I've a website that users submit their personal data to, and I'm thinking of encrypting these data using AES-256 and their password is used as a key for that encryption and then I store the encrypted data in a MySQL database...
Now if the user changes his/her password, how would I change the key of the encrypted data?
Should I gather all the data from the database, decrypt their data with the old key, and then encrypting it again with a new key?
You don't need to re-encrypt all of the user's data when they change their password.
Generate a secret key to encrypt a user's data; call this the "content encryption key." Derive a key from the user's password; call this the "key encryption key." Encrypt the "content encryption key" using the "key encryption key." Store the encrypted key along with the salt and the number of iterations used for key derivation.
If they change their password, decrypt the content encryption key with the old password, and re-encrypt it with a key derived from the new password. You should choose a new salt for the new password, and make sure you store it along with the new encrypted key.
Because the content encryption key is randomly chosen from a huge space, you can safely use ECB as the cipher mode when encrypting it.
Don't simply hash the password, even if you use salt or even if you use an as-yet-unbroken algorithm. You need to repeat the hashing operation thousands of times. There are libraries for doing this (correctly) on most platforms. Use a key derivation algorithm (PBKDF2, from PKCS #5) to create a secret key from a password.
This concept follows the draft for password-based S/MIME encryption.
One possibility to consider decouples the key used to encrypt the data from the key used to gain access to the data. Done carefully, this allows the user to change their password as often as they desire, while you only change one record in the database. Separately, you can schedule changes to the key(s) encrypting their data when it is convenient for you.
How does it work?
You encrypt the data D for user U with a randomly generated key, KU,D.
You encrypt the key KU,D with a separate key K1U,K generated from a random salt, S1U (which you keep a record of) and the user's password P1U (which you may or may not keep track of). The encrypted key is E1U.
You store S1U and K1U,K ready for when the user wants to access their data.
When user U wants to access their data, they provide you with their password, P1U, and you look up S1U and regenerate K1U,K from that data, and use that to decrypt E1U, giving you KU,D once more, with which you decrypt their actual data.
You ensure you can detect when the password given is correct so you don't spew forth binary gibberish if the users types the wrong password.
The advantage of this level of indirection comes when the user wants to change their password. If you don't use some technique analogous to this, you will have to get and validate the old password and the new password, decrypt all the data with the old password, and re-encrypt it all with the new password.
With the level of indirection, you still prompt the user for their old password (P1U) and their new password (P2U) and validate them, but you only have to decrypt E1U and then re-encrypt it with a new key K2U,K generated from a new salt S2U and the new password P2U. You do not have to touch the encrypted data at all.
With the level of indirection, the system S can also keep a second encrypted copy of the data key KU,D, encrypted with the system's password. If it becomes necessary or desirable to change the key used for encrypting the data, the system can use its encrypted copy of the key to do so. It can keep a record of which key was last recorded by the user in their key, so when the user returns to look at the data, it can arrange to to change the stored key K2U,D because at that time, it has their password (the rest of the time, it does not).
This is a mild variation on some of the ideas in "Cryptography in the Database: The Last Line of Defense" by Kevin Kenan. The KnU,K keys are examples of a KEK, a Key-Encrypting Key. You could also read about key families in the book, which would help with the management of encrypted data.
First, you generally shouldn't use the password as an AES key. Maybe something like a cryptographic hash (not MD5) of the password + a salt (you would store the salt but not the hash in this case).
One thing you could do is encrypt each user's files with a random key, then encrypt that key with the hashed+salted password. If the user changes passwords, you only have to re-encrypt the key.
That’s silly.
AES uses a 256-bit key, so when you say that you will be using their password for the key, it won’t be nearly as long as the key size requirement.
Related
I am creating an application where I need to store client's information(Like their API Keys and API Secret to access my service, along with other confidential information).
Now, in the database, I want to store these in the encrypted format. In this regard, I decided to with symmetric key cryptography, AES in specific to encrypt the details.
However, for security purposes I want to use a different AES encryption key on a per client basis, so that even if the DB is compromised, all the data cannot be decrypted using a single key.
However, due to obvious reasons, I do not want to store my private keys in the DB with the encrypted informations.
So, I cannot seem to decide how to store my keys, especially since I need to have a binding that which key belongs to which client.
How can I achieve this, and which is the best approach in scenarios like this?
Use a KDF to derive an encryption key from the users password and then use this key to encrypt their private information.
When any action is to be taken that requires their API secret or whatever other private data you are storing, simply request the users password and use it to derive the key again and use the key to decrypt.
If you want users to be able to change their password, add an intermediary random key for each user and use this key to encrypt their data. Use the key derived from their password to encrypt the random key. Then when changing the users password, you only need to decrypt and re-encrypt the random key.
I store some data which is encrypted, and the key is generated based on the user's password (after hashing and salting) using PBKDF2.
