On Wed, May 29, 2024 at 11:09 AM Alessandro Vesely <ves...@tana.it> wrote:
> On Wed 29/May/2024 19:29:27 +0200 John Levine wrote: > > It appears that Alessandro Vesely <ves...@tana.it> said: > >>My verifier, in particular, works every time on my messages. It doesn't > mean > >>it doesn't work at scale. > > > > Nor, of course, does it mean that it does. > > However, if it doesn't work for a given list, it's always possible to add > more > stuff in the header that will help the verifier restore the original > values and > evaluate if the amount of change the list applied is acceptable. Since > the > signer and the verifier is the same program, it's easy to coordinate. > I'm generally an advocate of experimenting with the notion of at least attempting reversible mutations, but I just realized that there might be data that the notion is a futile one. "z=" has been around since RFC 4871. The "z=" tag, when used, typically contains an encoding of the entire original header. This could be used to recover a signature that was invalidated by a header field modification of some kind. Has anyone heard of a verifier actually doing so? OpenDKIM can do this. It won't then switch the result to a valid one, but it will at least tell you what change was made to the header that invalidated the signature so you can pass that information back to the signer if you wish. I thought this was a valuable thing to add at the time, but I don't think I've ever heard of anyone trying to extend it to change the validation result. All of that is meant to say that the idea of undoing mutations you're able to identify has existed for a while, at least in one implementation. However, since it hasn't been identified as an interesting capability in the intervening years, it would seem to support Barry's claim that a broken signature oughta just stay broken. -MSK
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