EHE: nonce misuse-resistant message authentication

We propose a nonce misuse-resistant message authentication scheme called EHE (Encrypt-Hash-Encrypt). In EHE, a message-dependent polynomial is evaluated at the point which is an encrypted nonce. The resulting polynomial hash value is encrypted again and becomes an authentication tag. We prove the prf-security of the EHE scheme and extend it to two authenticated encryption modes which follow the “encrypt-then-authenticate” paradigm

On the Post-Quantum Security of Classical Authenticated Encryption Schemes

We study the post-quantum security of authenticated encryption (AE) schemes, designed with classical security in mind. Under superposition attacks, many CBC-MAC variants have been broken, and AE modes employing those variants, such as EAX and GCM, thus fail at authenticity. As we show, the same modes are IND-qCPA insecure, i.e., they fail to provide privacy under superposition attacks. However, a constrained version of GCM is IND-qCPA secure, and a nonce-based variant of the CBC-MAC is secure under superposition queries. Further, the combination of classical authenticity and classical chosen-plaintext privacy thwarts attacks with superposition chosen-ciphertext and classical chosen-plaintext queries -a security notion that we refer to as IND-qdCCA. And nonce-based key derivation allows generically turning an IND-qdCCA secure scheme into an IND-qCCA secure scheme

Elastic-Tweak: A Framework for Short Tweak Tweakable Block Cipher

Tweakable block cipher (TBC), a stronger notion than standard block ciphers, has wide-scale applications in symmetric-key schemes. At a high level, it provides flexibility in design and (possibly) better security bounds. In multi-keyed applications, a TBC with short tweak values can be used to replace multiple keys. However, the existing TBC construction frameworks, including TWEAKEY and XEX, are designed for general purpose tweak sizes. Specifically, they are not optimized for short tweaks, which might render them inefficient for certain resource constrained applications. So a dedicated paradigm to construct short-tweak TBCs (tBC) is highly desirable. In this paper, as a first contribution, we present a dedicated framework, called the Elastic-Tweak framework (ET in short), to convert any reasonably secure SPN block cipher into a secure tBC. We apply the ET framework on GIFT and AES to construct efficient tBCs, named TweGIFT and TweAES. These short-tweak TBCs have already been employed in recent NIST lightweight competition candidates, LOTUS-LOCUS and ESTATE. As our second contribution, we show some concrete applications of ET-based tBCs, which are better than their block cipher counterparts in terms of key size, state size, number of block cipher calls, and short message processing. Some notable applications include, Twe-FCBC (reduces the key size of FCBC and gives better security than CMAC), Twe-LightMAC Plus (better rate than LightMAC Plus), Twe-CLOC, and Twe-SILC (reduces the number of block cipher calls and simplifies the design of CLOC and SILC)

Automated Analysis of Protocols that use Authenticated Encryption: How Subtle AEAD Differences can impact Protocol Security

Many modern security protocols such as TLS, WPA2, WireGuard, and Signal use a cryptographic primitive called Authenticated Encryption (optionally with Authenticated Data), also known as an AEAD scheme. AEAD is a variant of symmetric encryption that additionally provides authentication. While authentication may seem to be a straightforward additional requirement, it has in fact turned out to be complex: many different security notions for AEADs are still being proposed, and several recent protocol-level attacks exploit subtle behaviors that differ among real-world AEAD schemes. We provide the first automated analysis method for protocols that use AEADs that can systematically find attacks that exploit the subtleties of the specific type of AEAD used. This can then be used to analyze specific protocols with a fixed AEAD choice, or to provide guidance on which AEADs might be (in)sufficient to make a protocol design secure. We develop generic symbolic AEAD models, which we instantiate for the Tamarin prover. Our approach can automatically and efficiently discover protocol attacks that could previously only be found using manual inspection, such as the Salamander attack on Facebook’s message franking, and attacks on SFrame and YubiHSM. Furthermore, our analysis reveals undesirable behaviors of several other protocols

Context Discovery and Commitment Attacks: How to Break CCM, EAX, SIV, and More

A line of recent work has highlighted the importance of context commitment security, which asks that authenticated encryption with associated data (AEAD) schemes will not decrypt the same adversarially-chosen ciphertext under two different, adversarially-chosen contexts (secret key, nonce, and associated data). Despite a spate of recent attacks, many open questions remain around context commitment; most obviously nothing is known about the commitment security of important schemes such as CCM, EAX, and SIV. We resolve these open questions, and more. Our approach is to, first, introduce a new framework that helps us more granularly define context commitment security in terms of what portions of a context are adversarially controlled. We go on to formulate a new notion, called context discoverability security, which can be viewed as analogous to preimage resistance from the hashing literature. We show that unrestricted context commitment security (the adversary controls all of the two contexts) implies context discoverability security for a class of schemes encompassing most schemes used in practice. Then, we show new context discovery attacks against a wide set of AEAD schemes, including CCM, EAX, SIV, GCM, and OCB3, and, by our general result, this gives new unrestricted context commitment attacks against them. Finally, we consider restricted context commitment security for the original SIV mode, for which no prior attack techniques work (including our context discovery based ones). We are nevertheless able to give a novel $O(2^{n/3})$ attack using Wagner\u27s k-tree algorithm for the generalized birthday problem

