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The Setup

I'm making a password manager for fun. It stores a random encryption key, encrypted with a key derived from the master password, and then it stores the data, which is a list of (site, username, password) triples, encrypted with the encryption key. Here, encryption means calling libsodium's secretbox, which is authenticated encryption.

The Problem

I wanted to avoid decrypting every single password just to get a single one. Since the database still needs to be searchable, all the metadata still needs to be unencrypted in one go. So I came up with two different strategies:

  1. Keep the passwords in a separate dictionary, indexed by hash(encryption_key ++ data).
  2. Encrypt the password before storing it in the triple, which is then encrypted again. This idea sounds terrible, because double encryption scares me.

My questions are:

  1. Is my setup secure?
  2. Is it reasonable to avoid decrypting every password?
  3. Which of the two aforementioned strategies are secure, and is there a simpler way to encrypt passwords only as necessary?

Sorry if this question is too specific, I'd be happy to try to make it more general.

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  • $\begingroup$ So the idea of the first strategy is to break the mapping data <=> password without the encryption key but still technically decrypting the passwords? $\endgroup$ – SEJPM Aug 11 '18 at 17:55
  • $\begingroup$ I wrote that because otherwise someone could use this file to verify the person's online username, but I guess it helps with that, too. $\endgroup$ – Jotaro Kujo Aug 11 '18 at 19:23
  • $\begingroup$ Could you please elaborate a bit on what "the triple" is from your second strategy and what is encrypted how for the first strategy? $\endgroup$ – SEJPM Aug 12 '18 at 8:45
  • $\begingroup$ Double encryption with independent keys should be just fine. My first thought is I'd just derive separate metadata and password encryption keys from the master key (with crypto_kdf if you're using libsodium), encrypt those two types of data with them, and not worry about double encryption. $\endgroup$ – Luis Casillas Aug 13 '18 at 22:15
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I understand your design goal, but I see an issue in that you propose to encrypt the metadata alongside the passwords. Searching is made more complicated because you have to decrypt the database to expose the metadata to search for the associated password.

Instead, consider storing only encrypted, unique passwords in your database, indexed by a secure hash of the site/username metadata. For any new site that requires a password, locate a slot indexed by $H(meta)$. Generate a unique password for that site, encrypt it with the master, and store it at the hash slot. Give the plaintext to the website.

To access the password later, reverse the steps. This lets you encrypt only the passwords, and uses a strong hash function to protect the mapping between the site and the password even in the event of a disclosure of the master password.

[A few edits later,] Here is a protocol that might work. It assumes you have a secure hash function $H$, a wallet $W$ indexable as a dictionary with no entries to start, and a secure master password $m$. Assume $E_p(data)$ means to encrypt data with key $p$, and $D_p(data)$ means to decrypt. Let $meta$ be a tuple representing the site name, user name, cookie, or anything else that makes sense to include for retrieving a password.

When visiting a site $s$:

  1. Let $i \leftarrow H(meta)$.
  2. If $i \not \in W$, then this is the first visit to the site (with this meta tuple); generate unique password $p_s$ (conforming to the policy at $s$) and encrypt with $E_m(p_s)$ and store at $W[i]$.
  3. If $i \in W$, then site has been visited before, therefore retrieve site password $p_s \leftarrow D_m(W[i])$.

The benefits are that you decrypt only a single password for each site $s$. The master password $m$ is used only for site password protection, not for the metadata. The identity of the sites and meta information are protected with a hash function. With a fast implementation of $W$, lookup can be as fast as $O(1)$. A dictionary indexed by a hash can grow dynamically, better than my previous edit.

On the downside, if the adversary has access to $W$ and meta information on $s$, he can tell that you have an account on that site. By encrypting the entire database including metadata, your original design is secure against this threat.

I haven't thought more than a day on this, so there may be a weakness - hopefully someone will comment and I'll continue to revise it.

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  • $\begingroup$ Sorry if I misunderstood, but doesn't that make it impossible to search through the list of passwords? That would be inconvenient for the user. $\endgroup$ – Jotaro Kujo Aug 11 '18 at 19:18
  • $\begingroup$ The goal is to not search through the list of passwords. The goal is to encrypt the passwords. In my design, you use a hash of the desired metadata to index into a list consisting only of encrypted blobs. If you don't know the metadata, you should not be able to decrypt the password. (This handles the case of hash collisions for non-matching sites as well.) $\endgroup$ – Russ Aug 12 '18 at 1:58
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    $\begingroup$ "I haven't thought too long on this" Neither have I, but one thing that comes to mind if you're using H(meta) to both index and encrypt passwords: remember to re-encrypt/-index if the meta data ever changes. $\endgroup$ – TripeHound Aug 12 '18 at 6:42
  • $\begingroup$ @TripeHound, the latest version is much different and addresses this problem. Thanks for the insight! $\endgroup$ – Russ Aug 12 '18 at 11:34

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