Differences

This shows you the differences between two versions of the page.

--- pgpfan:seip [2025/08/25 19:02] – [Protection] Missing line end b.walzer
+++ pgpfan:seip [2026/03/14 17:55] (current) – [The Output Feedback (OFB) Block Cipher Mode] Missed a detail. b.walzer
@@ Line 14: / Line 14: @@
 //<sub>Output Feedback (OFB)</sub>//
-We start with a 128 bit Initialization Vector (IV) (any previously unused 128 bits) and encrypt it using the key to produce the first 128 bits of the Bitstream. Then we take the first segment of the Bitstream and encrypt it. We encrypt the IV over and over again until we have a long enough Bitstream. Then we XOR the Bitstream with the Plaintext to produce the Ciphertext. The interesting thing here is that we are not actually using the encryption function to encrypt anything we care about. We are using the encryption function to do something else and then use that result to do the actual encryption. We don't even need a separate decryption function for OFB. When we want the plaintext back we just use the key to generate the Bitstream again and XOR it with the Ciphertext. That is based on a helpful property of the XOR function. If you XOR something with the same bitstream twice, you end up with what you started with.
+We start with a 128 bit Initialization Vector (IV) (any previously unused 128 bits) and encrypt it using the key to produce the first 128 bits of the Bitstream. Then we take the first segment of the Bitstream and encrypt it. We encrypt the IV over and over again until we have a long enough Bitstream. Then we XOR the Bitstream with the Plaintext to produce the Ciphertext. The interesting thing here is that we are not actually using the encryption function to encrypt anything we care about. We are using the encryption function to do something else and then use that result to do the actual encryption. We don't even need a separate decryption function for OFB. When we want the plaintext back we just use the key and IV to generate the Bitstream again and XOR it with the Ciphertext. That is based on a helpful property of the XOR function. If you XOR something with the same bitstream twice, you end up with what you started with.
 The OFB mode has a fairly serious failure possibility. If you use the same IV and key on two or more messages then you get the same bitstream which causes, essentially, one-time pad key reuse. An unhelpful property of the XOR function is that when you XOR two ciphertexts that were produced with the same bitstream, the bitstream cancels out. You are left with the two plaintexts XORed together. Separating such combined plaintexts is usually not very hard.