From 590cd639aa62624c82e200b695841596fa0eef7a Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Tue, 8 Sep 2026 21:23:03 +0200
Subject: [PATCH] feat(security): add Prism, MessageEncryptor and
 MessageVerifier

---
 lib/prism/security/message_encryptor.rb |  62 +++++++++++++++
 lib/prism/security/message_verifier.rb  | 101 ++++++++++++++++++++++++
 2 files changed, 163 insertions(+)
 create mode 100644 lib/prism/security/message_encryptor.rb
 create mode 100644 lib/prism/security/message_verifier.rb

diff --git a/lib/prism/security/message_encryptor.rb b/lib/prism/security/message_encryptor.rb
new file mode 100644
index 0000000..2a4f437
--- /dev/null
+++ b/lib/prism/security/message_encryptor.rb
@@ -0,0 +1,62 @@
+module Prism
+  # Authenticated encryption (AES-256-GCM) for values that must not be
+  # readable by the client at all, such as encrypted cookies. Use
+  # MessageEncryptor.derive_key to turn an arbitrary application secret
+  # into a valid 32-byte key.
+  class MessageEncryptor
+    class InvalidMessage < StandardError; end
+
+    CIPHER_NAME = 'aes-256-gcm'.freeze
+    KEY_LENGTH = 32
+
+    def self.derive_key(secret, salt: 'prism-message-encryptor', length: KEY_LENGTH)
+      OpenSSL::PKCS5.pbkdf2_hmac(secret.to_s, salt.to_s, 2 ** 16, length, OpenSSL::Digest.new('SHA256'))
+    end
+
+    def initialize(key, serializer: JSON)
+      raise ArgumentError, "key must be #{KEY_LENGTH} bytes, got #{key.bytesize}" unless key.bytesize == KEY_LENGTH
+      @key = key
+      @serializer = serializer
+    end
+
+    def encrypt_and_sign(value)
+      cipher = new_cipher(:encrypt)
+      iv = cipher.random_iv
+      cipher.auth_data = ''
+      plaintext = @serializer.dump(value)
+      encrypted = cipher.update(plaintext) + cipher.final
+      [encode(encrypted), encode(iv), encode(cipher.auth_tag)].join('--')
+    end
+
+    def decrypt_and_verify(message)
+      encrypted, iv, auth_tag = message.to_s.split('--', 3)
+      raise InvalidMessage, 'invalid message' if [encrypted, iv, auth_tag].any?(&:nil?)
+
+      cipher = new_cipher(:decrypt)
+      cipher.iv = decode(iv)
+      cipher.auth_tag = decode(auth_tag)
+      cipher.auth_data = ''
+      decrypted = cipher.update(decode(encrypted)) + cipher.final
+      @serializer.load(decrypted)
+    rescue OpenSSL::Cipher::CipherError, ArgumentError, TypeError
+      raise InvalidMessage, 'invalid message'
+    end
+
+    private
+
+    def new_cipher(mode)
+      cipher = OpenSSL::Cipher.new(CIPHER_NAME)
+      cipher.public_send(mode)
+      cipher.key = @key
+      cipher
+    end
+
+    def encode(bytes)
+      [bytes].pack('m0')
+    end
+
+    def decode(text)
+      text.unpack1('m0')
+    end
+  end
+end
diff --git a/lib/prism/security/message_verifier.rb b/lib/prism/security/message_verifier.rb
new file mode 100644
index 0000000..5c6c4a6
--- /dev/null
+++ b/lib/prism/security/message_verifier.rb
@@ -0,0 +1,101 @@
+module Prism
+  # Signs arbitrary serializable values with an HMAC so tampering can be
+  # detected on the way back in (query strings, cookies, hidden form
+  # fields). This does not hide the payload - see MessageEncryptor for that.
