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I must admit, I hardly remembered.
S.E.T. - short for Secure Electronic Transaction(s) - was an attempt by VISA (and others) to establish secure protocols for electronic payments.
I think we even implemented this in an online shop, around 2000. At least we had a plan to do so, not sure if we really did...
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And while you're at it, check out WAP as well, should go to the same