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What is this method/algorithm/construction called? Dunno; this is a new one on me. Is it as secure as CBC implemented the normal way? Should be. Modeled as an abstract 'take plaintext, output ciphertext' model, this method (with a random last ciphertext bits) has precisely the same ciphertext output distribution as CBC mode (with a random IV), ...


2

First, the advice: What are the best-practices to store the message length / strip away padding? Use standard padding, like PKCS#7 padding. It handles finding the length uniquely for you. Use encrypt-then-MAC to prevent padding oracle attacks. (Or better yet, don't use CBC. Use an authenticated encryption mode like GCM, or use CTR+MAC which doesn't ...


1

Your mode is essentially equivalent to CFB mode, except that: you've reversed the order of the blocks in the message, and you're using the block cipher in the opposite direction than usual. Neither of those differences should have any direct security implications (since all standard block ciphers have the same security properties in both directions), ...



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