Robust Authenticated-Encryption AEZ and the Problem That It Solves.

With a scheme for robust authenticated-encryption a user can select an arbitrary value lambda >= 0 and then encrypt a plaintext of any length into a ciphertext that's lambda characters longer. The scheme must provide all the privacy and authenticity possible for the requested lambda. We formalize and investigate this idea, and construct a well-optimized solution, AEZ, from the AES round function. Our scheme encrypts strings at almost the same rate as OCB-AES or CTR-AES (on Haswell, AEZ has a peak speed of about 0.7 cpb). To accomplish this we employ an approach we call prove-then-prune: prove security and then instantiate with a scaled-down primitive (e.g., reducing rounds for blockcipher calls).