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As I am fairly new to cryptography, I would like to understand how to, in a simple way, implement a system that would achieve the following: the user would have to setup a password, which would then be used to:

1.) encrypt the data provided by the user and save it in an encrypted form and 2.) to authenticate the user when using the system the next time and decrypt his data.

When searching for viable solutions I found the following two approaches:

a.) The password is hashed and the hash is later used to authenticate the user. The combination of the plain text password and its hash is used to generate a key for the encryption and decryption.

b.) The password is hashed and a key is derived from its hash to encrypt the data. The authentication is solved by comparing some know plain text to a part of the decrypted data. If there is a match, the user entered the correct password.

I would be grateful for a simple and clear explanation.

I would be also thankful for any references on this specific topic.

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  • $\begingroup$ Where did you see both? They are not good approaches since the user may want to change the passwords. You create a random encryption key per file, and encrypt the key with the key-encryption key generated from password like PBKDFDF2 or better Argon2id. You can see here in more detail. When data at rest, the files are protected with random keys and only your password can open it. A good password like from dicewire is recommended. $\endgroup$
    – kelalaka
    Commented Oct 14, 2021 at 7:29
  • $\begingroup$ @kelalaka Can you please explain how to also solve the user authentication part with the same password. Thank you. $\endgroup$
    – simbr
    Commented Oct 14, 2021 at 7:39
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    $\begingroup$ It is just the standard password hashing How to securely hash passwords?. You have one password and separate the domains by salt and info. $\endgroup$
    – kelalaka
    Commented Oct 14, 2021 at 7:44
  • $\begingroup$ Do NOT send everything that is needed for decryption to any server, do decryption locally. Otherwise a compromised server can intercept it and decrypt eveything. $\endgroup$
    – jjj
    Commented Oct 14, 2021 at 10:28

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