Have there been any publicly known exploits of a cryptographic break in a widely used cryptographic system to actually read encrypted information (or falsely authenticate) since the Ultra program in World War II?
I want to define my terms as precisely as possible to clarify what I mean. An example of what I'm looking for needs to satisfy three requirements:
It needs to be a true cryptographic break, stemming from mathematical cryptanalysis, as opposed to a side-channel attack, insider attack, implementation error, faulty random number generator, brute-force attack, etc. In other words, it must exploit a fundamental weakness of the underlying mathematical algorithm that was not known at the time. I know that the line between an "implementation error" and a "fundamental weakness" is somewhat subjective, but a good rule of thumb is that if the latter is publicly revealed, then the basic cipher can't easily be fixed up and must be abandoned. (Another rule of thumb is that a contemporary cryptography expert would have needed to think quite hard in order to understand how the exploit worked; it would not have been trivial to explain the exploit to them.)
The break must have actually been executed in the real world to actually read encrypted information "in the wild" or falsely authenticate without the sender's knowledge/permission. A demonstration that a break would be plausible to execute in practice doesn't count.
The algorithm must have been widely used, e.g. in an Internet, commercial, or governmental setting. Again, what counts as "widely" is a bit subjective, but it doesn't count if Alice invents her own cryptography algorithm for fun, uses it to send an encrypted message to Bob, and then their friend Charlie breaks it. A good rule of thumb for what counts as "widely" is that this cryptography algorithm was used by people who had no direct connection to the algorithm's creator.
(Now, I acknowledge that by the very nature of cryptography, anyone who has developed such a break would have a strong incentive not to publicize that fact, so any such breaks may not be public knowledge. But I'm curious if any of them have been publicly revealed.)