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1. The Curse of Small Domains: New Attacks on Format-Preserving Encryption

   https://doi.org/10.1007/978-3-319-96884-1_8  

   Viet Tung Hoang, Stefano Tessaro (August 2018, Advances in Cryptology – CRYPTO 2018)

   Format-preserving encryption (FPE) produces ciphertexts which have the same format as the plaintexts. Building secure FPE is very challenging, and recent attacks (Bellare, Hoang, Tessaro, CCS ’16; Durak and Vaudenay, CRYPTO ’17) have highlighted security deficiencies in the recent NIST SP800-38G standard. This has left the question open of whether practical schemes with high security exist. In this paper, we continue the investigation of attacks against FPE schemes. Our first contribution are new known-plaintext message recovery attacks against Feistel-based FPEs (such as FF1/FF3 from the NIST SP800-38G standard) which improve upon previous work in terms of amortized complexity in multi-target scenarios, where multiple ciphertexts are to be decrypted. Our attacks are also qualitatively better in that they make no assumptions on the correlation between the targets to be decrypted and the known plaintexts. We also surface a new vulnerability specific to FF3 and how it handles odd length domains, which leads to a substantial speedup in our attacks. We also show the first attacks against non-Feistel based FPEs. Specifically, we show a strong message-recovery attack for FNR, a construction proposed by Cisco which replaces two rounds in the Feistel construction with a pairwise-independent permutation, following the paradigm by Naor and Reingold (JoC, ’99). We also provide a strong ciphertext-only attack against a variant of the DTP construction by Brightwell and Smith, which is deployed by Protegrity within commercial applications. All of our attacks show that existing constructions fall short of achieving desirable security levels. For Feistel and the FNR schemes, our attacks become feasible on small domains, e.g., 8 bits, for suggested round numbers. Our attack against the DTP construction is practical even for large domains. We provide proof-of-concept implementations of our attacks that verify our theoretical findings. 

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2. IDCrypt: A Multi-User Searchable Symmetric Encryption Scheme for Cloud Applications

   https://doi.org/10.1109/ACCESS.2017.2786026  

   Wang, Guofeng; Liu, Chuanyi; Dong, Yingfei; Han, Peiyi; Pan, Hezhong; Fang, Binxing (January 2018, IEEE Access)

   Searchable Encryption (SE) has been extensively examined by both academic and industry researchers. While many academic SE schemes show provable security, they usually expose some query information (e.g., search patterns) to achieve high efficiency. However, several inference attacks have exploited such leakage, e.g., a query recovery attack can convert opaque query trapdoors to their corresponding keywords based on some prior knowledge. On the other hand, many proposed SE schemes require significant modification of existing applications, which makes them less practical, weak in usability, and difficult to deploy. In this paper, we introduce a secure and practical SE scheme with provable security strength for cloud applications, called IDCrypt, which improves the search efficiency and enhanced the security strength of SE using symmetric cryptography. We further point out the main challenges in securely searching on multiple indexes and sharing encrypted data between multiple users. To address the above issues, we propose a token-adjustment scheme to preserve the search functionality among multi-indexes, and a key sharing scheme which combines Identity-Based Encryption (IBE) and Public-Key Encryption (PKE). Our experimental results show that the overhead of IDCrypt is fairly low. 

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3. 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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4. 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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5. New Approaches to Traitor Tracing with Embedded Identities

   https://doi.org/10.1007/978-3-030-36033-7_6  

   Goyal, Rishab; Koppula, Venkata; Waters, Brent (November 2019, New Approaches to Traitor Tracing with Embedded Identities)

   In a traitor tracing (TT) system for n users, every user has his/her own secret key. Content providers can encrypt messages using a public key, and each user can decrypt the ciphertext using his/her secret key. Suppose some of the n users collude to construct a pirate decoding box. Then the tracing scheme has a special algorithm, called 𝖳𝗋𝖺𝖼𝖾 , which can identify at least one of the secret keys used to construct the pirate decoding box. Traditionally, the trace algorithm output only the ‘index’ associated with the traitors. As a result, to use such systems, either a central master authority must map the indices to actual identities, or there should be a public mapping of indices to identities. Both these options are problematic, especially if we need public tracing with anonymity of users. Nishimaki, Wichs, and Zhandry (NWZ) [Eurocrypt 2016] addressed this problem by constructing a traitor tracing scheme where the identities of users are embedded in the secret keys, and the trace algorithm, given a decoding box D, can recover the entire identities of the traitors. We call such schemes ‘Embedded Identity Traitor Tracing’ schemes. NWZ constructed such schemes based on adaptively secure functional encryption (FE). Currently, the only known constructions of FE schemes are based on nonstandard assumptions such as multilinear maps and iO. In this work, we study the problem of embedded identities TT based on standard assumptions. We provide a range of constructions based on different assumptions such as public key encryption (PKE), bilinear maps and the Learning with Errors (LWE) assumption. The different constructions have different efficiency trade offs. In our PKE based construction, the ciphertext size grows linearly with the number of users; the bilinear maps based construction has sub-linear (𝑛√ ) sized ciphertexts. Both these schemes have public tracing. The LWE based scheme is a private tracing scheme with optimal ciphertexts (i.e., log(𝑛)). Finally, we also present other notions of traitor tracing, and discuss how they can be build in a generic manner from our base embedded identity TT scheme. 

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