require 'openssl'
require 'base64'
require 'digest/md5'
require 'digest/sha1'

class SSHKey
  SSH_TYPES = {
    "ssh-rsa" => "rsa",
    "ssh-dss" => "dsa",
    "ssh-ed25519" => "ed25519",
    "ecdsa-sha2-nistp256" => "ecdsa",
    "ecdsa-sha2-nistp384" => "ecdsa",
    "ecdsa-sha2-nistp521" => "ecdsa",
  }
  SSH_CONVERSION = {"rsa" => ["e", "n"], "dsa" => ["p", "q", "g", "pub_key"]}
  SSH2_LINE_LENGTH = 70 # +1 (for line wrap '/' character) must be <= 72

  class << self
    # Generate a new keypair and return an SSHKey object
    #
    # The default behavior when providing no options will generate a 2048-bit RSA
    # keypair.
    #
    # ==== Parameters
    # * options<~Hash>:
    #   * :type<~String> - "rsa" or "dsa", "rsa" by default
    #   * :bits<~Integer> - Bit length
    #   * :comment<~String> - Comment to use for the public key, defaults to ""
    #   * :passphrase<~String> - Encrypt the key with this passphrase
    #
    def generate(options = {})
      type   = options[:type] || "rsa"

      # JRuby modulus size must range from 512 to 1024
      default_bits = type == "rsa" ? 2048 : 1024

      bits   = options[:bits] || default_bits
      cipher = OpenSSL::Cipher.new("AES-128-CBC") if options[:passphrase]

      case type.downcase
      when "rsa" then new(OpenSSL::PKey::RSA.generate(bits).to_pem(cipher, options[:passphrase]), options)
      when "dsa" then new(OpenSSL::PKey::DSA.generate(bits).to_pem(cipher, options[:passphrase]), options)
      else
        raise "Unknown key type: #{type}"
      end
    end

    # Validate an existing SSH public key
    #
    # Returns true or false depending on the validity of the public key provided
    #
    # ==== Parameters
    # * ssh_public_key<~String> - "ssh-rsa AAAAB3NzaC1yc2EA...."
    #
    def valid_ssh_public_key?(ssh_public_key)
      ssh_type, encoded_key = parse_ssh_public_key(ssh_public_key)
      sections = unpacked_byte_array(ssh_type, encoded_key)
      case ssh_type
        when "ssh-rsa", "ssh-dss"
          sections.size == SSH_CONVERSION[SSH_TYPES[ssh_type]].size
        when "ssh-ed25519"
          sections.size == 1 && sections[0].num_bytes == 32 # https://tools.ietf.org/id/draft-bjh21-ssh-ed25519-00.html#rfc.section.4
        when "ecdsa-sha2-nistp256", "ecdsa-sha2-nistp384", "ecdsa-sha2-nistp521"
          sections.size == 2                                # https://tools.ietf.org/html/rfc5656#section-3.1
        else
          false
      end
    rescue
      false
    end

    # Bits
    #
    # Returns ssh public key bits or false depending on the validity of the public key provided
    #
    # ==== Parameters
    # * ssh_public_key<~String> - "ssh-rsa AAAAB3NzaC1yc2EA...."
    #
    def ssh_public_key_bits(ssh_public_key)
      unpacked_byte_array( *parse_ssh_public_key(ssh_public_key) ).last.num_bytes * 8
    end

    # Fingerprints
    #
    # Accepts either a public or private key
    #
    # MD5 fingerprint for the given SSH key
    def md5_fingerprint(key)
      if key.match(/PRIVATE/)
        new(key).md5_fingerprint
      else
        Digest::MD5.hexdigest(decoded_key(key)).gsub(fingerprint_regex, '\1:\2')
      end
    end
    alias_method :fingerprint, :md5_fingerprint

    # SHA1 fingerprint for the given SSH key
    def sha1_fingerprint(key)
      if key.match(/PRIVATE/)
        new(key).sha1_fingerprint
      else
        Digest::SHA1.hexdigest(decoded_key(key)).gsub(fingerprint_regex, '\1:\2')
      end
    end

    # SHA256 fingerprint for the given SSH key
    def sha256_fingerprint(key)
      if key.match(/PRIVATE/)
        new(key).sha256_fingerprint
      else
        Base64.encode64(Digest::SHA256.digest(decoded_key(key))).gsub("\n", "")
      end
    end

    # Convert an existing SSH public key to SSH2 (RFC4716) public key
    #
    # ==== Parameters
    # * ssh_public_key<~String> - "ssh-rsa AAAAB3NzaC1yc2EA...."
    # * headers<~Hash> - The Key will be used as the header-tag and the value as the header-value
    #
    def ssh_public_key_to_ssh2_public_key(ssh_public_key, headers = nil)
      raise PublicKeyError, "invalid ssh public key" unless SSHKey.valid_ssh_public_key?(ssh_public_key)

