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As a theorist, I often motivate the need for strong cryptography via simplistic methods, such as "If this did not exist, your online bank transactions would be vulnerable". This is of course true if "this did not exist" means that we are in Minicrypt (or some weaker "world"), but I also sometimes use this to motivate switching off of particular weak cryptographic primitives. In this setting, it seems like social/legal conventions would mitigate the potential harm to the user --- for example, simple ways to attempt to leverage the total break of a cryptographic primitive (say transferring a ton of money into your personal bank account) would easily be undone through legal avenues.

In truth, I suspect that most of the hypotheticals that I can think of for how to nefariously exploit a broken cryptographic primitive in the wild would be easily reversed by law enforcement (perhaps I am just a bad cybercriminal though). That, coupled with my perception that "most" cybercrime does not rely on breaking the "actual" cryptographic primitives that are used (and rather implementation errors/social engineering), makes me curious if cybercriminals have ever attacked underlying cryptographic primitives as part of their criminal enterprise. I would be most interested in novel attacks / attacks against primitives that were thought to be strong beforehand, but as I suspect these do not exist I would also be interested in attacks against primitives that are known to be weak, provided they were against some notable company (where "notable" essentially means "something an average person may have heard of").

I want to exclude attacks by governments in this question, as these (especially via the NSA) tend to be fairly well-documented already, and seem to mostly sidestep the "law enforcement could/would undo the damage" argument.

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    $\begingroup$ Err, the second to last paragraph has no place in this site, AFAIK. $\endgroup$
    – kelalaka
    Apr 11, 2021 at 18:13
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    $\begingroup$ Did SolarWind involve actuak cryptography breaking though? I thought it was "just" a supply chain attack. I dont want to include things like that because their root cause could be "simpler" things like social engineering $\endgroup$
    – Mark
    Apr 11, 2021 at 19:06
  • $\begingroup$ And I include the second to last paragraph because if one doesn't, there are simple answers that perform much better in theory than they likely would "in real life". For example, one could MiTM banking credentials and transfer tons of money into your personal banking account. This attack is somewhat compelling for why we need strong cryptography, but also would be trivially "countered" by law enforcement, so is not actually a compelling example (although is vaguely related to an example which may be compelling). $\endgroup$
    – Mark
    Apr 11, 2021 at 19:13
  • $\begingroup$ I read it like show me the ways :). Well, banks check unusual activities, they have tons of mechanisms apart from cryptography. Also, poor entropy sources are another way to attack. $\endgroup$
    – kelalaka
    Apr 11, 2021 at 19:20
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    $\begingroup$ @kelalaka yeah, maybe I'll try rewriting the question. I think in hindsight what I really want to get at is applications of non-trivial cryptanalysis to cybercrime in particular. Among theorists there is a sense that cryptography itself usually doesn't break, and security vulnerabilities come from implementation issues/side-channels/social engineering (whether or not that is cryptography breaking is up to the reader). Clearly there are some threats of groups actually breaking cryptography, but they are usually government groups that people are worried about. $\endgroup$
    – Mark
    Apr 11, 2021 at 19:35

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An example that literally made the headlines in France in March 2000 involves factorization of the 321-bit RSA modulus that was a safeguard to the security of most debit/credit cards issued by French banks.

The incident is known as "YesCard". It started to surface publicly circa 1998. An individual factored the modulus used¹ to verify the issuer-made certificate of cards (certifying PAN, expiration date..). By proxy of a lawyer, he contacted the card issuing authority, trying to monetize his work. In order to prove his point, he made a handful of counterfeit Smart Cards and actually used them in metro tickets vending machine(s). He was caught and got a 10 months suspended sentence (judgment in French). In 2000 the factorization of the same key was posted (in French) on a public forum² and soon after, counterfeit Smart Cards burgeoned. These worked with any PIN, hence the name YesCard (in French) (other account in English). For a while, they caused some monetary loss, at least in vending machines, from pizzas to ski passes.

¹ That's even though 321-bit was clearly much too short. That fact was well-known in the Smart Card and banking terminal industry since 1990 at least, and publicly reported as early as 1988 (source, in French). I'm among several who tried to formally alert the bank issuers. Our pleas have not been enough to make the system change; only public exposure and actual fraud was. I got involved when the company I then worked for acquired (circa 1990 IIRC) the supplier of the DSP56000-based tamper-resistant device that computed the issuer certificates, and a maker of bank terminals. In a technology review I identified that the 512-bit (IIRC) practical upper limit of modulus in the tamper-resistant device was way too short. I was horrified to discover that the modulus actually used was 321-bit, and available to anyone purchasing the non-confidential specification for a nominal fee. I insisted the issue was formally reported to the customers using these modules. The letter went thru sales and legal thus ended worded as a disclaimer, but was still technically clear. Order for supplementary tamper-resistant devices from bank issuers followed nevertheless.

² While writing this answer, I find an account (in French) that the factors where "available on the internet" since autumn 1998.

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I would also be interested in attacks against primitives that are known to be weak, provided they were against some notable company (where "notable" essentially means "something an average person may have heard of").

The first example that comes to mind in TJX's use of WEP 5 years after the break was announced; that lead to millions of credit cards being stolen.

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