International Association
for Cryptologic Research

We describe OCB3, the final version of OCB, a blockcipher mode for authenticated encryption (AE). We prove the construction secure, up to the birthday bound, assuming its underlying blockcipher is secure as a strong-PRP. We study the scheme’s software performance, comparing its speed, on multiple platforms, to a variety of other AE schemes. We reflect on the history and development of the mode.

The customary formulation of authenticated encryption (AE) requires the decrypting party to supply the correct nonce with each ciphertext it decrypts. To enable this, the nonce is often sent in the clear alongside the ciphertext. But doing this can forfeit anonymity and degrade usability. Anonymity can also be lost by transmitting associated data (AD) or a session-ID (used to identify the operative key). To address these issues, we introduce anonymous AE, wherein ciphertexts must conceal their origin even when they are understood to encompass everything needed to decrypt (apart from the receiver’s secret state). We formalize a type of anonymous AE we call anAE, anonymous nonce-based AE, which generalizes and strengthens conventional nonce-based AE, nAE. We provide an efficient construction for anAE, NonceWrap, from an nAE scheme and a blockcipher. We prove NonceWrap secure. While anAE does not address privacy loss through traffic-flow analysis, it does ensure that ciphertexts, now more expansively construed, do not by themselves compromise privacy.

Often the simplest way of specifying game-based cryptographic definitions is apparently barred because the adversary would have some trivial win. Disallowing or invalidating these wins can lead to complex or unconvincing definitions. We suggest a generic way around this difficulty. We call it indistinguishability up to correctness, or IND$$\vert $$C. Given games $${{\text {G}}}$$ and $${{\text {H}}}$$ and a correctness condition $${{\text {C}}}$$ we define an advantage measure $${\mathbf {Adv}_{{{\text {G}}},{{\text {H}}},{{\text {C}}}}^{{\text {indc}}}}$$ wherein $${{{\text {G}}}}$$/$${{{\text {H}}}}$$ distinguishing attacks are effaced to the extent that they are inevitable due to $${{\text {C}}}$$. We formalize this in the language of oracle silencing, an alternative to exclusion-style and penalty-style definitions. We apply our ideas to a domain where game-based definitions have been cumbersome: stateful authenticated-encryption (sAE). We rework existing sAE notions and encompass new ones, like replay-free AE permitting a specified degree of out-of-order message delivery.