Universal Forgery Attack against GCM-RUP

International audienceAuthenticated encryption (AE) schemes are widely used to secure communications because they can guarantee both confidentiality and authenticity of a message. In addition to the standard AE security notion, some recent schemes offer extra robustness, i.e. they maintain security in some misuse scenarios. In particular, Ashur, Dunkelman and Luykx proposed a generic AE construction at CRYPTO'17 that is secure even when releasing unverified plaintext (the RUP setting), and a concrete instantiation, GCM-RUP. The designers proved that GCM-RUP is secure up to the birthday bound in the nonce-respecting model. In this paper, we perform a birthday-bound universal forgery attack against GCM-RUP, matching the bound of the proof. While there are simple distinguishing attacks with birthday complexity on GCM-RUP, our attack is much stronger: we have a partial key recovery leading to universal forgeries. For reference, the best known universal forgery attack against GCM requires 2 2n/3 operations, and many schemes do not have any known universal forgery attacks faster than 2 n. This suggests that GCM-RUP offers a different security trade-off than GCM: stronger protection in the RUP setting, but more fragile when the data complexity reaches the birthday bound. In order to avoid this attack, we suggest a minor modification of GCM-RUP that seems to offer better robustness at the birthday bound

The Summation-Truncation Hybrid: Reusing Discarded Bits for Free

A well-established PRP-to-PRF conversion design is truncation: one evaluates an $n$-bit pseudorandom permutation on a certain input, and truncates the result to $a$ bits. The construction is known to achieve tight $2^{n-a/2}$ security. Truncation has gained popularity due to its appearance in the GCM-SIV key derivation function (ACM CCS 2015). This key derivation function makes four evaluations of AES, truncates the outputs to $n/2$ bits, and concatenates these to get a $2n$-bit subkey. In this work, we demonstrate that truncation is wasteful. In more detail, we present the Summation-Truncation Hybrid (STH). At a high level, the construction consists of two parallel evaluations of truncation, where the truncated $(n-a)$-bit chunks are not discarded but rather summed together and appended to the output. We prove that STH achieves a similar security level as truncation, and thus that the $n-a$ bits of extra output is rendered for free. In the application of GCM-SIV, the current key derivation can be used to output $3n$ bits of random material, or it can be reduced to three primitive evaluations. Both changes come with no security loss

Automated Analysis of Protocols that use Authenticated Encryption: How Subtle AEAD Differences can impact Protocol Security

Many modern security protocols such as TLS, WPA2, WireGuard, and Signal use a cryptographic primitive called Authenticated Encryption (optionally with Authenticated Data), also known as an AEAD scheme. AEAD is a variant of symmetric encryption that additionally provides authentication. While authentication may seem to be a straightforward additional requirement, it has in fact turned out to be complex: many different security notions for AEADs are still being proposed, and several recent protocol-level attacks exploit subtle behaviors that differ among real-world AEAD schemes. We provide the first automated analysis method for protocols that use AEADs that can systematically find attacks that exploit the subtleties of the specific type of AEAD used. This can then be used to analyze specific protocols with a fixed AEAD choice, or to provide guidance on which AEADs might be (in)sufficient to make a protocol design secure. We develop generic symbolic AEAD models, which we instantiate for the Tamarin prover. Our approach can automatically and efficiently discover protocol attacks that could previously only be found using manual inspection, such as the Salamander attack on Facebook’s message franking, and attacks on SFrame and YubiHSM. Furthermore, our analysis reveals undesirable behaviors of several other protocols

Toward a Fully Secure Authenticated Encryption Scheme From a Pseudorandom Permutation (Full Version)

In this paper, we propose a new block cipher-based authenticated encryption scheme, dubbed the Synthetic Counter with Masking~(SCM) mode. SCM follows the NSIV paradigm proposed by Peyrin and Seurin~(CRYPTO 2016), where a keyed hash function accepts a nonce N with associated data and a message, yielding an authentication tag T, and then the message is encrypted by a counter-like mode using both T and N. Here we move one step further by encrypting nonces; in the encryption part, the inputs to the block cipher are determined by T, counters, and an encrypted nonce, and all its outputs are also masked by an (additional) encrypted nonce, yielding keystream blocks. As a result, we obtain, for the first time, a block cipher-based authenticated encryption scheme of rate 1/2 that provides n-bit security with respect to the query complexity (ignoring the influence of message length) in the nonce-respecting setting, and at the same time guarantees graceful security degradation in the faulty nonce model, when the underlying n-bit block cipher is modeled as a secure pseudorandom permutation. Seen as a slight variant of GCM-SIV, SCM is also parallelizable and inverse-free, and its performance is still comparable to GCM-SIV