions that all operate on an n-bit state. It is particularly efficient for short messages, it does not rely on a nonce, and it provides maximal robustness to a lack of secure state. Whereas SUNDAE is not secure under release of unverified plaintext (a fairly simple attack can be mounted in constant time), ANYDAE is. We make handy use of the AERUP security model to prove that ANYDAE achieves both conventional security as RUP security, provided that certain modest conditions on the mixing functions are met. We describe two simple instances, called MONDAE and TUESDAE, that conform to these conditions and that are competitive with SUNDAE, in terms of efficiency and optimality.

2020

ASIACRYPT

Minimizing the Two-Round Tweakable Even-Mansour Cipher
📺
Abstract

In CRYPTO 2015, Cogliati et al. have proposed one-round tweakable Even-Mansour (\textsf{1-TEM}) cipher constructed out of a single $n$-bit public permutation $\pi$ and a uniform and almost XOR-universal hash function \textsf{H} as $(k, t, x) \mapsto \textsf{H}_k(t) \oplus \pi(\textsf{H}_k(t) \oplus x)$, where $t$ is the tweak, and $x$ is the $n$-bit message. Authors have shown that its two-round extension, which we refer to as \textsf{2-TEM}, obtained by cascading $2$-independent instances of the construction gives $2n/3$-bit security and $r$-round cascading gives $rn/r+2$-bit security. In ASIACRYPT 2015, Cogliati and Seurin have shown that four-round tweakable Even-Mansour cipher, which we refer to as \textsf{4-TEM}, constructed out of four independent $n$-bit permutations $\pi_1, \pi_2, \pi_3, \pi_4$ and two independent $n$-bit keys $k_1, k_2$, defined as $$k_1 \oplus t \oplus \pi_4(k_2 \oplus t \oplus \pi_3(k_1 \oplus t \oplus \pi_2(k_2 \oplus t \oplus \pi_1(k_1 \oplus t \oplus x)))), $$ is secure upto $2^{2n/3}$ adversarial queries. In this paper, we have shown that if we replace two independent permutations of \textsf{2-TEM} (Cogliati et al., CRYPTO 2015) with a single $n$-bit public permutation, then the resultant construction still guarrantees security upto $2^{2n/3}$ adversarial queries. Using the results derived therein, we also show that replacing the permutation $(\pi_4, \pi_3)$ with $(\pi_1, \pi_2)$ in the above equation preserves security upto $2^{2n/3}$ adversarial queries.

2019

EUROCRYPT

Beyond Birthday Bound Secure MAC in Faulty Nonce Model
📺
Abstract

Encrypt-then-MAC (EtM) is a popular mode for authenticated encryption (AE). Unfortunately, almost all designs following the EtM paradigm, including the AE suites for TLS, are vulnerable against nonce misuse. A single repetition of the nonce value reveals the hash key, leading to a universal forgery attack. There are only two authenticated encryption schemes following the EtM paradigm which can resist nonce misuse attacks, the GCM-RUP (CRYPTO-17) and the $$\mathsf {GCM/2}^{+} $$ (INSCRYPT-12). However, they are secure only up to the birthday bound in the nonce respecting setting, resulting in a restriction on the data limit for a single key. In this paper we show that nEHtM, a nonce-based variant of EHtM (FSE-10) constructed using a block cipher, has a beyond birthday bound (BBB) unforgeable security that gracefully degrades under nonce misuse. We combine nEHtM with the CENC (FSE-06) mode of encryption using the EtM paradigm to realize a nonce-based AE, CWC+. CWC+ is very close (requiring only a few more xor operations) to the CWC AE scheme (FSE-04) and it not only provides BBB security but also gracefully degrading security on nonce misuse.

2018

CRYPTO

Encrypt or Decrypt? To Make a Single-Key Beyond Birthday Secure Nonce-Based MAC
📺
Abstract

At CRYPTO 2016, Cogliati and Seurin have proposed a highly secure nonce-based MAC called Encrypted Wegman-Carter with Davies-Meyer ($$\textsf {EWCDM}$$EWCDM) construction, as $$\textsf {E}_{K_2}\bigl (\textsf {E}_{K_1}(N)\oplus N\oplus \textsf {H}_{K_h}(M)\bigr )$$EK2(EK1(N)⊕N⊕HKh(M)) for a nonce N and a message M. This construction achieves roughly $$2^{2n/3}$$22n/3 bit MAC security with the assumption that $$\textsf {E}$$E is a PRP secure n-bit block cipher and $$\textsf {H}$$H is an almost xor universal n-bit hash function. In this paper we propose Decrypted Wegman-Carter with Davies-Meyer ($$\textsf {DWCDM}$$DWCDM) construction, which is structurally very similar to its predecessor $$\textsf {EWCDM}$$EWCDM except that the outer encryption call is replaced by decryption. The biggest advantage of $$\textsf {DWCDM}$$DWCDM is that we can make a truly single key MAC: the two block cipher calls can use the same block cipher key $$K=K_1=K_2$$K=K1=K2. Moreover, we can derive the hash key as $$K_h=\textsf {E}_K(1)$$Kh=EK(1), as long as $$|K_h|=n$$|Kh|=n. Whether we use encryption or decryption in the outer layer makes a huge difference; using the decryption instead enables us to apply an extended version of the mirror theory by Patarin to the security analysis of the construction. $$\textsf {DWCDM}$$DWCDM is secure beyond the birthday bound, roughly up to $$2^{2n/3}$$22n/3 MAC queries and $$2^n$$2n verification queries against nonce-respecting adversaries. $$\textsf {DWCDM}$$DWCDM remains secure up to $$2^{n/2}$$2n/2 MAC queries and $$2^n$$2n verification queries against nonce-misusing adversaries.

