Abstract
This report summarizes our results from security analysis covering all 57 competitions for authenticated encryption: security, applicability, and robustness (CAESAR) first-round candidates and over 210 implementations. We have manually identified security issues with three candidates, two of which are more serious, and these ciphers have been withdrawn from the competition. We have developed a testing framework, BRUTUS, to facilitate automatic detection of simple security lapses and susceptible statistical structures across all ciphers. From this testing, we have security usage notes on four submissions and statistical notes on a further four. We highlight that some of the CAESAR algorithms pose an elevated risk if employed in real-life protocols due to a class of adaptive-chosen-plaintext attacks. Although authenticated encryption with associated data are often defined (and are best used) as discrete primitives that authenticate and transmit only complete messages, in practice, these algorithms are easily implemented in a fashion that outputs observable ciphertext data when the algorithm has not received all of the (attacker-controlled) plaintext. For an implementor, this strategy appears to offer seemingly harmless and compliant storage and latency advantages. If the algorithm uses the same state for secret keying information, encryption, and integrity protection, and the internal mixing permutation is not cryptographically strong, an attacker can exploit the ciphertext–plaintext feedback loop to reveal secret state information or even keying material. We conclude that the main advantages of exhaustive, automated cryptanalysis are that it acts as a very necessary sanity check for implementations and gives the cryptanalyst insights that can be used to focus more specific attack methods on given candidates.
Highlights
Authenticated encryption with associated data (AEAD) algorithms provide message confidentiality and integrity protection with a single cryptographic primitive
We describe the development of our framework for automated cryptanalysis, BRUTUS, in Sect. 4, together with security usage notes obtained
PAES has been withdrawn from the CAESAR competition
Summary
Authenticated encryption with associated data (AEAD) algorithms provide message confidentiality and integrity protection with a single cryptographic primitive. As such, they offer functionality similar to combining a stream or block cipher with a message authentication code (MAC) on protocol level. Similar approaches have been taken by other popular security protocols such as IPSec [20,21] and SSH [52] This separation has been exploited by numerous real-life attacks [10,33,45]. The candidates can be classified according to their robustness against adaptive-chosen-plaintext attacks, which generally do not apply to AES-GCM. This is done, and we conclude in Sect.
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