class Net::SSH::Transport::Algorithms

Implements the higher-level logic behind an SSH key-exchange. It handles both the initial exchange, as well as subsequent re-exchanges (as needed). It also encapsulates the negotiation of the algorithms, and provides a single point of access to the negotiated algorithms.

You will never instantiate or reference this directly. It is used internally by the transport layer.

Constants

ALGORITHMS

Define all algorithms, with the deprecated, supported by Net::SSH.

DEFAULT_ALGORITHMS

Define the default algorithms, in order of preference, supported by Net::SSH.

Attributes

algorithms[R]

The hash of algorithms preferred by the client, which will be told to the server during algorithm negotiation.

compression_client[R]

The type of compression to use to compress packets being sent by the client.

compression_server[R]

The type of compression to use to decompress packets arriving from the server.

encryption_client[R]

The type of the cipher to use to encrypt packets sent from the client to the server.

encryption_server[R]

The type of the cipher to use to decrypt packets arriving from the server.

hmac_client[R]

The type of HMAC to use to sign packets sent by the client.

hmac_server[R]

The type of HMAC to use to validate packets arriving from the server.

host_key[R]

The type of host key that will be used for this session.

kex[R]

The kex algorithm to use settled on between the client and server.

language_client[R]

The language that will be used in messages sent by the client.

language_server[R]

The language that will be used in messages sent from the server.

options[R]

The hash of options used to initialize this object

session[R]

The underlying transport layer session that supports this object

session_id[R]

The session-id for this session, as decided during the initial key exchange.

Public Class Methods

allowed_packet?(packet) click to toggle source

Returns true if the given packet can be processed during a key-exchange.

# File lib/net/ssh/transport/algorithms.rb, line 141
def self.allowed_packet?(packet)
  (1..4).include?(packet.type) ||
  (6..19).include?(packet.type) ||
  (21..49).include?(packet.type)
end
new(session, options = {}) click to toggle source

Instantiates a new Algorithms object, and prepares the hash of preferred algorithms based on the options parameter and the ALGORITHMS constant.

# File lib/net/ssh/transport/algorithms.rb, line 149
def initialize(session, options = {})
  @session = session
  @logger = session.logger
  @options = options
  @algorithms = {}
  @pending = @initialized = false
  @client_packet = @server_packet = nil
  prepare_preferred_algorithms!
end

Public Instance Methods

[](key) click to toggle source

A convenience method for accessing the list of preferred types for a specific algorithm (see algorithms).

# File lib/net/ssh/transport/algorithms.rb, line 193
def [](key)
  algorithms[key]
end
accept_kexinit(packet) click to toggle source

Called by the transport layer when a KEXINIT packet is received, indicating that the server wants to exchange keys. This can be spontaneous, or it can be in response to a client-initiated rekey request (see rekey!). Either way, this will block until the key exchange completes.

# File lib/net/ssh/transport/algorithms.rb, line 180
def accept_kexinit(packet)
  info { "got KEXINIT from server" }
  @server_data = parse_server_algorithm_packet(packet)
  @server_packet = @server_data[:raw]
  if !pending?
    send_kexinit
  else
    proceed!
  end
end
allow?(packet) click to toggle source

Returns true if no exchange is pending, and otherwise returns true or false depending on whether the given packet is of a type that is allowed during a key exchange.

# File lib/net/ssh/transport/algorithms.rb, line 209
def allow?(packet)
  !pending? || Algorithms.allowed_packet?(packet)
end
host_key_format() click to toggle source
# File lib/net/ssh/transport/algorithms.rb, line 218
def host_key_format
  case host_key
  when /^([a-z0-9-]+)-cert-v\d{2}@openssh.com$/
    Regexp.last_match[1]
  else
    host_key
  end
end
initialized?() click to toggle source

Returns true if the algorithms have been negotiated at all.

# File lib/net/ssh/transport/algorithms.rb, line 214
def initialized?
  @initialized
end
pending?() click to toggle source

Returns true if a key-exchange is pending. This will be true from the moment either the client or server requests the key exchange, until the exchange completes. While an exchange is pending, only a limited number of packets are allowed, so event processing essentially stops during this period.

# File lib/net/ssh/transport/algorithms.rb, line 202
def pending?
  @pending
end
rekey!() click to toggle source

Request a rekey operation. This will return immediately, and does not actually perform the rekey operation. It does cause the session to change state, however–until the key exchange finishes, no new packets will be processed.