2018

TOSC

Double-block Hash-then-Sum: A Paradigm for Constructing BBB Secure PRF
📺
Abstract

SUM-ECBC (Yasuda, CT-RSA 2010) is the first beyond birthday bound (BBB) secure block cipher based deterministic MAC. After this work, some more BBB secure deterministic MACs have been proposed, namely PMAC_Plus (Yasuda, CRYPTO 2011), 3kf9 (Zhang et al., ASIACRYPT 2012) and LightMAC_Plus (Naito, ASIACRYPT 2017). In this paper, we have abstracted out the inherent design principle of all these BBB secure MACs and present a generic design paradigm to construct a BBB secure pseudo random function, namely Double-block Hash-then- Sum or in short (DbHtS). A DbHtS construction, as the name implies, computes a double block hash on the message and then sum the encrypted output of the two hash blocks. Our result renders that if the underlying hash function meets certain security requirements (namely cover-free and block-wise universal advantage is low), DbHtS construction provides 2n/3-bit security. We demonstrate the applicability of our result by instantiating all the existing beyond birthday secure deterministic MACs (e.g., SUM-ECBC, PMAC_Plus, 3kf9, LightMAC_Plus) as well as a simple two-keyed variant for each of them and some algebraic hash based constructions.

2017

TOSC

Tight Security Analysis of EHtM MAC
Abstract

The security of a probabilistic Message Authentication Code (MAC) usually depends on the uniqueness of the random salt which restricts the security to birthday bound of the salt size due to the collision on random salts (e.g XMACR). To overcome the birthday bound limit, the natural approach to use (a) either a larger random salt (e.g MACRX3 uses 3n bits of random salt where n is the input and output size of the underlying non-compressing pseudorandom function or PRF) or (b) a PRF with increased domain size (e.g RWMAC or Randomized WMAC). Enhanced Hashthen- Mask (EHtM), proposed by Minematsu in FSE 2010, is the first probabilistic MAC scheme that provides beyond birthday bound security without increasing the randomness of the salt and the domain size of the non-compressing PRF. The author proved the security of EHtM as long as the number of MAC query is smaller than 22n/3 where n is the input size of the underlying non-compressing PRF. In this paper, we provide the exact security bound of EHtM and prove that this construction offers security up to 23n/4 MAC queries. The exactness is shown by demonstrating a matching attack.

2017

TOSC

Single Key Variant of PMAC_Plus
Abstract

At CRYPTO 2011, Yasuda proposed the PMAC_Plus message authentication code based on an n-bit block cipher. Its design principle inherits the well known PMAC parallel network with a low additional cost. PMAC_Plus is a rate-1 construction like PMAC (i.e., one block cipher call per n-bit message block) but provides security against all adversaries (under black-box model) making queries altogether consisting of roughly upto 22n/3 blocks (strings of n-bits). Even though PMAC_Plus gives higher security than the standard birthday bound security, with currently available best bound, it provides weaker security than PMAC for certain choices of adversaries. Moreover, unlike PMAC, PMAC_Plus operates with three independent block cipher keys. In this paper, we propose 1k-PMAC_Plus, the first rate-1 single keyed block cipher based BBB (Beyond Birthday Bound) secure (in standard model) deterministic MAC construction without arbitrary field multiplications. 1k-PMAC_Plus, as the name implies, is a simple one-key variant of PMAC_Plus. In addition to the key reduction, we obtain a higher security guarantee than what was proved originally for PMAC_Plus, thus an improvement in two directions.

#### Coauthors

- Ritam Bhaumik (1)
- Donghoon Chang (1)
- Nilanjan Datta (6)
- Ashwin Jha (1)
- Bart Mennink (1)
- Nicky Mouha (1)
- Mridul Nandi (8)
- Goutam Paul (5)
- Somitra Sanadhya (1)
- Ferdinand Sibleyras (1)
- Suprita Talnikar (1)
- Kan Yasuda (1)
- Liting Zhang (2)