      _source_format, source_key = parse_ssh_public_key(ssh_public_key)

      # Add a 'Comment' Header Field unless others are explicitly passed in
      if source_comment = ssh_public_key.split(source_key)[1]
        headers = {'Comment' => source_comment.strip} if headers.nil? && !source_comment.empty?
      end
      header_fields = build_ssh2_headers(headers)

      ssh2_key = "---- BEGIN SSH2 PUBLIC KEY ----\n"
      ssh2_key << header_fields unless header_fields.nil?
      ssh2_key << source_key.scan(/.{1,#{SSH2_LINE_LENGTH}}/).join("\n")
      ssh2_key << "\n---- END SSH2 PUBLIC KEY ----"
    end

    private

    def unpacked_byte_array(ssh_type, encoded_key)
      prefix = [ssh_type.length].pack("N") + ssh_type
      decoded = Base64.decode64(encoded_key)

      # Base64 decoding is too permissive, so we should validate if encoding is correct
      unless Base64.encode64(decoded).gsub("\n", "") == encoded_key && decoded.slice!(0, prefix.length) == prefix
        raise PublicKeyError, "validation error"
      end

      data = []
      until decoded.empty?
        front = decoded.slice!(0,4)
        size = front.unpack("N").first
        segment = decoded.slice!(0, size)
        unless front.length == 4 && segment.length == size
          raise PublicKeyError, "byte array too short"
        end
        data << OpenSSL::BN.new(segment, 2)
      end
      return data
    end

    def decoded_key(key)
      Base64.decode64(parse_ssh_public_key(key).last)
    end

    def fingerprint_regex
      /(.{2})(?=.)/
    end

    def parse_ssh_public_key(public_key)
      raise PublicKeyError, "newlines are not permitted between key data" if public_key =~ /\n(?!$)/

      parsed = public_key.split(" ")
      parsed.each_with_index do |el, index|
        return parsed[index..(index+1)] if SSH_TYPES[el]
      end
      raise PublicKeyError, "cannot determine key type"
    end

    def build_ssh2_headers(headers = {})
      return nil if headers.nil? || headers.empty?

      headers.keys.sort.collect do |header_tag|
        # header-tag must be us-ascii & <= 64 bytes and header-data must be UTF-8 & <= 1024 bytes
        raise PublicKeyError, "SSH2 header-tag '#{header_tag}' must be US-ASCII" unless header_tag.each_byte.all? {|b| b < 128 }
        raise PublicKeyError, "SSH2 header-tag '#{header_tag}' must be <= 64 bytes" unless header_tag.size <= 64
        raise PublicKeyError, "SSH2 header-value for '#{header_tag}' must be <= 1024 bytes" unless headers[header_tag].size <= 1024

        header_field = "#{header_tag}: #{headers[header_tag]}"
        header_field.scan(/.{1,#{SSH2_LINE_LENGTH}}/).join("\\\n")
      end.join("\n") << "\n"
    end
  end

  attr_reader :key_object, :type
  attr_accessor :passphrase, :comment

  # Create a new SSHKey object
  #
  # ==== Parameters
  # * private_key - Existing RSA or DSA private key
  # * options<~Hash>
  #   * :comment<~String> - Comment to use for the public key, defaults to ""
  #   * :passphrase<~String> - If the key is encrypted, supply the passphrase
  #   * :directives<~Array> - Options prefixed to the public key
  #
  def initialize(private_key, options = {})
    @passphrase = options[:passphrase]
    @comment    = options[:comment] || ""
    self.directives = options[:directives] || []
    begin
      @key_object = OpenSSL::PKey::RSA.new(private_key, passphrase)
      @type = "rsa"
    rescue
      @key_object = OpenSSL::PKey::DSA.new(private_key, passphrase)
      @type = "dsa"
    end
  end

  # Fetch the RSA/DSA private key
  #
  # rsa_private_key and dsa_private_key are aliased for backward compatibility
  def private_key
    key_object.to_pem
  end
  alias_method :rsa_private_key, :private_key
  alias_method :dsa_private_key, :private_key

  # Fetch the encrypted RSA/DSA private key using the passphrase provided
  #
  # If no passphrase is set, returns the unencrypted private key
  def encrypted_private_key
    return private_key unless passphrase
    key_object.to_pem(OpenSSL::Cipher.new("AES-128-CBC"), passphrase)
  end

  # Fetch the RSA/DSA public key
  #
  # rsa_public_key and dsa_public_key are aliased for backward compatibility
  def public_key
    key_object.public_key.to_pem
  end
  alias_method :rsa_public_key, :public_key
  alias_method :dsa_public_key, :public_key

  # SSH public key
  def ssh_public_key
    [directives.join(",").strip, SSH_TYPES.invert[type], Base64.encode64(ssh_public_key_conversion).gsub("\n", ""), comment].join(" ").strip
  end

