Verify signed data vault offline - ruby

Question

I'm sending to a client a public_key and a payload plus a signature.
I'm trying to verify the signature on the client using the public_key and payload but I can't find a way to get the data verified. Every time I receive false as a response regardless of what I'm doing.
I'm using vault to sign the payload
def sign(box_identifier, data) do
Vaultex.Client.write("transit/sign/#{box_identifier}",
%{
input: data,
hash_algorithm: "sha2-256"
},
#authentication_strategy,
{#token})
end
Vault.Transit.sign("coucou", Base.encode64("test"))
_____________________________________________________________
digest = OpenSSL::Digest::SHA256.new
key = OpenSSL::PKey::RSA.new File.read 'key.pem'
key.public_key.verify digest, signature, "test"
-----BEGIN PUBLIC KEY-----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-----END PUBLIC KEY-----
Am I even capable of validating the signature offline when its signed by vault ?

Answer 1

Yes, it's possible to verify the signature offline. My answer is going to mention some Ruby specifics, since I didn't notice this was Elixir until after I started researching a bit more.
By default, when signing with a RSA key, Vault uses the PSS algorithm for the signature. The verify method on OpenSSL::PKey::PKey is expecting PKCS #1 v1.5. Here's some useful info about the pros and cons of the two.
In addition to this algorithm difference, you need to strip off the vault:v1: from the returned signature from Vault.
Here's some sample Ruby code to sign and then verify the signature:
transit_key = "test_key"
message = "test"
# Returns something like:
# vault:v1:B3reNpf8e/WyAYzBzyWz3oSUM...
signature = Vault.logical.write(
"transit/sign/#{transit_key}/sha2-256",
input: Base64.encode64(message),
signature_algorithm: "pkcs1v15"
).data[:signature]
signature = signature.split(":")[2]
# Gives us the PEM encoded public key
# -----BEGIN PUBLIC KEY-----
# ...
public_key = Vault.logical.read("transit/keys/#{transit_key}").data[:keys][:"1"][:public_key]
public_key = OpenSSL::PKey::RSA.new(public_key)
digest = OpenSSL::Digest::SHA256.new
puts public_key.verify(digest, Base64.decode64(signature), message) # returns true
puts public_key.verify(digest, Base64.decode64(signature), message + "modified") # returns false
Looks like as of Ruby 2.5, there's a new verify_pss method on the OpenSSL::PKey::RSA class.
signature = Vault.logical.write(
"transit/sign/#{transit_key}/sha2-256",
input: Base64.encode64(message),
signature_algorithm: "pss"
).data[:signature].split(":")[2]
puts public_key.verify_pss(digest, Base64.decode64(signature), message, salt_length: :auto, mgf1_hash: "SHA256") # returns true
puts public_key.verify_pss(digest, Base64.decode64(signature), message + "modified", salt_length: :auto, mgf1_hash: "SHA256") # returns false