Bitwarden send feature

khoidang · March 15, 2021, 7:20pm

So Bitwarden has a new feature that allows you to share things using secure links. They claim end-to-end AES-256 encryption, but I’m not sure how this works. What process is used to generate the key and how does the person opening the link get the key? And if it’s just a link, how is it actually secure (I get that you could limit how many times it can be opened)? Perhaps these are naive questions.

SgtAwesomesauce · March 15, 2021, 8:06pm

This is interesting. When I have some time, I’ll investigate it.

WolfTech716 · March 15, 2021, 8:36pm

Just noticed it myself this morning, I’ll be testing it out with my wife to see how it works.

khoidang · March 15, 2021, 9:08pm

Ok, so apparently Mozilla had created something similar in the past. I imagine Bitwarden’s implementation has similar steps.

github.com

mozilla/send/blob/master/docs/encryption.md

# File Encryption

Send use 128-bit AES-GCM encryption via the [Web Crypto API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Crypto_API) to encrypt files in the browser before uploading them to the server. The code is in [app/keychain.js](../app/keychain.js).

## Steps

### Uploading

1. A new secret key is generated with `crypto.getRandomValues`
2. The secret key is used to derive more keys via HKDF SHA-256
    - a series of encryption keys for the file, via [ECE](https://tools.ietf.org/html/rfc8188) (AES-GCM)
    - an encryption key for the file metadata (AES-GCM)
    - a signing key for request authentication (HMAC SHA-256)
3. The file and metadata are encrypted with their corresponding keys
4. The encrypted data and signing key are uploaded to the server
5. An owner token and the share url are returned by the server and stored in local storage
6. The secret key is appended to the share url as a [#fragment](https://en.wikipedia.org/wiki/Fragment_identifier) and presented to the UI

### Downloading

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Trooper_ish · March 15, 2021, 9:12pm

was just going to ask if that is where the cancelled firefox thing went… (Okay, maybe not cancelled)

SgtAwesomesauce · March 15, 2021, 9:21pm

Yeah, that’s how I would do it.

WolfTech716 · March 15, 2021, 11:31pm

So it works pretty well except for some odd reason the password wouldn’t take. I’m sure I was doing something and not realizing it, but so far I dig it.

khoidang · March 17, 2021, 4:20pm

Looks like Bitwarden has a high level overview of their implementation as well. Not as detailed as Mozilla’s but looks like they added it.
https://bitwarden.com/help/article/send-encryption/

khoidang · March 18, 2021, 10:03pm

And they followed it up with a blog post.

WolfTech716 · March 19, 2021, 4:48pm

I used it with one of my parents via text (set to open only once and expire within the hour) and it worked perfectly, can recommend it from a user perspective, very straightforward and easy to use.