  # SSH2 public key (RFC4716)
  #
  # ==== Parameters
  # * headers<~Hash> - Keys will be used as header-tags and values as header-values.
  #
  # ==== Examples
  # {'Comment' => '2048-bit RSA created by user@example'}
  # {'x-private-use-tag' => 'Private Use Value'}
  #
  def ssh2_public_key(headers = nil)
    self.class.ssh_public_key_to_ssh2_public_key(ssh_public_key, headers)
  end

  # Fingerprints
  #
  # MD5 fingerprint for the given SSH public key
  def md5_fingerprint
    Digest::MD5.hexdigest(ssh_public_key_conversion).gsub(/(.{2})(?=.)/, '\1:\2')
  end
  alias_method :fingerprint, :md5_fingerprint

  # SHA1 fingerprint for the given SSH public key
  def sha1_fingerprint
    Digest::SHA1.hexdigest(ssh_public_key_conversion).gsub(/(.{2})(?=.)/, '\1:\2')
  end

  # SHA256 fingerprint for the given SSH public key
  def sha256_fingerprint
    Base64.encode64(Digest::SHA256.digest(ssh_public_key_conversion)).gsub("\n", "")
  end

  # Determine the length (bits) of the key as an integer
  def bits
    self.class.ssh_public_key_bits(ssh_public_key)
  end

  # Randomart
  #
  # Generate OpenSSH compatible ASCII art fingerprints
  # See http://www.opensource.apple.com/source/OpenSSH/OpenSSH-175/openssh/key.c (key_fingerprint_randomart function)
  #
  # Example:
  # +--[ RSA 2048]----+
  # |o+ o..           |
  # |..+.o            |
  # | ooo             |
  # |.++. o           |
  # |+o+ +   S        |
  # |.. + o .         |
  # |  . + .          |
  # |   . .           |
  # |    Eo.          |
  # +-----------------+
  def randomart
    fieldsize_x = 17
    fieldsize_y = 9
    x = fieldsize_x / 2
    y = fieldsize_y / 2
    raw_digest = Digest::MD5.digest(ssh_public_key_conversion)
    num_bytes = raw_digest.bytesize

    field = Array.new(fieldsize_x) { Array.new(fieldsize_y) {0} }

    raw_digest.bytes.each do |byte|
      4.times do
        x += (byte & 0x1 != 0) ? 1 : -1
        y += (byte & 0x2 != 0) ? 1 : -1

        x = [[x, 0].max, fieldsize_x - 1].min
        y = [[y, 0].max, fieldsize_y - 1].min

        field[x][y] += 1 if (field[x][y] < num_bytes - 2)

        byte >>= 2
      end
    end

    field[fieldsize_x / 2][fieldsize_y / 2] = num_bytes - 1
    field[x][y] = num_bytes
    augmentation_string = " .o+=*BOX@%&#/^SE"
    output = "+--#{sprintf("[%4s %4u]", type.upcase, bits)}----+\n"
    fieldsize_y.times do |y|
      output << "|"
      fieldsize_x.times do |x|
        output << augmentation_string[[field[x][y], num_bytes].min]
      end
      output << "|"
      output << "\n"
    end
    output << "+#{"-" * fieldsize_x}+"
    output
  end

  def directives=(directives)
    @directives = Array[directives].flatten.compact
  end
  attr_reader :directives

  private

  # SSH Public Key Conversion
  #
  # All data type encoding is defined in the section #5 of RFC #4251.
  # String and mpint (multiple precision integer) types are encoded this way:
  # 4-bytes word: data length (unsigned big-endian 32 bits integer)
  # n bytes: binary representation of the data

  # For instance, the "ssh-rsa" string is encoded as the following byte array
  # [0, 0, 0, 7, 's', 's', 'h', '-', 'r', 's', 'a']
  def ssh_public_key_conversion
    typestr = SSH_TYPES.invert[type]
    methods = SSH_CONVERSION[type]
    pubkey = key_object.public_key
    methods.inject([7].pack("N") + typestr) do |pubkeystr, m|
      # Given pubkey.class == OpenSSL::BN, pubkey.to_s(0) returns an MPI
      # formatted string (length prefixed bytes). This is not supported by
      # JRuby, so we still have to deal with length and data separately.
      val = pubkey.send(m)

      # Get byte-representation of absolute value of val
      data = val.to_s(2)

      first_byte = data[0,1].unpack("c").first
      if val < 0
        # For negative values, highest bit must be set
        data[0] = [0x80 & first_byte].pack("c")
      elsif first_byte < 0
        # For positive values where highest bit would be set, prefix with \0
        data = "\0" + data
      end
      pubkeystr + [data.length].pack("N") + data
    end
  end

  class PublicKeyError < StandardError; end
end
