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If you have a website that need to store user's credentials for other sites, how do you store those credentials securely? An example is Mint.com where you give them your username/password for your bank accounts. Based on this quora answer, Mint.com uses a combination of software and specialized hardware which doesn't sound cheap or easy to build. What are some simpler pure-software solutions for storing credentials? Are there any open source software that's designed for this?