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1. Succinctly-Committing Authenticated Encryption

   Bellare, Mihir; Hoang, Viet Tung (August 2024, Springer)

   Recent attacks and applications have led to the need for symmetric encryption schemes that, in addition to providing the usual authenticity and privacy, are also committing. In response, many committing authenticated encryption schemes have been proposed. However, all known schemes, in order to provide s bits of committing security, suffer an expansion---this is the length of the ciphertext minus the length of the plaintext---of 2s bits. This incurs a cost in bandwidth or storage. (We typically want s=128, leading to 256-bit expansion.) However, it has been considered unavoidable due to birthday attacks. We show how to bypass this limitation. We give authenticated encryption (AE) schemes that provide s bits of committing security, yet suffer expansion only around s as long as messages are long enough, namely more than s bits. We call such schemes succinct. We do this via a generic, ciphertext-shortening transform called SC: given an AE scheme with 2s-bit expansion, SC returns an AE scheme with s-bit expansion while preserving committing security. SC is very efficient; an AES-based instantiation has overhead just two AES calls. As a tool, SC uses a collision-resistant invertible PRF called HtM, that we design, and whose analysis is technically difficult. To add the committing security that SC assumes to a base scheme, we also give a transform CTY that improves Chan and Rogaway's CTX. Our results hold in a general framework for authenticated encryption that includes both classical AEAD and AE2 (also called nonce-hiding AE) as special cases, so that we in particular obtain succinctly-committing AE schemes for both these settings. 

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2. Succinctly-Committing Authenticated Encryption

   https://doi.org/10.1007/978-3-031-68385-5_10  

   Bellare, Mihir; Hoang, Viet Tung (August 2024, Springer Nature Switzerland)

   Recent attacks and applications have led to the need for symmetric encryption schemes that, in addition to providing the usual authenticity and privacy, are also committing. In response, many committing authenticated encryption schemes have been proposed. However, all known schemes, in order to provide s bits of committing security, suffer an expansion---this is the length of the ciphertext minus the length of the plaintext---of 2s bits. This incurs a cost in bandwidth or storage. (We typically want s=128, leading to 256-bit expansion.) However, it has been considered unavoidable due to birthday attacks. We show how to bypass this limitation. We give authenticated encryption (AE) schemes that provide s bits of committing security, yet suffer expansion only around s as long as messages are long enough, namely more than s bits. We call such schemes succinct. We do this via a generic, ciphertext-shortening transform called SC: given an AE scheme with 2s-bit expansion, SC returns an AE scheme with s-bit expansion while preserving committing security. SC is very efficient; an AES-based instantiation has overhead just two AES calls. As a tool, SC uses a collision-resistant invertible PRF called HtM, that we design, and whose analysis is technically difficult. To add the committing security that SC assumes to a base scheme, we also give a transform CTY that improves Chan and Rogaway's CTX. Our results hold in a general framework for authenticated encryption that includes both classical AEAD and AE2 (also called nonce-hiding AE) as special cases, so that we in particular obtain succinctly-committing AE schemes for both these settings. 

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3. Efficient Schemes for Committing Authenticated Encryption

   https://doi.org/10.1007/978-3-031-07085-3_29  

   Bellare, Mihir; Hoang, Viet Tung (January 2022, Annual International Conference on the Theory and Applications of Cryptology and Information Security (EUROCRYPT 2022))

   This paper provides efficient authenticated-encryption (AE) schemes in which a ciphertext is a commitment to the key. These are extended, at minimal additional cost, to schemes where the ciphertext is a commitment to all encryption inputs, meaning key, nonce, associated data and message. Our primary schemes are modifications of GCM (for basic, unique-nonce AE security) and AES-GCM-SIV (for misuse-resistant AE security) and add both forms of commitment without any increase in ciphertext size. We also give more generic, but somewhat more costly, solutions. 

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4. Robust AE With Committing Security

   Hoang, Viet Tung; Menda, Sanketh (December 2024, Springer)

   There has been a recent interest to develop and standardize Robust Authenticated Encryption schemes. NIST, for example, is considering an Accordion mode for (wideblock) tweakable blockcipher, with Robust AE as a primary application. At the same time, recent attacks and applications suggest that encryption context needs to be committed. Indeed, committing security is also a design consideration in Accordion mode. In this work, we give a modular solution for this problem. We first show how to transform any wideblock tweakable blockcipher TE to a Robust AE scheme SE that commits just the key. The overhead is cheap, just a few finite-field multiplications and blockcipher calls. If one wants to commit the entire encryption context, one can simply hash the context to derive a 256-bit subkey, and uses SE on that subkey. The use of 256-bit key on SE only means that it has to rely on AES-256 but doesn't require TE to have 256-bit key. Our approach frees the Accordion designs from consideration of committing security. Moreover, it gives a big saving for several key-committing applications that don't want to pay the inherent hashing cost of full committing. 

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5. Anonymous AE

   Chan, John; Rogaway, Phillip (November 2019, Advances in Cryptology -- ASIACRYPT 2019)

   The customary formulation of authenticated encryption (AE) requires the decrypting party to supply the correct nonce with each ciphertext it decrypts. To enable this, the nonce is often sent in the clear alongside the ciphertext. But doing this can forfeit anonymity and degrade usability. Anonymity can also be lost by transmitting associated data (AD) or a session-ID (used to identify the operative key). To address these issues, we introduce anonymous AE, wherein ciphertexts must conceal their origin even when they are understood to encompass everything needed to decrypt (apart from the receiver’s secret state). We formalize a type of anonymous AE we call anAE, anonymous nonce-based AE, which generalizes and strengthens conventional nonce-based AE, nAE. We provide an efficient construction for anAE, NonceWrap, from an nAE scheme and a blockcipher. We prove NonceWrap secure. While anAE does not address privacy loss through traffic-flow analysis, it does ensure that ciphertexts, now more expansively construed, do not by themselves compromise privacy. 

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