How to sign and verify a message with HMAC-SHA256 in Ruby

1require "openssl"
2 
3module Webhook
4  module_function
5 
6  # Sender: HMAC-SHA256 over the exact body bytes, sent as "sha256=<hex>"
7  def sign(secret, raw_body)
8    "sha256=#{OpenSSL::HMAC.hexdigest('SHA256', secret, raw_body)}"
9  end
10 
11  # Receiver: recompute over the raw body and compare in constant time
12  def verify(secret, raw_body, header)
13    return false if header.nil? # a missing header fails, it never skips the check
14 
15    OpenSSL.secure_compare(sign(secret, raw_body), header) # false, not an error, for another length
16  end
17end
18 

1require "json"
2require "sinatra/base"
3require_relative "webhook"
4 
5class WebhookApp < Sinatra::Base
6  set :secret, ENV.fetch("WEBHOOK_SECRET") # fails at start-up when unset
7 
8  post "/webhook" do
9    raw_body = request.body.read # the bytes as received, before any JSON parsing
10    signature = request.env["HTTP_X_HUB_SIGNATURE_256"] # how Rack names X-Hub-Signature-256
11    halt 401, "bad signature\n" unless Webhook.verify(settings.secret, raw_body, signature)
12 
13    event = JSON.parse(raw_body)
14    "accepted #{event['action']} ##{event['number']}\n"
15  end
16end
17 

How it works

1. OpenSSL::HMAC.hexdigest('SHA256', secret, raw_body) returns the 64 lower-case hex digits; the header is sha256= followed by them.
2. OpenSSL.secure_compare(a, b) hashes both strings and compares the digests in constant time, so it returns false — never raises — for inputs of different lengths. Plain == stops at the first differing character, and its timing tells a forger how much of a guess was right.
3. In Sinatra, request.body.read returns the body bytes as received; the route verifies them before JSON.parse. Rack exposes the header as request.env["HTTP_X_HUB_SIGNATURE_256"].
4. ENV.fetch("WEBHOOK_SECRET") raises at start-up when the secret is missing, instead of signing with an empty string.
5. The run checks GitHub's example (Hello, World!, secret It's a Secret to Everybody → sha256=757107ea…3e17) and drives the route with Rack::MockRequest.

Gotchas

• OpenSSL.fixed_length_secure_compare raises ArgumentError: inputs must be of equal length for a truncated header; use secure_compare, which returns false, unless you check lengths first.
• The MAC covers bytes, not JSON: the pretty-printed event re-generated with JSON.generate(JSON.parse(body)) is rejected with 401. Verify request.body.read, not a re-serialized params hash.
• A missing header must fail the request (verify returns false for nil), not be read as "nothing to check".

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