Verifying web crypto signatures in erlang/Elixir

Crowdhailer

Creator of Raxx

I’m experimenting with using the web crypto API. and want to verify signatures from the browser in Elixir (or erlang, so far I have used only modules that are also available in erlang).

The steps in the browser are:

generate a key pair.
export public-key in PEM format
Sign message
Base64 encode the signature
Send the PEM public-key and encoded signature to the backend

The steps to verify on the backend:

decode PEM
decode signature
use :public_key.verify

The exact code I run is available in this gist.

This files can be used by copy pasting sign.js into the browser console and verify.ex into an iex session.

Doing this the verify function will return false.

I have tried several variations of how I encode/decode. I do need these encode steps because I eventually want to send the signature in the authorization header of a request.
Any help would be greatly appreciated.

12 comments

#erlang #crypto #web

3 4872 12

2021-11-19 17:00:07 UTC

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voltone

The JS API returns a ‘raw’ signature, whereas OTP’s :public_key API expects a DER-encoded signature.

Try this:

message = "abc"

encoded_signature = "AOqjfvb1P0fYdbnq+f3XsbOR/Lylq4csC5e1Ks4cKRTcWKQyH4CVr/XRHCmZDEWOpaA8gwuM3Z3qD21vMaixySUPASTDnd1J8shNx51MCbONIReTNuH53kzxykLpf0riSyMbEBQtie4/pxlrolUOsDja2f/ikgM/lGXtCTXz2faV4m2Y"

public_PEM = "-----BEGIN PUBLIC KEY-----
MIGbMBAGByqGSM49AgEGBSuBBAAjA4GGAAQB7PNQVbADLNyobtijE5NVZUvHs74h
iMntCbp0C8pdU1IQRWlAfDeEs/iuxA32VARw9Q5/0mim8Si8JcpCJnhS0u8AESMf
Ux3WqzHhB33t4q3iPsJbM7zmN91QNnbYErrGqEDCmSruPpKw1iK5dJ3/xQZbkpmR
ztoVwrZoCoGUu+WTqEI=
-----END PUBLIC KEY-----"

raw_signature = Base.decode64!(encoded_signature)
size = div(byte_size(raw_signature), 2)
<<r::binary-size(size), s::binary-size(size)>> = raw_signature
signature = <<48, 129, 136, 2, size, r::binary, 2, size, s::binary>>

[key_entry] = :public_key.pem_decode(public_PEM)
public_key = :public_key.pem_entry_decode(key_entry)

:public_key.verify(
  message,
  :sha256,
  signature,
  public_key
)
# returns true

Post #1

voltone

To clean up that code, here’s a module that can help convert from raw format to DER format using :public_key:

I might add it to x509 at some point…

Post #2

Crowdhailer

Creator of Raxx

Thanks for this. So to use the functions bellow I would use?

signature = ECDSASignature.new(raw_signature) |>ECDSASignature.to_der()

Also does the :public_key module not expect DER encoding for RSA keys? My original code seams to work for rsa signatures

Post #3

Exadra37

I am curious to know what are you trying to achieve with this approach of sending the signature in the authorization header of a request?

Post #4

voltone

Correct

An RSA signature is just a single integer, which is simply encoded as a binary by all implementations. An ECDSA signature consists of two integers. Some implementations wrap those in an ASN.1 ‘Sequence’ and others just concatenate them (which may require zero-padding of the smaller of the two, to remove ambiguity about where to split).

Post #5

Crowdhailer

Creator of Raxx

Just some experiments at the moment, but in general i’m looking at alternatives to passwords

Post #6

Exadra37

Client side generated secrets can be reverse engineered in mobile apps, that ship their code obfuscated in binaries, and in the web is even more easy to reverse engineer this process, thus anything generated programmatically in the client side cannot be trusted for authentication purposes.

Post #7

Crowdhailer

Creator of Raxx

What about Crypto.getRandomValues()?

Post #8

Exadra37

Sorry I was not meaning the code used to generate the secret… What I was trying to say is that any secret generated in the client side can be reused by an automated script to access the backend as the genuine web app or mobile app.

Another approach is that the automated script can use the same code of your app to generate the authorization secret, thus your backend will trust in requests from automated bots, when you want it to only trust in requests from your genuine web app or mobile app.

Post #9