# Copyright (c) 2018 Yegor Bugayenko
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the 'Software'), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

require 'openssl'
require 'base64'

# The RSA key (either private or public).
# Author:: Yegor Bugayenko (yegor256@gmail.com)
# Copyright:: Copyright (c) 2018 Yegor Bugayenko
# License:: MIT
module Zold
  # A key
  class Key
    def initialize(file: nil, text: nil)
      unless file.nil?
        path = File.expand_path(file)
        raise "Can't find RSA key at #{file} (#{path})" unless File.exist?(path)
        @body = File.read(path)
      end
      return if text.nil?
      @body = [
        '-----BEGIN PUBLIC KEY-----',
        text.gsub(/(?<=\G.{64})/, "\n"),
        '-----END PUBLIC KEY-----'
      ].join("\n")
    end

    def to_s
      rsa.to_s.strip
    end

    def to_pub
      to_s.delete("\n").gsub(/-{5}[ A-Z]+-{5}/, '')
    end

    def sign(text)
      Base64.encode64(rsa.sign(OpenSSL::Digest::SHA256.new, text)).delete("\n")
    end

    def verify(signature, text)
      rsa.verify(OpenSSL::Digest::SHA256.new, Base64.decode64(signature), text)
    end

    private

    def rsa
      text = @body.strip
      unless text.start_with?('-----BEGIN')
        text = OpenSSHKeyConverter.decode_pubkey(text.split[1])
      end
      begin
        OpenSSL::PKey::RSA.new(text)
      rescue OpenSSL::PKey::RSAError => e
        raise "Can't read RSA key (#{e.message}): #{text}"
      end
    end
  end
end

# Stolen from: https://gist.github.com/tombh/f66de84fd3a63e670ad9
module OpenSSHKeyConverter
  # The components in a openssh .pub / known_host RSA public key.
  RSA_COMPONENTS = ['ssh-rsa', :e, :n].freeze
  # The components in a openssh .pub / known_host DSA public key.
  DSA_COMPONENTS = ['ssh-dss', :p, :q, :g, :pub_key].freeze

  # Encodes a key's public part in the format found in .pub & known_hosts files.
  def self.encode_pubkey(key)
    case key
    when OpenSSL::PKey::RSA
      components = RSA_COMPONENTS
    when OpenSSL::PKey::DSA
      components = DSA_COMPONENTS
    else
      raise "Unsupported key type #{key.class.name}"
    end
    components.map! { |c| c.is_a?(Symbol) ? encode_mpi(key.send(c)) : c }
    # ruby tries to be helpful and adds new lines every 60 bytes :(
    [pack_pubkey_components(components)].pack('m').delete("\n")
  end

  # Decodes an openssh public key from the format of .pub & known_hosts files.
  def self.decode_pubkey(string)
    components = unpack_pubkey_components Base64.decode64(string)
    case components.first
    when RSA_COMPONENTS.first
      ops = RSA_COMPONENTS.zip components
      key = OpenSSL::PKey::RSA.new
    when DSA_COMPONENTS.first
      ops = DSA_COMPONENTS.zip components
      key = OpenSSL::PKey::DSA.new
    else
      raise "Unsupported key type #{components.first}"
    end
    ops.each do |o|
      next unless o.first.is_a? Symbol
      key.send "#{o.first}=", decode_mpi(o.last)
    end
    key
  end

  # Loads a serialized key from an IO instance (File, StringIO).
  def self.load_key(io)
    key_from_string io.read
  end

  # Reads a serialized key from a string.
  def self.key_from_string(serialized_key)
    header = first_line serialized_key
    if header.index 'RSA'
      OpenSSL::PKey::RSA.new serialized_key
    elsif header.index 'DSA'
      OpenSSL::PKey::DSA.new serialized_key
    else
      raise 'Unknown key type'
    end
  end

  # Extracts the first line of a string.
  def self.first_line(string)
    string[0, string.index(/\r|\n/) || string.len]
  end

  # Unpacks the string components in an openssh-encoded pubkey.
  def self.unpack_pubkey_components(str)
    cs = []
    i = 0
    while i < str.length
      len = str[i, 4].unpack('N').first
      cs << str[i + 4, len]
      i += 4 + len
    end
    cs
  end

  # Packs string components into an openssh-encoded pubkey.
  def self.pack_pubkey_components(strings)
    (strings.map { |s| [s.length].pack('N') }).zip(strings).flatten.join
  end

  # Decodes an openssh-mpi-encoded integer.
  def self.decode_mpi(mpi_str)
    mpi_str.unpack('C*').inject(0) { |a, e| (a << 8) | e }
  end

  # Encodes an openssh-mpi-encoded integer.
  def self.encode_mpi(n)
    chars = []
    n = n.to_i
    chars << (n & 0xff) && n >>= 8 while n != 0
    chars << 0 if chars.empty? || chars.last >= 0x80
    chars.reverse.pack('C*')
  end
end