support arbitrary headers during encryption

Author: zarqman

README.md

@@ -73,6 +73,19 @@ plaintext = JWE.decrypt(encrypted, key)
 puts plaintext #"The quick brown fox jumps over the lazy dog."
 ```
 
+This example sets an extra header.
+
+```ruby
+require 'jwe'
+
+keys = {
+  'id-1' => OpenSSL::PKey::RSA.generate(2048)
+}
+payload = "The quick brown fox jumps over the lazy dog."
+
+encrypted = JWE.encrypt(payload, keys['id-1'], headers: {kid: 'id-1'})
+puts encrypted
+
 ## Available Algorithms
 
 The RFC 7518 JSON Web Algorithms (JWA) spec defines the algorithms for [encryption](https://tools.ietf.org/html/rfc7518#section-5.1)

lib/jwe.rb

@@ -19,14 +19,15 @@ class InvalidData < RuntimeError; end
   VALID_ENC = ['A128CBC-HS256', 'A192CBC-HS384', 'A256CBC-HS512', 'A128GCM', 'A192GCM', 'A256GCM'].freeze
   VALID_ZIP = ['DEF'].freeze
 
-  def self.encrypt(payload, key, alg: 'RSA-OAEP', enc: 'A128GCM', zip: nil)
+  def self.encrypt(payload, key, alg: 'RSA-OAEP', enc: 'A128GCM', zip: nil, headers: {})
     raise ArgumentError.new("\"#{alg}\" is not a valid alg method") unless VALID_ALG.include?(alg)
     raise ArgumentError.new("\"#{enc}\" is not a valid enc method") unless VALID_ENC.include?(enc)
     raise ArgumentError.new("\"#{zip}\" is not a valid zip method") unless zip.nil? || zip == '' || VALID_ZIP.include?(zip)
     raise ArgumentError.new('The key must not be nil or blank') if key.nil? || (key.is_a?(String) && key.strip == '')
 
     header = { alg: alg, enc: enc }
     header[:zip] = zip if zip && zip != ''
+    header.merge!(headers) if headers.is_a?(Hash)
 
     cipher = Enc.for(enc).new
     cipher.cek = key if alg == 'dir'

spec/jwe_spec.rb

@@ -29,6 +29,18 @@
     end
   end
 
+  describe 'when using extra headers' do
+    it 'roundtrips' do
+      encrypted = JWE.encrypt(plaintext, rsa_key, headers: {kid: 'some-kid-1'})
+      result = JWE.decrypt(encrypted, rsa_key)
+      header, _ = JWE::Serialization::Compact.decode(encrypted)
+      header = JSON.parse(header)
+ 
+      expect(header['kid']).to eq 'some-kid-1'
+      expect(result).to eq plaintext
+    end
+  end
+
   it 'raises when passed a bad alg' do
     expect { JWE.encrypt(plaintext, rsa_key, alg: 'TEST') }.to raise_error(ArgumentError)
   end