require 'base64'
require 'uri'
require 'json'
require 'omniauth'

module OmniAuth
  module Auth0
    # JWT Validator class
    class JWTValidator
      attr_accessor :issuer, :domain

      # Initializer
      # @param options object
      #   options.domain - Application domain.
      #   options.issuer - Application issuer (optional).
      #   options.client_id - Application Client ID.
      #   options.client_secret - Application Client Secret.

      def initialize(options, authorize_params = {})
        @domain = uri_string(options.domain)

        # Use custom issuer if provided, otherwise use domain
        @issuer = @domain
        @issuer = uri_string(options.issuer) if options.respond_to?(:issuer)

        @client_id = options.client_id
        @client_secret = options.client_secret
      end

      def verify_signature(jwt)
        head = token_head(jwt)

        # Make sure the algorithm is supported and get the decode key.
        if head[:alg] == 'RS256'
          [rs256_decode_key(head[:kid]), head[:alg]]
        elsif head[:alg] == 'HS256'
          [@client_secret, head[:alg]]
        else
          raise OmniAuth::Auth0::TokenValidationError.new("Signature algorithm of #{head[:alg]} is not supported. Expected the ID token to be signed with RS256 or HS256")
        end
      end

      # Verify a JWT.
      # @param jwt string - JWT to verify.
      # @param authorize_params hash - Authorization params to verify on the JWT
      # @return hash - The verified token, if there were no exceptions.
      def verify(jwt, authorize_params = {})
        if !jwt
          raise OmniAuth::Auth0::TokenValidationError.new('ID token is required but missing')
        end

        parts = jwt.split('.')
        if parts.length != 3
          raise OmniAuth::Auth0::TokenValidationError.new('ID token could not be decoded')
        end

        key, alg = verify_signature(jwt)
        id_token, header = JWT.decode(jwt, key, false)
        verify_claims(id_token, authorize_params)

        return id_token
      end

      # Get the decoded head segment from a JWT.
      # @return hash - The parsed head of the JWT passed, empty hash if not.
      def token_head(jwt)
        jwt_parts = jwt.split('.')
        return {} if blank?(jwt_parts) || blank?(jwt_parts[0])

        json_parse(Base64.decode64(jwt_parts[0]))
      end

      # Get the JWKS from the issuer and return a public key.
      # @param x5c string - X.509 certificate chain from a JWKS.
      # @return key - The X.509 certificate public key.
      def jwks_public_cert(x5c)
        x5c = Base64.decode64(x5c)

        # https://docs.ruby-lang.org/en/2.4.0/OpenSSL/X509/Certificate.html
        OpenSSL::X509::Certificate.new(x5c).public_key
      end

      # Return a specific key from a JWKS object.
      # @param key string - Key to find in the JWKS.
      # @param kid string - Key ID to identify the right JWK.
      # @return nil|string
      def jwks_key(key, kid)
        return nil if blank?(jwks[:keys])

        matching_jwk = jwks[:keys].find { |jwk| jwk[:kid] == kid }
        matching_jwk[key] if matching_jwk
      end

      private
      def rs256_decode_key(kid)
        jwks_x5c = jwks_key(:x5c, kid)

        if jwks_x5c.nil?
          raise OmniAuth::Auth0::TokenValidationError.new("Could not find a public key for Key ID (kid) '#{kid}''")
        end

        jwks_public_cert(jwks_x5c.first)
      end

      # Get a JWKS from the domain
      # @return void
      def jwks
        jwks_uri = URI(@domain + '.well-known/jwks.json')
        @jwks ||= json_parse(Net::HTTP.get(jwks_uri))
      end

      # Rails Active Support blank method.
      # @param obj object - Object to check for blankness.
      # @return boolean
      def blank?(obj)
        obj.respond_to?(:empty?) ? obj.empty? : !obj
      end

      # Parse JSON with symbolized names.
      # @param json string - JSON to parse.
      # @return hash
      def json_parse(json)
        JSON.parse(json, symbolize_names: true)
      end

