Active Support Message Verifier

MessageVerifier makes it easy to generate and verify messages which are signed to prevent tampering.

In a Rails application, you can use Rails.application.message_verifier to manage unique instances of verifiers for each use case. Learn more.

This is useful for cases like remember-me tokens and auto-unsubscribe links where the session store isn’t suitable or available.

First, generate a signed message:

cookies[:remember_me] = Rails.application.message_verifier(:remember_me).generate([@user.id, 2.weeks.from_now])

Later verify that message:

id, time = Rails.application.message_verifier(:remember_me).verify(cookies[:remember_me])
if time.future?
  self.current_user = User.find(id)
end

Signing is not encryption

The signed messages are not encrypted. The payload is merely encoded (Base64 by default) and can be decoded by anyone. The signature is just assuring that the message wasn’t tampered with. For example:

message = Rails.application.message_verifier('my_purpose').generate('never put secrets here')
# => "BAhJIhtuZXZlciBwdXQgc2VjcmV0cyBoZXJlBjoGRVQ=--a0c1c0827919da5e949e989c971249355735e140"
Base64.decode64(message.split("--").first) # no key needed
# => 'never put secrets here'

If you also need to encrypt the contents, you must use ActiveSupport::MessageEncryptor instead.

Confine messages to a specific purpose

It’s not recommended to use the same verifier for different purposes in your application. Doing so could allow a malicious actor to re-use a signed message to perform an unauthorized action. You can reduce this risk by confining signed messages to a specific :purpose.

token = @verifier.generate("signed message", purpose: :login)

Then that same purpose must be passed when verifying to get the data back out:

@verifier.verified(token, purpose: :login)    # => "signed message"
@verifier.verified(token, purpose: :shipping) # => nil
@verifier.verified(token)                     # => nil

@verifier.verify(token, purpose: :login)      # => "signed message"
@verifier.verify(token, purpose: :shipping)   # => raises ActiveSupport::MessageVerifier::InvalidSignature
@verifier.verify(token)                       # => raises ActiveSupport::MessageVerifier::InvalidSignature

Likewise, if a message has no purpose it won’t be returned when verifying with a specific purpose.

token = @verifier.generate("signed message")
@verifier.verified(token, purpose: :redirect) # => nil
@verifier.verified(token)                     # => "signed message"

@verifier.verify(token, purpose: :redirect)   # => raises ActiveSupport::MessageVerifier::InvalidSignature
@verifier.verify(token)                       # => "signed message"

Expiring messages

By default messages last forever and verifying one year from now will still return the original value. But messages can be set to expire at a given time with :expires_in or :expires_at.

@verifier.generate("signed message", expires_in: 1.month)
@verifier.generate("signed message", expires_at: Time.now.end_of_year)

Messages can then be verified and returned until expiry. Thereafter, the verified method returns nil while verify raises ActiveSupport::MessageVerifier::InvalidSignature.

Rotating keys

MessageVerifier also supports rotating out old configurations by falling back to a stack of verifiers. Call rotate to build and add a verifier so either verified or verify will also try verifying with the fallback.

By default any rotated verifiers use the values of the primary verifier unless specified otherwise.

You’d give your verifier the new defaults:

verifier = ActiveSupport::MessageVerifier.new(@secret, digest: "SHA512", serializer: JSON)

Then gradually rotate the old values out by adding them as fallbacks. Any message generated with the old values will then work until the rotation is removed.

verifier.rotate(old_secret)          # Fallback to an old secret instead of @secret.
verifier.rotate(digest: "SHA256")    # Fallback to an old digest instead of SHA512.
verifier.rotate(serializer: Marshal) # Fallback to an old serializer instead of JSON.

Though the above would most likely be combined into one rotation:

verifier.rotate(old_secret, digest: "SHA256", serializer: Marshal)

Namespace

Class

Methods

Class Public methods

new(secret, **options)

Initialize a new MessageVerifier with a secret for the signature.

Options

:digest

Digest used for signing. The default is "SHA1". See OpenSSL::Digest for alternatives.

:serializer

The serializer used to serialize message data. You can specify any object that responds to dump and load, or you can choose from several preconfigured serializers: :marshal, :json_allow_marshal, :json, :message_pack_allow_marshal, :message_pack.

The preconfigured serializers include a fallback mechanism to support multiple deserialization formats. For example, the :marshal serializer will serialize using Marshal, but can deserialize using Marshal, ActiveSupport::JSON, or ActiveSupport::MessagePack. This makes it easy to migrate between serializers.

The :marshal, :json_allow_marshal, and :message_pack_allow_marshal serializers support deserializing using Marshal, but the others do not. Beware that Marshal is a potential vector for deserialization attacks in cases where a message signing secret has been leaked. If possible, choose a serializer that does not support Marshal.

The :message_pack and :message_pack_allow_marshal serializers use ActiveSupport::MessagePack, which can roundtrip some Ruby types that are not supported by JSON, and may provide improved performance. However, these require the msgpack gem.

When using Rails, the default depends on config.active_support.message_serializer. Otherwise, the default is :marshal.

:url_safe

By default, MessageVerifier generates RFC 4648 compliant strings which are not URL-safe. In other words, they can contain “+” and “/”. If you want to generate URL-safe strings (in compliance with “Base 64 Encoding with URL and Filename Safe Alphabet” in RFC 4648), you can pass true.

:force_legacy_metadata_serializer

Whether to use the legacy metadata serializer, which serializes the message first, then wraps it in an envelope which is also serialized. This was the default in Rails 7.0 and below.

