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require 'active_support/base64'
require 'active_support/core_ext/object/blank'
module ActiveSupport
# +MessageVerifier+ makes it easy to generate and verify messages which are signed
# to prevent tampering.
#
# This is useful for cases like remember-me tokens and auto-unsubscribe links where the
# session store isn't suitable or available.
#
# Remember Me:
# cookies[:remember_me] = @verifier.generate([@user.id, 2.weeks.from_now])
#
# In the authentication filter:
#
# id, time = @verifier.verify(cookies[:remember_me])
# if time < Time.now
# self.current_user = User.find(id)
# end
#
# By default it uses Marshal to serialize the message. If you want to use another
# serialization method, you can set the serializer attribute to something that responds
# to dump and load, e.g.:
#
# @verifier.serializer = YAML
class MessageVerifier
class InvalidSignature < StandardError; end
def initialize(secret, options = {})
@secret = secret
@digest = options[:digest] || 'SHA1'
@serializer = options[:serializer] || Marshal
end
def verify(signed_message)
raise InvalidSignature if signed_message.blank?
data, digest = signed_message.split("--")
if data.present? && digest.present? && secure_compare(digest, generate_digest(data))
@serializer.load(ActiveSupport::Base64.decode64(data))
else
raise InvalidSignature
end
end
def generate(value)
data = ActiveSupport::Base64.encode64s(@serializer.dump(value))
"#{data}--#{generate_digest(data)}"
end
private
# constant-time comparison algorithm to prevent timing attacks
def secure_compare(a, b)
return false unless a.bytesize == b.bytesize
l = a.unpack "C#{a.bytesize}"
res = 0
b.each_byte { |byte| res |= byte ^ l.shift }
res == 0
end
def generate_digest(data)
require 'openssl' unless defined?(OpenSSL)
OpenSSL::HMAC.hexdigest(OpenSSL::Digest.const_get(@digest).new, @secret, data)
end
end
end
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