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I want user's to be able to select a "remember me" box on my website so they need not log in each time they come. So, I need to store a unique ID in a cookie to identify them. Is it safe to hash their password with sha512 and a long salt in PHP and store that value in the cookie? If the cookie was stolen, would their password be at risk? Obviously it must be connected to their password somehow, otherwise if the cookie value was guessed or stolen, the user would not be able to stop someone else logging in.
Also, is it advisable to use a GUID at all as the unique identifier?
Thanks, Ben
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You should never store sensitive data in a cookie, such as user names, passwords, credit card numbers, and so on. Do not put anything in a cookie that should not be in the hands of a user or of someone who might somehow steal the cookie. Similarly, be suspicious of information you get out of a cookie.
Why Is Cookie Hashing Important? Cookie hashing provides increased security and privacy over non-hashed cookies because a cookie hash can only be decoded and read by the website that created it.
Hashing and encryption both provide ways to keep sensitive data safe. However, in almost all circumstances, passwords should be hashed, NOT encrypted. Hashing is a one-way function (i.e., it is impossible to "decrypt" a hash and obtain the original plaintext value). Hashing is appropriate for password validation.
Hashing allows passwords to be stored in a format that can't be reversed at any reasonable amount of time or cost for a hacker. Hashing algorithms turn the plaintext password into an output of characters of a fixed length.
Remember, the hash of the password is effectively the same as their password. Somebody who stole the hash would have the same access to the user's account as if they had stolen their password. Therefore it is not advisable to store a hash of the user's password in a cookie unless there was some other information not stored with the cookie that is used to authenticate (i.e. 2-factor authentication).
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