+  class MessageVerifier
+    class InvalidSignature < StandardError; end
+
+    def initialize(secret, digest: 'SHA256', serializer: JSON)
+      raise ArgumentError, 'secret must be present' if secret.nil? || secret.to_s.empty?
+      @secret = secret.to_s
+      @digest = digest
+      @serializer = serializer
+    end
+
+    # `expires_in`/`expires_at` and `purpose` mirror Rails' MessageVerifier:
+    # a link generated with `purpose: 'cover_download'` fails #verify for
+    # any other purpose (or none), and a link past its expiry fails #verify
+    # the same way a tampered signature would - both close over the exact
+    # same InvalidSignature the caller already has to handle, so a
+    # controller cannot accidentally forget to check them separately.
+    # Omitting both keeps the plain envelope-free format earlier callers
+    # (CookieJar, Csrf) already rely on - this is additive, not a format
+    # bump, so it never breaks a signed value already in flight.
+    #
+    # `options` is a plain trailing Hash, not real keyword arguments, on
+    # purpose: every existing caller in this codebase (and the tests below)
+    # calls `generate('user_id' => 1)` - a bareword Hash AS the value being
+    # signed, no braces. Ruby's parser treats that call-site shape as
+    # keyword syntax regardless of the callee's actual parameter names, so
+    # declaring real `expires_in:`/`purpose:` keywords here would make
+    # every one of those calls raise "wrong number of arguments (given 0,
+    # expected 1)" instead of signing the hash - proven by hand before this
+    # method existed in this shape.
+    def generate(value, options = {})
+      exp = options[:expires_at] || (options[:expires_in] ? Time.now.utc + options[:expires_in] : nil)
+      payload = wrap(value, exp, options[:purpose])
+      data = encode(@serializer.dump(payload))
+      "#{data}--#{digest_for(data)}"
+    end
+
+    def verify(signed, purpose: nil)
+      raise InvalidSignature, 'invalid message' unless valid_message?(signed)
+      data, = signed.split('--', 2)
+      unwrap(@serializer.load(decode(data)), purpose)
+    rescue ArgumentError, TypeError
+      raise InvalidSignature, 'invalid message'
+    end
+
+    def verified(signed, purpose: nil)
+      verify(signed, purpose: purpose)
+    rescue InvalidSignature
+      nil
+    end
+
+    def valid_message?(signed)
+      return false if signed.nil? || !signed.include?('--')
+      data, signature = signed.split('--', 2)
+      return false if data.nil? || data.empty? || signature.nil? || signature.empty?
+      secure_compare(signature, digest_for(data))
+    end
+
+    private
+
+    ENVELOPE_KEY = '_prism_envelope'
+
+    def wrap(value, exp, purpose)
+      return value if exp.nil? && purpose.nil?
+      { ENVELOPE_KEY => true, 'value' => value, 'exp' => exp&.utc&.iso8601, 'pur' => purpose&.to_s }
+    end
+
+    def unwrap(payload, purpose)
+      unless payload.is_a?(Hash) && payload[ENVELOPE_KEY]
+        raise InvalidSignature, 'message requires a purpose' if purpose
+        return payload
+      end
+      raise InvalidSignature, 'message expired' if payload['exp'] && Time.parse(payload['exp']) <= Time.now.utc
+      raise InvalidSignature, 'purpose mismatch' unless payload['pur'] == purpose&.to_s
+      payload['value']
+    end
+
+    def digest_for(data)
+      OpenSSL::HMAC.hexdigest(@digest, @secret, data)
+    end
+
+    def encode(text)
+      [text].pack('m0')
+    end
+
+    def decode(text)
+      text.unpack1('m0')
+    end
+
+    def secure_compare(left, right)
+      return false unless left.bytesize == right.bytesize
+      left_bytes = left.unpack("C#{left.bytesize}")
+      difference = 0
+      right.each_byte.with_index { |byte, index| difference |= byte ^ left_bytes[index] }
+      difference.zero?
+    end
+  end
+end