[nayuki]

As a less extreme example, an implementation of the AES cipher in Excel formulas (no macros): https://www.nayuki.io/page/aes-cipher-internals-in-excel

[bonzini]

That reminds me of when my mother-in-law asked me if you could compute IBAN check digits in Excel. The suggestion by IT had been to cut-and-paste customer data into a random "compute IBAN check digit" website, one customer at a time, because they couldn't be arsed to update the database automatically.