I’m working on adding the option to encrypt uploaded files on my website by giving each file a “password”, but I’m not sure of the best way to keep track of the encryption keys for each individual file. For example, when retrieving the file if the user enters the wrong password I’m pretty sure it’ll download the file, but it won’t be unencrypted properly. Is it best to store the hashed & salted passwords for each file in a database and match up there first? Or is there a better way to do it?
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If you store the encryption keys as hashed (and salted) values, there is no way to retrieve the original encryption key. If you hash something (with a strong hashing algorithm), you cannot get the original back.
The best mechanism depends on the operating environment (OS, plus potentially other software installed), and on the requirements.
Depending on the specific requirements, it may be best not to store the encryption key at all. The point is that the user wants to protect data and the user knows the key. If it is possible to perform the encryption on the client side, the key would never have to traverse the network at all (e.g. encrypt in JavaScript). If the encryption must happen on the server, ensure the key is sent via an https connection, use it in memory to encrypt the file, and remove from memory (if your language supports immediately removing something from memory… e.g. in C# use a SecureString).
If you must keep the encryption key (which is a security issue on several levels), you will have to state more information about the operating environment.