I can implement password change requests simply by decrypting the data with the old password, then reencrypting the data with the new password.
However, how do I go about implementing password reset, when the user forgets their password? Should I be storing things differently to support this use case? If so, how?
I can implement password change requests simply by decrypting the data
with the old password, then reencrypting the data with the new
password.
However, how do I go about implementing password reset, when the user
forgets their password? Should I be storing things differently to
support this use case? If so, how?
Typically you use a Content Encryption Key or CEK. The CEK is a random key, and you use it to key you block cipher and MAC that protects the file. Each file gets its own CEK and other crypto parameters, like and IV or nonce.
Then you use a Key Encryption Key or KEK. In you case, the KEK would be the output of you PBKDF. The KEK encrypts the CEK.
For recovery, you create a recovery key. Then you encrypt the CEK again under the recovery key, and move the encrypted CEK somewhere safe.
When it comes time to change passwords, you just re-encrypt the CEK under the new PBKDF derivation. No need to mess with the recovery key since its safe somewhere else. (Or as safe as it can be with governments issuing NSLs with no oversight).
I have the need to store private keys for multiple users, so that my server application can sign files on their behalf.
I want to store the private keys securely, but I couldn't find best practices around this. If I was storing a password I would salt+hash the password to make a hash that can't be easily turned back into the password. However, with a private key I need to store it in a way I can later retrieve it.
I was thinking I would encrypt the private key and then store it in my database. I originally thought each key would be encrypted with a different password (based on some properties of the user). However, those properties would most likely be stored in the database, so if my database got leaked then the attacker has everything.
I could encrypt all private keys with a single password that is only known to my application. Then an attacker would have to steal my database, and my application to do any harm.
Is there a technique/best practice I'm missing?
You could encrypt the private key with a symmetric key based on the users password. Simply store an additional salt and perform the password "hash" to get a separate key. Then use that as key for encrypting the private key. Note that it is required to use a Password Based Key Derivation Function (PBKDF) such as PBKDF2, bcrypt or scrypt to create a secure password hash (given the normal security level of a password).
If the user is not online at the time that a signature needs to be generated, then you should indeed protect the passwords in a sense that only you / our backoffice can decrypt the keys. You can use some user ID + your own secret key to calculate an encryption/decryption key. You may even want to generate a separate RSA key pair to perform hybrid encryption decryption.
Storing private keys on behalf of users is a very dangerous practice. There are a lot of ways for the private key to become exposed (e.g. side channel attacks). To do it professionally you should really be using an HSM somewhere in the process. If this is for any serious data, please consult a professional and a lawyer.
Given a raw password
create an unique salt
append it to the raw password
brcypt / SHA512 this combination using disposable secret key that changes over time
stores the encrypted password and salt in the user table
To verify identity
append the salt to the raw password
bcrypt / SHA512 the combination verify
checks the hashed against the db hashed value
In the verify part, what if the current secret key is no longer the same?
Should I always keep a list of old key and iterate them through to verify that that old password is generated with one of the old keys? If verifier returns true, I will update the new encrypted password.
Also, how can I be sure the salt is unique per-password-per-user?
Is this all I need to do?
Any thing missing? Thanks.
Making sure the salt is unique is easy - you could just hash together the username and the time when the password was last changed.
As for encryption, that's only necessary if for some reason you want to store the user's password, not only be able to verify it. It's more common and more secure to only store a hash of the password and salt. No secret key is necessary and even if an attacker compromises the database, they will have troubling recovering passwords. The only downside is that you can't tell users their passwords, you can only reset them.
So I need to be able to actually decrypt the password because some old websites don't use OAuth and I need to have some of my users have access to them through there API. Therefore one way hashing does not work for me. I have been told that the best way to store the passwords with AES and using the salted hash of the passwords as the key. How do I store all the keys and where do I store the salt? Basically how would I exactly implement this.
Thank You
If I understand you correctly you have the following situation. Users log in to your system with a username and password. Your system then needs to impersonate these users by logging into another system that you do not control using the user's username and password for that system which they have given to you.
If that is correct, then it might be reasonable to encrypt their credentials for the other websites using AES. In this case, I would not store the key used to encrypt those crendentials. The password that the user uses to access your system should be used as the key, and it should not be stored anywhere. In this way, you have a chance of protecting your users privacy (depending on the design of the rest of the system, of course).
Since you are encrypting rather than hashing, and the encryption key would be differnet for each user, salting is not necessary.
Also, I would encrypt the full credentials, not just the passwords. Your users are showing an incredible amount of trust by giving you their credentials in the first place, so I would do everything possible to justify that trust!
Your approach is essentially to use AES as a hash function but this will not allow you to decrypt the passwords (short of brute force or some yet-to-be-discovered vulnerability).
You might want to consider asymetric key encryption instead (e.g. RSA). You'll need to encrypt the passwords with the public key of each person you expect would need to decrypt it (which would include the user).