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Block cipher decryption tool aes to cbc7/17/2023 Fortunately (or unfortunately, depending of your point of view), in many file formats the first some bytes are fixed (or almost fixed, or can be guessed based on the rest of the file). On decryption, we will apply the block cipher to each ciphertext block, and then XOR the result with the previous ciphertext block (or the initialization vector, for the first block).Īs all ciphertext blocks are known, and we have the key, we can reliably decrypt everything other than the first block, even if no or a wrong initialization vector is given.įor the first block, we actually have no information at all, without the initialization vector, other than that it has the same length as any other block (16 bytes for AES): Every 16-bytes plaintext can be reached by supplying the fitting IV. With CBC (Cipher block chaining) mode, before encryption, each block is XOR-ed with the ciphertext of the previous block, to randomize the input to the block cipher (and avoid encrypting the same block twice with the same key, as this would give the same output, and tell the attacker something about the plaintext).Īs the first block has no previous block, here the initialization vector will be used instead.
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