POEx: a beyond-birthday-bound-secure on-line cipher. On-line ciphers are convenient building blocks for realizing efficient single- pass encryption. In particular, the trend to limit the consequences of nonce reuses rendered them popular in recent authenticated encryption schemes. While encryption schemes, such as POE, COPE, or the ciphers within ElmE/ElmD concentrated on efficiency, their security guarantees and that of all earlier on-line ciphers is limited by the birthday bound, and so are those of the AE schemes built upon them. This work proposes POEx, a beyond-birthday-bound-secure on-line cipher which employs one call to a tweakable block cipher and one call to a 2n-bit universal hash function per message block. POEx builds upon the recently proposed XTX tweak extender by Iwata and Minematsu. We prove the security of our construction and discuss possible instantiations.

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