# File lib/net/ssh/transport/algorithms.rb, line 170
def rekey!
  @client_packet = @server_packet = nil
  @initialized = false
  send_kexinit
end
start() click to toggle source

Start the algorithm negotation

# File lib/net/ssh/transport/algorithms.rb, line 160
def start
  raise ArgumentError, "Cannot call start if it's negotiation started or done" if @pending || @initialized

  send_kexinit
end

Private Instance Methods

build_client_algorithm_packet() click to toggle source

Given the algorithms map of preferred algorithm types, this constructs a KEXINIT packet to send to the server. It does not actually send it, it simply builds the packet and returns it.

# File lib/net/ssh/transport/algorithms.rb, line 363
def build_client_algorithm_packet
  kex         = algorithms[:kex].join(",")
  host_key    = algorithms[:host_key].join(",")
  encryption  = algorithms[:encryption].join(",")
  hmac        = algorithms[:hmac].join(",")
  compression = algorithms[:compression].join(",")
  language    = algorithms[:language].join(",")

  Net::SSH::Buffer.from(:byte, KEXINIT,
                        :long, [rand(0xFFFFFFFF), rand(0xFFFFFFFF), rand(0xFFFFFFFF), rand(0xFFFFFFFF)],
                        :mstring, [kex, host_key, encryption, encryption, hmac, hmac],
                        :mstring, [compression, compression, language, language],
                        :bool, false, :long, 0)
end
compose_algorithm_list(supported, option, append_all_supported_algorithms = false) click to toggle source

Composes the list of algorithms by taking supported algorithms and matching with supplied options.

# File lib/net/ssh/transport/algorithms.rb, line 290
def compose_algorithm_list(supported, option, append_all_supported_algorithms = false)
  return supported.dup unless option

  list = []
  option = Array(option).compact.uniq

  if option.first && option.first.start_with?('+', '-')
    list = supported.dup

    appends = option.select { |opt| opt.start_with?('+') }.map { |opt| opt[1..-1] }
    deletions = option.select { |opt| opt.start_with?('-') }.map { |opt| opt[1..-1] }

    list.concat(appends)

    deletions.each do |opt|
      if opt.include?('*')
        opt_escaped = Regexp.escape(opt)
        algo_re = /\A#{opt_escaped.gsub('\*', '[A-Za-z\d\-@\.]*')}\z/
        list.delete_if { |existing_opt| algo_re.match(existing_opt) }
      else
        list.delete(opt)
      end
    end

    list.uniq!
  else
    list = option

    if append_all_supported_algorithms
      supported.each { |name| list << name unless list.include?(name) }
    end
  end

  unsupported = []
  list.select! do |name|
    is_supported = supported.include?(name)
    unsupported << name unless is_supported
    is_supported
  end

  lwarn { %(unsupported algorithm: `#{unsupported}') } unless unsupported.empty?

  list
end
exchange_keys() click to toggle source

Instantiates one of the Transport::Kex classes (based on the negotiated kex algorithm), and uses it to exchange keys. Then, the ciphers and HMACs are initialized and fed to the transport layer, to be used in further communication with the server.

# File lib/net/ssh/transport/algorithms.rb, line 437
def exchange_keys
  debug { "exchanging keys" }

  algorithm = Kex::MAP[kex].new(self, session,
                                client_version_string: Net::SSH::Transport::ServerVersion::PROTO_VERSION,
                                server_version_string: session.server_version.version,
                                server_algorithm_packet: @server_packet,
                                client_algorithm_packet: @client_packet,
                                need_bytes: kex_byte_requirement,
                                minimum_dh_bits: options[:minimum_dh_bits],
                                logger: logger)
  result = algorithm.exchange_keys

  secret   = result[:shared_secret].to_ssh
  hash     = result[:session_id]
  digester = result[:hashing_algorithm]

  @session_id ||= hash

  key = Proc.new { |salt| digester.digest(secret + hash + salt + @session_id) }

  iv_client = key["A"]
  iv_server = key["B"]
  key_client = key["C"]
  key_server = key["D"]
  mac_key_client = key["E"]
  mac_key_server = key["F"]

  parameters = { shared: secret, hash: hash, digester: digester }

  cipher_client = CipherFactory.get(encryption_client, parameters.merge(iv: iv_client, key: key_client, encrypt: true))
  cipher_server = CipherFactory.get(encryption_server, parameters.merge(iv: iv_server, key: key_server, decrypt: true))

  mac_client = HMAC.get(hmac_client, mac_key_client, parameters)
  mac_server = HMAC.get(hmac_server, mac_key_server, parameters)

  session.configure_client cipher: cipher_client, hmac: mac_client,
                           compression: normalize_compression_name(compression_client),
                           compression_level: options[:compression_level],
                           rekey_limit: options[:rekey_limit],
                           max_packets: options[:rekey_packet_limit],
                           max_blocks: options[:rekey_blocks_limit]

  session.configure_server cipher: cipher_server, hmac: mac_server,
                           compression: normalize_compression_name(compression_server),
                           rekey_limit: options[:rekey_limit],
                           max_packets: options[:rekey_packet_limit],
                           max_blocks: options[:rekey_blocks_limit]