If you don’t pass a truthy value, the default is set using config.active_support.use_message_serializer_for_metadata.

📝 Source code 
# File activesupport/lib/active_support/message_verifier.rb, line 165
    def initialize(secret, **options)
      raise ArgumentError, "Secret should not be nil." unless secret
      super(**options)
      @secret = secret
      @digest = options[:digest]&.to_s || "SHA1"
    end
🔎 See on GitHub

Instance Public methods

generate(value, **options)

Generates a signed message for the provided value.

The message is signed with the MessageVerifier‘s secret. Returns Base64-encoded message joined with the generated signature.

verifier = ActiveSupport::MessageVerifier.new("secret")
verifier.generate("signed message") # => "BAhJIhNzaWduZWQgbWVzc2FnZQY6BkVU--f67d5f27c3ee0b8483cebf2103757455e947493b"

Options

:expires_at

The datetime at which the message expires. After this datetime, verification of the message will fail.

message = verifier.generate("hello", expires_at: Time.now.tomorrow)
verifier.verified(message) # => "hello"
# 24 hours later...
verifier.verified(message) # => nil
verifier.verify(message)   # => raises ActiveSupport::MessageVerifier::InvalidSignature
:expires_in

The duration for which the message is valid. After this duration has elapsed, verification of the message will fail.

message = verifier.generate("hello", expires_in: 24.hours)
verifier.verified(message) # => "hello"
# 24 hours later...
verifier.verified(message) # => nil
verifier.verify(message)   # => raises ActiveSupport::MessageVerifier::InvalidSignature
:purpose

The purpose of the message. If specified, the same purpose must be specified when verifying the message; otherwise, verification will fail. (See verified and verify.)

📝 Source code 
# File activesupport/lib/active_support/message_verifier.rb, line 304
    def generate(value, **options)
      create_message(value, **options)
    end
🔎 See on GitHub

valid_message?(message)

Checks if a signed message could have been generated by signing an object with the MessageVerifier‘s secret.

verifier = ActiveSupport::MessageVerifier.new("secret")
signed_message = verifier.generate("signed message")
verifier.valid_message?(signed_message) # => true

tampered_message = signed_message.chop # editing the message invalidates the signature
verifier.valid_message?(tampered_message) # => false
📝 Source code 
# File activesupport/lib/active_support/message_verifier.rb, line 181
    def valid_message?(message)
      !!catch_and_ignore(:invalid_message_format) { extract_encoded(message) }
    end
🔎 See on GitHub

verified(message, **options)

Decodes the signed message using the MessageVerifier‘s secret.

verifier = ActiveSupport::MessageVerifier.new("secret")

signed_message = verifier.generate("signed message")
verifier.verified(signed_message) # => "signed message"

Returns nil if the message was not signed with the same secret.

other_verifier = ActiveSupport::MessageVerifier.new("different_secret")
other_verifier.verified(signed_message) # => nil

Returns nil if the message is not Base64-encoded.

invalid_message = "f--46a0120593880c733a53b6dad75b42ddc1c8996d"
verifier.verified(invalid_message) # => nil

Raises any error raised while decoding the signed message.

incompatible_message = "test--dad7b06c94abba8d46a15fafaef56c327665d5ff"
verifier.verified(incompatible_message) # => TypeError: incompatible marshal file format

Options

:purpose

The purpose that the message was generated with. If the purpose does not match, verified will return nil.

message = verifier.generate("hello", purpose: "greeting")
verifier.verified(message, purpose: "greeting") # => "hello"
verifier.verified(message, purpose: "chatting") # => nil
verifier.verified(message)                      # => nil

message = verifier.generate("bye")
verifier.verified(message)                      # => "bye"
verifier.verified(message, purpose: "greeting") # => nil
📝 Source code 
# File activesupport/lib/active_support/message_verifier.rb, line 222
    def verified(message, **options)
      catch_and_ignore :invalid_message_format do
        catch_and_raise :invalid_message_serialization do
          catch_and_ignore :invalid_message_content do
            read_message(message, **options)
          end
        end
      end
    end
🔎 See on GitHub

verify(message, **options)

Decodes the signed message using the MessageVerifier‘s secret.

verifier = ActiveSupport::MessageVerifier.new("secret")
signed_message = verifier.generate("signed message")

verifier.verify(signed_message) # => "signed message"

Raises InvalidSignature if the message was not signed with the same secret or was not Base64-encoded.

other_verifier = ActiveSupport::MessageVerifier.new("different_secret")
other_verifier.verify(signed_message) # => ActiveSupport::MessageVerifier::InvalidSignature

Options

:purpose

The purpose that the message was generated with. If the purpose does not match, verify will raise ActiveSupport::MessageVerifier::InvalidSignature.

message = verifier.generate("hello", purpose: "greeting")
verifier.verify(message, purpose: "greeting") # => "hello"
verifier.verify(message, purpose: "chatting") # => raises InvalidSignature
verifier.verify(message)                      # => raises InvalidSignature

message = verifier.generate("bye")
verifier.verify(message)                      # => "bye"
verifier.verify(message, purpose: "greeting") # => raises InvalidSignature
📝 Source code 
# File activesupport/lib/active_support/message_verifier.rb, line 260
    def verify(message, **options)
      catch_and_raise :invalid_message_format, as: InvalidSignature do
        catch_and_raise :invalid_message_serialization do
          catch_and_raise :invalid_message_content, as: InvalidSignature do
            read_message(message, **options)
          end
        end
      end
    end
🔎 See on GitHub