Security

Online Ciphers From Tweakable Blockciphers

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Executive Summary

Informally, a cryptographic transform is said to be online if it can be computed by an algorithm that reads in the (unknown number of) input bits - in order, one at a time - as it writes out the corresponding output bits - again in order, one at a time - never using more than a constant amount of memory or incurring more than a constant amount of latency. Most block-cipher modes of operation are online - for example, modes like CBC, HMAC, and GCM certainly are. But one kind of transformation is not online, and can never be online: a general cipher, one secure in the customary sense of a PRP (a Pseudo-Random Permutation)

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