I'm going to rephrase a question I asked earlier, as I don't think anyone understood what I meant.

Basically, I'm writing a web based password locker, which can handle multiple accounts; users can share passwords (useful if you're in a website company and need to share login details to, for example, 1&1 or Fasthosts).

I've been doing tons of research into encryption algorithms, CSPRNGs, hashing algorithms, and key stretching. I don't want to implement my own encryption or hashing algorithms. Instead, I want to use existing algorithms and apply them in a way that I've understood from my research.

I'm not a cryptographer, so I may have misunderstood something, or something I've read about has actually been found to be insecure, or it was an old way of doing things that something better replaced. I'd like to discuss what the thoughts are about implementing hashing for storing the passwords of user accounts and encryption for storing the login details of websites that users can share.


First of all, I need to store hashes of account passwords (these are the accounts that log into the application to view the shared passwords).
After reading around, my understanding is that to make a hash "more secure", you need to use scrypt or bcrypt or pbkdv2. I've found an implementation of these 3 algorithms in CryptSharp. (Hopefully, this is a secure implementation of these algorithms.) After further research, it looks like sCrypt is a more secure way of creating a hash.

Is scrypt a secure way to create a hash of a user's password, or should I use one of the other two? (Maybe sCrypt hasn't been tried and tested as much as the other two.) How can I tell if the implementation of sCrypt in CryptSharp is a good secure implementation?


As the data will be shared between users, I shall need to make sure that the system uses a master encryption key for the entire project, and that this is stored securely.

My thinking here is generating an encryption key and initialization vector (3 of them; I'll explain why). These will be generated with RNGCryptoServiceProvider, and they will be concatenated into Key1|IV1|Key2|IV2|Key3|IV3 so that they can be split and used with the encryption algorithms.

I need some way of storing this concatenated string of IVs and Keys. After some research, I came across Microsoft's Data Protection API (ProtectedMemory.Protect(secret, MemoryProtectionScope.SameLogon)). My understanding with this is that the App Pool will need to run under a user account, and "load profile" set to true. The resulting encrypted string will then be stored in an XML setting file.

With regards to encrypting the passwords to be shared, I read about cascading encryption (used in TrueCrypt). The encryption algorithms TrueCrypt uses are; AES, TwoFish, and Serpent. I've found an implementation of these which, again, I hope is a secure implementation. This is where the 3 IVs and Keys will be used; so effectively the data will be encrypted like Serpent(TwoFish(AES(data, key1), key2), key3).

Just to be clear, this is the process flow of encryption within the system:

  1. Upon installation of the system, the system will use RNGCryptoServiceProvider to create 3 encryption keys and 3 IVs.
  2. These 3 keys and IVs will be concatenated into a single string (Key1|IV1|Key2|IV2|Key3|IV3), so I can split them and use them in the 3 encryption algorithms.
  3. This string will then be encrypted using Data Protection API and stored in a settings file.
  4. All shared login detailsare encrypted using "cascading encryption" of Serpent(TwoFish(AES(data, key1), key2), key3). The keys and IVs will be decrypted on-the-fly, instead of being stored in memory.

When it's time to decrypt the shared login details:

  1. The 3 keys and decrypted using DPAPI
  2. Encryption is reversed: AES(TwoFish(Serpent(data, key3), key2), key1)

Is "cascading encryption" still a good idea? Is Data Protection API a good way to encrypt the keys and IVs?

So like I say, I know how to build the entire system and implement my understanding of hashing and encryption above into the system, but is what I've discussed the right way of doing things? Am I over-looking something? Did I misunderstand something? Am I using an old hat way of doing things?

