Add RsaOaep256 algorithm (#31)

Author: grantpaulson6

README.md

@@ -96,7 +96,7 @@ Only a subset of these algorithms is implemented in this gem. Striked elements a
 Key management:
 * RSA1_5
 * RSA-OAEP (default)
-* ~~RSA-OAEP-256~~
+* RSA-OAEP-256 (if OpenSSL::VERSION >= '3.0')
 * A128KW
 * A192KW
 * A256KW

lib/jwe/alg.rb

@@ -5,6 +5,7 @@
 require 'jwe/alg/a256_kw'
 require 'jwe/alg/dir'
 require 'jwe/alg/rsa_oaep'
+require 'jwe/alg/rsa_oaep_256' if OpenSSL::VERSION >= '3.0'
 require 'jwe/alg/rsa15'
 
 module JWE

lib/jwe/alg/rsa_oaep_256.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module JWE
+  module Alg
+    # RSA-OAEP-256 key encryption algorithm.
+    class RsaOaep256
+      attr_accessor :key
+
+      def initialize(key)
+        self.key = key
+      end
+
+      def encrypt(cek)
+        key.encrypt(cek, { rsa_padding_mode: 'oaep', rsa_oaep_md: 'sha256' })
+      end
+
+      def decrypt(encrypted_cek)
+        key.decrypt(encrypted_cek, { rsa_padding_mode: 'oaep', rsa_oaep_md: 'sha256' })
+      end
+    end
+  end
+end

spec/jwe/alg_spec.rb

@@ -55,6 +55,29 @@
   end
 end
 
+if OpenSSL::VERSION >= '3.0'
+  describe JWE::Alg::RsaOaep256 do
+    let(:alg) { JWE::Alg::RsaOaep256.new(key) }
+
+    describe '#encrypt' do
+      it 'returns an encrypted string' do
+        expect(alg.encrypt('random key')).to_not eq 'random key'
+      end
+    end
+
+    it 'decrypts the encrypted key to the original key' do
+      ciphertext = alg.encrypt('random key')
+      expect(alg.decrypt(ciphertext)).to eq 'random key'
+    end
+  end
+else
+  describe JWE::Alg do
+    it 'raises an error for rsa-oaep-256 if openssl < 3.0' do
+      expect { JWE::Alg.for('rsa-oaep-256') }.to raise_error(JWE::NotImplementedError)
+    end
+  end
+end
+
 describe JWE::Alg::Rsa15 do
   let(:alg) { JWE::Alg::Rsa15.new(key) }