## CryptoDB

### Kazuhiko Minematsu

#### Publications

Year
Venue
Title
2019
JOFC
This paper presents a lightweight blockcipher-based authenticated encryption mode mainly focusing on minimizing the implementation size, i.e., hardware gates or working memory on software. The mode is called $\textsf {COFB}$COFB, for COmbined FeedBack. $\textsf {COFB}$COFB uses an n-bit blockcipher as the underlying primitive and relies on the use of a nonce for security. In addition to the state required for executing the underlying blockcipher, $\textsf {COFB}$COFB needs only n / 2 bits state as a mask. Till date, for all existing constructions in which masks have been applied, at least n bit masks have been used. Thus, we have shown the possibility of reducing the size of a mask without degrading the security level much. Moreover, it requires one blockcipher call to process one input block. We show $\textsf {COFB}$COFB is provably secure up to $O(2^{n/2}/n)$O(2n/2/n) queries which is almost up to the standard birthday bound. We first present an idealized mode $\textsf {iCOFB}$iCOFB along with the details of its provable security analysis. Next, we extend the construction to the practical mode COFB. We instantiate COFB with two 128-bit blockciphers, AES-128 and GIFT-128, and present their implementation results on FPGAs. We present two implementations, with and without CAESAR hardware API. When instantiated with AES-128 and implemented without CAESAR hardware API, COFB achieves only a few more than 1000 Look-Up-Tables (LUTs) while maintaining almost the same level of provable security as standard AES-based AE, such as GCM. When instantiated with GIFT-128, COFB performs much better in hardware area. It consumes less than 1000 LUTs while maintaining the same security level. However, when implemented with CAESAR hardware API, there are significant overheads both in hardware area and in throughput. COFB with AES-128 achieves about 1475 LUTs. COFB with GIFT-128 achieves a few more than 1000 LUTs. Though there are overheads, still both these figures show competitive implementation results compared to other authenticated encryption constructions.
2019
TOSC
We define ZOCB and ZOTR for nonce-based authenticated encryption with associated data, and analyze their provable security. These schemes use a tweakable blockcipher (TBC) as the underlying primitive, and fully utilize its input to process a plaintext and associated data (AD). This property is commonly referred to as full absorption, and this has been explored for schemes based on a permutation or a pseudorandom function (PRF). Our schemes improve the efficiency of TBC-based counterparts of OCB and OTR called OCB3 (Krovetz and Rogaway, FSE 2011) and OTR (Minematsu, EUROCRYPT 2014). Specifically, ΘCB3 and OTR have an independent part to process AD, and our schemes integrate this process into the encryption part of a plaintext by using the tweak input of the TBC. Up to a certain length of AD, ZOCB and ZOTR completely eliminate the independent process for it. Even for longer AD, our schemes process it efficiently by fully using the tweak input of the TBC. For this purpose, based on previous tweak extension schemes for TBCs, we introduce a scheme called XTX*. To our knowledge, ZOCB and ZOTR are the first efficiency improvement of ΘCB3 and OTR in terms of the number of TBC calls. Compared to Sponge-based and PRF-based schemes, ZOCB and ZOTR allow fully parallel computation of the underlying primitive, and have a unique design feature that an authentication tag is independent of a part of AD. We present experimental results illustrating the practical efficiency gain and clarifying the efficiency cost for it with a concrete instantiation. The results show that for long input data, our schemes have gains, while we have efficiency loss for short input data.
2019
CRYPTO
We present practical attacks on OCB2. This mode of operation of a blockcipher was designed with the aim to provide particularly efficient and provably-secure authenticated encryption services, and since its proposal about 15 years ago it belongs to the top performers in this realm. OCB2 was included in an ISO standard in 2009.An internal building block of OCB2 is the tweakable blockcipher obtained by operating a regular blockcipher in $\text {XEX} ^*$ mode. The latter provides security only when evaluated in accordance with certain technical restrictions that, as we note, are not always respected by OCB2. This leads to devastating attacks against OCB2’s security promises: We develop a range of very practical attacks that, amongst others, demonstrate universal forgeries and full plaintext recovery. We complete our report with proposals for (provably) repairing OCB2. To our understanding, as a direct consequence of our findings, OCB2 is currently in a process of removal from ISO standards. Our attacks do not apply to OCB1 and OCB3, and our privacy attacks on OCB2 require an active adversary.
2017
CRYPTO
2017
TOSC
At CT-RSA 2017, List and Nandi proposed two variable input length pseudorandom functions (VI-PRFs) called PMACx and PMAC2x, and a deterministic authenticated encryption scheme called SIVx. These schemes use a tweakable block cipher (TBC) as the underlying primitive, and are provably secure up to the query complexity of 2n, where n denotes the block length of the TBC. In this paper, we falsify the provable security claims by presenting concrete attacks. We show that with the query complexity of O(2n/2), i.e., with the birthday complexity, PMACx, PMAC2x, and SIVx are all insecure.
2017
CHES
This paper presents a design of authenticated encryption (AE) focusing on minimizing the implementation size, i.e., hardware gates or working memory on software. The scheme is called $\textsf {COFB}$, for COmbined FeedBack. $\textsf {COFB}$ uses an n-bit blockcipher as the underlying primitive, and relies on the use of a nonce for security. In addition to the state required for executing the underlying blockcipher, $\textsf {COFB}$ needs only n / 2 bits state as a mask. Till date, for all existing constructions in which masks have been applied, at least n bit masks have been used. Thus, we have shown the possibility of reducing the size of a mask without degrading the security level much. Moreover, it requires one blockcipher call to process one input block. We show $\textsf {COFB}$ is provably secure up to $O(2^{n/2}/n)$ queries which is almost up to the standard birthday bound. We also present our hardware implementation results. Experimental implementation results suggest that our proposal has a good performance and the smallest footprint among all known blockcipher-based AE.
2016
TOSC
At CCS 2015, Gueron and Lindell proposed GCM-SIV, a provably secure authenticated encryption scheme that remains secure even if the nonce is repeated. While this is an advantage over the original GCM, we first point out that GCM-SIV allows a trivial distinguishing attack with about 248 queries, where each query has one plaintext block. This shows the tightness of the security claim and does not contradict the provable security result. However, the original GCM resists the attack, and this poses a question of designing a variant of GCM-SIV that is secure against the attack. We present a minor variant of GCM-SIV, which we call GCM-SIV1, and discuss that GCM-SIV1 resists the attack, and it offers a security trade-off compared to GCM-SIV. As the main contribution of the paper, we explore a scheme with a stronger security bound. We present GCM-SIV2 which is obtained by running two instances of GCM-SIV1 in parallel and mixing them in a simple way. We show that it is secure up to 285.3 query complexity, where the query complexity is measured in terms of the total number of blocks of the queries. Finally, we generalize this to show GCM-SIVr by running r instances of GCM-SIV1 in parallel, where r ≥ 3, and show that the scheme is secure up to 2128r/(r+1) query complexity. The provable security results are obtained under the standard assumption that the blockcipher is a pseudorandom permutation.
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
FSE
2014
EUROCRYPT
2014
EPRINT
2014
FSE
2013
FSE
2012
CRYPTO
2010
FSE
2010
FSE
2009
FSE
2007
FSE
2006
FSE

Eurocrypt 2020
FSE 2020
FSE 2018
FSE 2017