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Fix possible attacks against data confidentiality through LUKS2 online reencryption extension crash recovery. An attacker can modify on-disk metadata to simulate decryption in progress with crashed (unfinished) reencryption step and persistently decrypt part of the LUKS device. This attack requires repeated physical access to the LUKS device but no knowledge of user passphrases. The decryption step is performed after a valid user activates the device with a correct passphrase and modified metadata. There are no visible warnings for the user that such recovery happened (except using the luksDump command). The attack can also be reversed afterward (simulating crashed encryption from a plaintext) with possible modification of revealed plaintext. The problem was caused by reusing a mechanism designed for actual reencryption operation without reassessing the security impact for new encryption and decryption operations. While the reencryption requires calculating and verifying both key digests, no digest was needed to initiate decryption recovery if the destination is plaintext (no encryption key). Also, some metadata (like encryption cipher) is not protected, and an attacker could change it. Note that LUKS2 protects visible metadata only when a random change occurs. It does not protect against intentional modification but such modification must not cause a violation of data confidentiality. The fix introduces additional digest protection of reencryption metadata. The digest is calculated from known keys and critical reencryption metadata. Now an attacker cannot create correct metadata digest without knowledge of a passphrase for used keyslots. For more details, see LUKS2 On-Disk Format Specification version 1.1.0.
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