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Unformatted text preview: tly. Assume the last block has j bits. After encrypting the last full block, encrypt the ciphertext again, select the left-most j bits of the encrypted ciphertext, and XOR that with the short block to generate the ciphertext. Figure 9.4 illustrates this. The weakness here is that while Mallory cannot recover the last plaintext block, he can change it systematically by changing individual bits in the ciphertext. If the last few bits of the ciphertext contain essential information, this is a weakness. If the last bits simply contain housekeeping information, it isn’t a problem. Ciphertext stealing is a better way (see Figure 9.5) . Pn-1 is the last full plaintext block, and Pn is the final, short, plaintext block. Cn-1 is the last full ciphertext block, and Cn is the final, short, ciphertext block. C’ is just an intermediate result and is not part of the transmitted ciphertext. The benefit of this method is that all the bits of the plaintext message go through the encryption algorithm. Error Propagation
CBC mode can be characterized as feedback of the ciphertext at the encryption end and feedforward of the ciphertext at the decryption end. This has implications having to do with errors. A single bit error in a plaintext block will affect that ciphertext block and all subsequent ciphertext blocks. This isn’t significant because decryption will reverse that effect, and the recovered plaintext will have the same single error. Ciphertext errors are more common. They can easily result from a noisy communications path or a malfunction in the storage medium. In CBC mode, a single-bit error in the ciphertext affects one block and one bit of the recovered plaintext. The block containing the error is completely garbled. The subsequent block has a 1-bit error in the same bit position as the error. Figure 9.4 Encrypting the last short block in CBC mode. Figure 9.5 Ciphertext stealing in CBC mode. This property of taking a small ciphertext error and converting it into a large plaintext error is called error extension. It is a major annoyance. Blocks after the second are not affected...
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- Fall '10