I once used a system that stored old passwords as plaintext and searched for substring repetitions. Horrible.

Microsoft has that.

Not plaintext, but encrypted (not hashed) with the idea that they can be used for things like that.

https://docs.microsoft.com/en-us/windows/security/threat-pro...

This is a good idea, if the passwords are the ones people change *from* (i.e. once you change your password, it gets into that list). This way nobody can use the password anymore (with the idea that it is a weak now, for any reason).

This is selfish, though. If that database of passwords leaks, they are prime candidates to test on *other* sites.

If you encrypt the prior passwords using a key derived from the current password, you're enabling this sort of check on password change without really sacrificing security, don't you?

> If you encrypt the prior passwords using a key derived from the current password,

How can you do that with a prior password if you didn’t store it as plaintext when it was current? You can’t encrypt something you don’t have. Unless you are encrypting the old hash, not the password.

You would have it during the password change if you did old-new-new_again, yeah?

Yeah that was the idea. I guess a lot of apps don't actually do that and just email you password reset links, in which case you can't actually recover the old password. :<

This leaks data if a collision can be found and exploited.

Assuming the user uses the password in other places, this can be a bad thing.

The excellent (or horrible, depending on which end you're on) pam_pwquality[1] module for Linux allows rather fine-grained enforcement of how much a new password must differ from the old password.

1. https://www.systutorials.com/docs/linux/man/8-pam_pwquality/

That's why those of us in the security industry have to say "compliance is not security" whenever PCI is brought up.

PCI is just so asinine it made me want to poke my eyeballs out when I had to go through it (Level 1 Service provider). In some situations it actually prevents you from being more secure.

One of the reasons why I want the Credit card cartels to die.

well I've known more clever users, guess what you need to do if your password needs to change every 90 days, but you need to have a different password for at least 5 cycles?

correct, you do it like that:

- h@se2003 - h@se2006 - h@se2009 - h@se2012 - h@se2103 - h@se2106

you get the point ;-) bonus points for encoding the username into it and making it a thing for the whole company, yeah very secure!

This is how I used to do it. Just slap the year and Q on the end whenever it wanted me to change it. End of the quarter gets a bit tricky if things don't line up but it's mostly muscle memory by then

Sup3rdup3rp4ss2021Q2