      # Parse a URI into the desired string format
      # @param uri - the URI to parse
      # @return string
      def uri_string(uri)
        temp_domain = URI(uri)
        temp_domain = URI("https://#{uri}") unless temp_domain.scheme
        "#{temp_domain}/"
      end

      def verify_claims(id_token, authorize_params)
        leeway = authorize_params[:leeway] || 60
        max_age = authorize_params[:max_age]
        nonce = authorize_params[:nonce]

        verify_iss(id_token)
        verify_sub(id_token)
        verify_aud(id_token)
        verify_expiration(id_token, leeway)
        verify_iat(id_token)
        verify_nonce(id_token, nonce)
        verify_azp(id_token)
        verify_auth_time(id_token, leeway, max_age)
      end

      def verify_iss(id_token)
        issuer = id_token['iss']
        if !issuer
          raise OmniAuth::Auth0::TokenValidationError.new("Issuer (iss) claim must be a string present in the ID token")
        elsif @issuer != issuer
          raise OmniAuth::Auth0::TokenValidationError.new("Issuer (iss) claim mismatch in the ID token, expected (#{@issuer}), found (#{id_token['iss']})")
        end
      end

      def verify_sub(id_token)
        subject = id_token['sub']
        if !subject || !subject.is_a?(String) || subject.empty?
          raise OmniAuth::Auth0::TokenValidationError.new('Subject (sub) claim must be a string present in the ID token')
        end
      end

      def verify_aud(id_token)
        audience = id_token['aud']
        if !audience || !(audience.is_a?(String) || audience.is_a?(Array))
          raise OmniAuth::Auth0::TokenValidationError.new("Audience (aud) claim must be a string or array of strings present in the ID token")
        elsif audience.is_a?(Array) && !audience.include?(@client_id)
          raise OmniAuth::Auth0::TokenValidationError.new("Audience (aud) claim mismatch in the ID token; expected #{@client_id} but was not one of #{audience.join(', ')}")
        elsif audience.is_a?(String) && audience != @client_id
          raise OmniAuth::Auth0::TokenValidationError.new("Audience (aud) claim mismatch in the ID token; expected #{@client_id} but found #{audience}")
        end
      end

      def verify_expiration(id_token, leeway)
        expiration = id_token['exp']
        if !expiration || !expiration.is_a?(Integer)
          raise OmniAuth::Auth0::TokenValidationError.new("Expiration time (exp) claim must be a number present in the ID token")
        elsif expiration <= Time.now.to_i - leeway
          raise OmniAuth::Auth0::TokenValidationError.new("Expiration time (exp) claim error in the ID token; current time (#{Time.now}) is after expiration time (#{Time.at(expiration + leeway)})")
        end
      end

      def verify_iat(id_token)
        if !id_token['iat']
          raise OmniAuth::Auth0::TokenValidationError.new("Issued At (iat) claim must be a number present in the ID token")
        end
      end

      def verify_nonce(id_token, nonce)
        if nonce
          received_nonce = id_token['nonce']
          if !received_nonce
            raise OmniAuth::Auth0::TokenValidationError.new("Nonce (nonce) claim must be a string present in the ID token")
          elsif nonce != received_nonce
            raise OmniAuth::Auth0::TokenValidationError.new("Nonce (nonce) claim value mismatch in the ID token; expected (#{nonce}), found (#{received_nonce})")
          end
        end
      end

      def verify_azp(id_token)
        audience = id_token['aud']
        if audience.is_a?(Array) && audience.length > 1
          azp = id_token['azp']
          if !azp || !azp.is_a?(String)
            raise OmniAuth::Auth0::TokenValidationError.new("Authorized Party (azp) claim must be a string present in the ID token when Audience (aud) claim has multiple values")
          elsif azp != @client_id
            raise OmniAuth::Auth0::TokenValidationError.new("Authorized Party (azp) claim mismatch in the ID token; expected (#{@client_id}), found (#{azp})")
          end
        end
      end

      def verify_auth_time(id_token, leeway, max_age)
        if max_age
          auth_time = id_token['auth_time']
          if !auth_time || !auth_time.is_a?(Integer)
            raise OmniAuth::Auth0::TokenValidationError.new("Authentication Time (auth_time) claim must be a number present in the ID token when Max Age (max_age) is specified")
          elsif Time.now.to_i >  auth_time + max_age + leeway;
            raise OmniAuth::Auth0::TokenValidationError.new("Authentication Time (auth_time) claim in the ID token indicates that too much time has passed since the last end-user authentication. Current time (#{Time.now}) is after last auth time (#{Time.at(auth_time + max_age + leeway)})")
          end
        end
      end
    end
  end
end