  • 2
    Your new question isn't really better than the old one (just longer), and I think the misunderstanding is on your part. I didn't say that you're trying to invent your own algorithm. I said that you're trying to come up with your own protocol, which is just as bad. The mere fact that you ask us whether this is valid means that you've left the realms of established solutions and entered the do-it-yourself area. The only advice I can give you is: Don't do it. I realize that you don't like this answer, but anything else would simply be unprofessional.
    – Fleche
    Jul 4, 2014 at 12:17
  • I completely agree - the misunderstanding is on my part, which is why I tried to rephrase my question to hopefully expand on what I was trying to acheive. Also, your comment is actually helpful - I hadn't thought about that fact that I'm writting my own protocol, and that implementing my own protocol is as bad as implementing my own algoritm, so thanks making me aware of that.
    – binks
    Jul 4, 2014 at 12:31
  • I'll disagree slightly with Fleche and say actually, what you have isn't all that bad right up until you attempt to deploy encryption. Using bcrypt/scrypt is fine for hashing and storing the passwords.
    – AlexH
    Jul 4, 2014 at 14:43

1 Answer 1


I find your real question, which I consider it to be the following one, interesting.

How to share password between user of your service on Internet?

Let's start with some assumptions.

Architecture of your service

  • Each user have an account protected by a master password known only by themselves
  • Each user have a series of encrypted password associated with their account
  • The master password need to be hashed before storing it in the database

Now we need to be able to securely share these encrypted password.

There is a website that do something similar: LastPass.

One thing to note about LastPass is that all their encryption is done on the client side. LastPass uses your master password, the password you use to log into your account, to encrypt all your other password on the client side, your own machine.

This has the advantage that your master password never leaves your own machine and hence, unless your computer is compromised by a virus, it's impossible for anyone else than you to decrypt your other passwords, not even LastPass administrators. So your passwords are in the cloud but it's like if they were in your own head.

LastPass is also able to share password between users. I don't know how they do that, but I will take a random guess.

Using public/private key

Asymetric encryption is the key here. You can easily add a public key and a private key to the account. The public key need to be in plain text, so other are able to use it. The private key needs to be encrypted with your master password on the client side, like all your other passwords, before being stored on the server.

Now, if someone want to share a password with you, they just need to request your public key using your account name (or another alias); then they encrypt one of the passwords with your public key and send it to the website that will add it to your account.

When you log in, you will see that you have received a new password. You can then decrypt it on the client side using your private key, re-encrypt it using your master key, and store it back on the server.

  • 1
    From a high-level perspective, it always sounds trivial. Describing HTTPS in your style would look like this: “If you want to exchange data with a server in a secure way, you just request their public key and send them the encrypted data. Easy.” Well, we all know it's not easy at all. One of the problems with public-key cryptography is making sure that you indeed got the right public key. You can't just trust the server. And good luck implementing this correctly. I know that beginners like to challenge the “Don't roll your own” mantra. But there's a reason why almost everybody agrees on it.
    – Fleche
    Jul 4, 2014 at 13:54
  • @Fleche Here, you got a big advantage over implementing your HTTPS : you are already logged in to the server that store all your password which provide you a secure connection to transmit data and you can already trust the identity of that server. When that server gives you a public key, you can be sure it's the public key of the user you requested. If you requested the wrong user then it's your problem.
    – Gudradain
    Jul 4, 2014 at 13:58
  • @Gudradain you assume TOTAL security of the server, which is a factor of trust - it turns out you can use special whitespace characters in a username, it might appear identical to another user - thats only one example, but only any one thing needs to go wrong for the whole system to collapse, this applies more with password storage than virtually anything else (probably, no evidence/source just a very hungry gut) Jul 4, 2014 at 15:35
  • @user2867314 I assume TOTAL security of the server because that server is already storing all your passwords for nearly all the websites that you visit. If the server is not secure, you are in much bigger trouble than sending 1 password to the wrong person. Also, as I said before, if you decide to send your password to the wrong person then there is nothing anyone can do to stop you. You are the one typing the username so you are the only one to blame for a mistake.
    – Gudradain
    Jul 4, 2014 at 17:18
  • 1
    None of this makes a lot of sense. You started off by stressing that services like LastPass do not have access to the passwords, because all encryption happens client-side. That means you do not have to trust the server. Now you're suddenly switchting positions and describe the exact opposite: The server stores all passwords, and you need to blindly trust it. I don't even see how public-key cryptography fits into your new model. This isn't getting anywhere, and it's a great example why homegrown cryptography is harmful.
    – Fleche
    Jul 4, 2014 at 18:17

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