- It used to be common practice to use the last ciphertext block from one message as IV for the next message. This seemed like a good idea at the time, because the alternative is to generate a random IV for each new message, which requires a strong, fast PRNG, and strong, fast PRNGs didn't grow on trees back when this scheme was devised. By reusing the last ciphertext block from the previous message, a costly random IV was only required for the very first message.
- This practice was discovered to be a bad idea because in n - 1 out of n cases (where n is the block size in bytes), the last plaintext block of any message encrypted with a block cipher contains somewhat predictable padding.
- The flawed IV logic was replicated in several parts of the OpenBSD source tree, and the fix was implemented in some of them, but not all.
- The person who implemented this flawed logic was at that time a NetSec employee, but he had been involved in the development of OpenBSD's IPSec stack for years before he was hired, and, as previously mentioned, he was only following common practice.
- The same person implemented the obvious fix (generating a new, random IV for every message) once the attack was discovered.
- The person responsible for those parts of the tree in which the fix was not implemented is one of the people fingered by Perry, but his tenure started after Perry had left and ended before the attack was discovered.
- Anyone with any amount of experience in a large F/OSS project, or any large software development effort for that matter, can tell you that this kind of oversight is the rule rather than the exception. Although there is no evidence that he did not intentionally “forget” to fix his code, it is far more likely that he simply did not realize that the fix that had already been committed did not extend to his own code, or that he wasn't paying attention, and nobody else noticed.
I'm sure I don't need to remind anyone what this is about... The latest news: Theo now says that it is probable that NetSec was indeed contracted to insert backdoor code into OpenBSD, but after a month of review and changelog archeology, there is still no sign that they succeeded or even attempted to push tainted code into the tree. The audit (which is still ongoing) did uncover one serious bug, but there is no reason to believe that it was planted deliberately. This relates to CBC mode, an encryption protocol in which each block of plaintext is combined with the ciphertext of the previous block before encryption to make it harder to attack ciphertext blocks individually. If I understand Theo's message correctly,