Almost Universal Forgery Attacks on the COPA and Marble Authenticated Encryption Algorithms.

The COPA authenticated encryption mode was proved to have a birthday-bound security on integrity, and its instantiation AES-COPA (v1/2) was claimed or conjectured to have a full security on tag guessing. The Marble (v1.0/1.1/1.2) authenticated encryption algorithm was claimed to have a full security on authenticity. Both AES-COPA (v1) and Marble (v1.0) were submitted to the Competition for Authenticated Encryption: Security, Applicability, and Robustness (CAESAR) in 2014, and Marble was revised twice (v1.1/1.2) in the first round of CAESAR, and AES-COPA (v1) was tweaked (v2) for the second round of CAESAR. In this paper, we cryptanalyse the basic cases of COPA, AES-COPA and Marble, that process messages of a multiple of the block size long; we present collision-based almost universal forgery attacks on the basic cases of COPA, AES-COPA (v1/2) and Marble (v1.0/1.1/1.2), and show that the basic cases of COPA and AES-COPA have roughly at most a birthday-bound security on tag guessing and the basic case of Marble has roughly at most a birthday-bound security on authenticity. The attacks on COPA and AES-COPA do not violate their birthday-bound security proof on integrity, but the attack on AES-COPA violates its full security claim or conjecture on tag guessing. Therefore, the full security claim or conjecture on tag guessing of AES-COPA and the full security claim on authenticity of Marble are incorrectly far overestimated in the sense of a general understanding of full security of these security notions. Designers should pay attention to these attacks when designing authenticated encryption algorithms with similar structures in the future, and should be careful when claiming the security of an advanced form of a security notion without making a corresponding proof after proving the security of the security notion only under its most fundamental form.