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

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.

Alternative serializers

By default MessageVerifier uses JSON to serialize the message. If you want to use another serialization method, you can set the serializer in the options hash upon initialization:

@verifier = ActiveSupport::MessageVerifier.new("secret", serializer: YAML)

MessageVerifier creates HMAC signatures using the SHA1 hash algorithm by default. If you want to use a different hash algorithm, you can change it by providing :digest key as an option while initializing the verifier:

@verifier = ActiveSupport::MessageVerifier.new("secret", digest: "SHA256")

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)

Generating URL-safe strings

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 url_safe: true to the constructor:

@verifier = ActiveSupport::MessageVerifier.new("secret", url_safe: true)
@verifier.generate("signed message") #=> URL-safe string