require 'openssl'

class Signer

  # Digest algorithms supported "out of the box"
  DIGEST_ALGORITHMS = {
    # SHA 1
    sha1: {
      name: 'SHA1',
      id: 'http://www.w3.org/2000/09/xmldsig#sha1',
      digester: lambda { OpenSSL::Digest::SHA1.new },
    },
    # SHA 256
    sha256: {
        name: 'SHA256',
        id: 'http://www.w3.org/2001/04/xmldsig-more#rsa-sha256',
        digester: lambda { OpenSSL::Digest::SHA256.new },
    },
    # GOST R 34-11 94
    gostr3411: {
      name: 'GOST R 34.11-94',
      id: 'http://www.w3.org/2001/04/xmldsig-more#gostr3411',
      digester: lambda {
        OpenSSL::Engine.load
        gost_engine = OpenSSL::Engine.by_id('gost')
        gost_engine.set_default(0xFFFF)
        gost_engine.digest('md_gost94')
      },
    },
  }

  # Class that holds +OpenSSL::Digest+ instance with some meta information for digesting in XML.
  class Digester

    # You may pass either a one of +:sha1+, +:sha256+ or +:gostr3411+ symbols
    # or +Hash+ with keys +:id+ with a string, which will denote algorithm in XML Reference tag
    # and +:digester+ with instance of class with interface compatible with +OpenSSL::Digest+ class.
    def initialize(algorithm)
      if algorithm.kind_of? Symbol
        @digest_info = DIGEST_ALGORITHMS[algorithm].dup
        @digest_info[:digester] = @digest_info[:digester].call
        @symbol = algorithm
      else
        @digest_info = algorithm
      end
    end

    attr_reader :symbol

    # Digest
    def digest(message)
      self.digester.digest(message)
    end

    alias call digest

    # Returns +OpenSSL::Digest+ (or derived class) instance
    def digester
      @digest_info[:digester].reset
    end

    # Human-friendly name
    def digest_name
      @digest_info[:name]
    end

    # XML-friendly name (for specifying in XML +DigestMethod+ node +Algorithm+ attribute)
    def digest_id
      @digest_info[:id]
    end

  end

end