How to verify a webhook with Ruby? - ruby-on-rails

Question

I'm using Ruby 2.5.5 and Rails 5.2. I'm trying to verify a Paddle webhook, but can't get the verification to work. I tried various ways, here is one of them. I'll go through it on my console:
Here is the data:
data = {
"alert_id"=>"1",
"alert_name"=>"alert_created",
"cancel_url"=>"https://...",
"checkout_id"=>"1",
"user_id"=>"1",
"p_signature"=>"fwWXqR9C..."
}
public_key = "-----BEGIN PUBLIC KEY-----
FAKEdfsdfsdfsdfsdfsdfsdf23423423423423KCAgEAnyBxU0cUW9NGQU1Ur0MD
/M+VxdMsv7PMXsZFAKEKEYlskdfjlskdjflsjlskdj8i7+D9xZT57VhIfv8hInGy
IQFmJRrWxgmHjSW4cKbjg6Qg0NgOEuPdEIixsQZ/AfIar4KNObDul0EXL92B6ru/
...
-----END PUBLIC KEY-----"
Then I do the following:
signature = Base64.decode64(data['p_signature'])
data.delete('p_signature')
data.each {|key, value|data[key] = String(value)}
data_sorted = data.sort_by{|key, value| key}
data_serialized = data_sorted.to_json
digest = OpenSSL::Digest::SHA1.new
pub_key = OpenSSL::PKey::RSA.new(public_key)
verified = pub_key.verify(digest, signature, data_serialized)
In the end verified is always false. What am I doing wrong? I asked a related question here, but couldn't get it yet to work.

Answer 1

Example in paddle documentation that you've linked to suggests serializing data to PHP format before verification using php-serialize gem, that differs from regular json:
# ... data = ...
data_sorted.to_json
=> "[[\"alert_id\",\"1\"],[\"alert_name\",\"alert_created\"],[\"cancel_url\",\"https://...\"],[\"checkout_id\",\"1\"],[\"user_id\",\"1\"]]"
PHP.serialize(data_sorted, true)
=> "a:5:{s:8:\"alert_id\";s:1:\"1\";s:10:\"alert_name\";s:13:\"alert_created\";s:10:\"cancel_url\";s:11:\"https://...\";s:11:\"checkout_id\";s:1:\"1\";s:7:\"user_id\";s:1:\"1\";}"
as you can see - it gives different result, so obviously signature will not match.
Actually, you can take their whole ruby example:
require 'base64'
require 'php_serialize' # https://github.com/jqr/php-serialize
require 'openssl'
public_key = '-----BEGIN PUBLIC KEY-----
MIICIjANBgkqh...'
# 'data' represents all of the POST fields sent with the request.
signature = Base64.decode64(data['p_signature'])
data.delete('p_signature')
data.each {|key, value|data[key] = String(value)}
data_sorted = data.sort_by{|key, value| key}
data_serialized = PHP.serialize(data_sorted, true)
digest = OpenSSL::Digest::SHA1.new
pub_key = OpenSSL::PKey::RSA.new(public_key).public_key
verified = pub_key.verify(digest, signature, data_serialized)

Answer 2

I can't explain it completely but I think the false is related to your signature or data_serialized parameter, I use the same digest. Another difference is that sign is called on the key in the example below:
verify(digest, signature, data)
from: https://ruby-doc.org/stdlib-2.6.3/libdoc/openssl/rdoc/OpenSSL/PKey/PKey.html
To verify the String signature, digest, an instance of OpenSSL::Digest, must be provided to re-compute the message digest of the original data, also a String. The return value is true if the signature is valid, false otherwise. A PKeyError is raised should errors occur. Any previous state of the Digest instance is irrelevant to the validation outcome, the digest instance is reset to its initial state during the operation.
Example:
data = 'Sign me!'
digest = OpenSSL::Digest::SHA256.new
pkey = OpenSSL::PKey::RSA.new(2048)
signature = pkey.sign(digest, data)
pub_key = pkey.public_key
puts pub_key.verify(digest, signature, data) # => true`