Paper 2024/1527

How to Recover the Full Plaintext of XCB

Abstract

XCB, a tweakable enciphering mode, is part of IEEE Std. 1619.2 for shared storage media. We show that all versions of XCB are not secure through three plaintext recovery attacks. A key observation is that XCB behaves like an LRW1-type tweakable block cipher for single-block messages, which lacks CCA security. The first attack targets one-block XCB, using three queries to recover the plaintext. The second one requires four queries to recover the plaintext that excludes one block. The last one requires seven queries to recover the full plaintext. The first attack applies to any scheme that follows the XCB structure, whereas the latter two attacks work on all versions of XCB, exploiting the separable property of the underlying universal hash function. We also discuss the impact of these vulnerabilities on XCB-based applications, such as disk encryption, nonce-based encryption, deterministic authenticated encryption and robust authenticated encryption, highlighting the risks due to XCB's failure to achieve STPRP security. To address these flaws, we propose the XCB* structure, an improved version of XCB that adds only two XOR operations. We prove that XCB* is STPRP-secure when using AXU hash functions, SPRPs, and a secure IV-based stream cipher.

Note: We revise the text in response to the recently uploaded paper by Bhati et al. (Breaking, Repairing and Enhancing XCBv2 into the Tweakable Enciphering Mode GEM, https://eprint.iacr.org/2024/1554). In the paper, they propose a partial plaintext recovery against XCBv2 and give a fixed version of XCB3v, along with its security proof. We observe that by making an additional decryption query, the attacker can retrieve the full plaintext. We have cited this paper and indicated that XCBv3 is not secure by our three attacks.