  @initialized = true
end
kex_byte_requirement() click to toggle source

Considers the sizes of the keys and block-sizes for the selected ciphers, and the lengths of the hmacs, and returns the largest as the byte requirement for the key-exchange algorithm.

# File lib/net/ssh/transport/algorithms.rb, line 421
def kex_byte_requirement
  sizes = [8] # require at least 8 bytes

  sizes.concat(CipherFactory.get_lengths(encryption_client))
  sizes.concat(CipherFactory.get_lengths(encryption_server))

  sizes << HMAC.key_length(hmac_client)
  sizes << HMAC.key_length(hmac_server)

  sizes.max
end
negotiate(algorithm) click to toggle source

Negotiates a single algorithm based on the preferences reported by the server and those set by the client. This is called by negotiate_algorithms.

# File lib/net/ssh/transport/algorithms.rb, line 406
def negotiate(algorithm)
  match = self[algorithm].find { |item| @server_data[algorithm].include?(item) }

  if match.nil?
    raise Net::SSH::Exception, "could not settle on #{algorithm} algorithm\n"\
      "Server #{algorithm} preferences: #{@server_data[algorithm].join(',')}\n"\
      "Client #{algorithm} preferences: #{self[algorithm].join(',')}"
  end

  return match
end
negotiate_algorithms() click to toggle source

Given the parsed server KEX packet, and the client’s preferred algorithm lists in algorithms, determine which preferred algorithms each has in common and set those as the selected algorithms. If, for any algorithm, no type can be settled on, an exception is raised.

# File lib/net/ssh/transport/algorithms.rb, line 382
def negotiate_algorithms
  @kex                = negotiate(:kex)
  @host_key           = negotiate(:host_key)
  @encryption_client  = negotiate(:encryption_client)
  @encryption_server  = negotiate(:encryption_server)
  @hmac_client        = negotiate(:hmac_client)
  @hmac_server        = negotiate(:hmac_server)
  @compression_client = negotiate(:compression_client)
  @compression_server = negotiate(:compression_server)
  @language_client    = negotiate(:language_client) rescue ""
  @language_server    = negotiate(:language_server) rescue ""

  debug do
    "negotiated:\n" +
      %i[kex host_key encryption_server encryption_client hmac_client hmac_server
         compression_client compression_server language_client language_server].map do |key|
        "* #{key}: #{instance_variable_get("@#{key}")}"
      end.join("\n")
  end
end
normalize_compression_name(name) click to toggle source

Given the SSH name for some compression algorithm, return a normalized name as a symbol.

# File lib/net/ssh/transport/algorithms.rb, line 491
def normalize_compression_name(name)
  case name
  when "none"             then false
  when "zlib"             then :standard
  when "zlib@openssh.com" then :delayed
  else raise ArgumentError, "unknown compression type `#{name}'"
  end
end
parse_server_algorithm_packet(packet) click to toggle source

Parses a KEXINIT packet from the server.

# File lib/net/ssh/transport/algorithms.rb, line 336
def parse_server_algorithm_packet(packet)
  data = { raw: packet.content }

  packet.read(16) # skip the cookie value

  data[:kex]                = packet.read_string.split(/,/)
  data[:host_key]           = packet.read_string.split(/,/)
  data[:encryption_client]  = packet.read_string.split(/,/)
  data[:encryption_server]  = packet.read_string.split(/,/)
  data[:hmac_client]        = packet.read_string.split(/,/)
  data[:hmac_server]        = packet.read_string.split(/,/)
  data[:compression_client] = packet.read_string.split(/,/)
  data[:compression_server] = packet.read_string.split(/,/)
  data[:language_client]    = packet.read_string.split(/,/)
  data[:language_server]    = packet.read_string.split(/,/)

  # TODO: if first_kex_packet_follows, we need to try to skip the
  # actual kexinit stuff and try to guess what the server is doing...
  # need to read more about this scenario.
  # first_kex_packet_follows = packet.read_bool

  return data
end
prepare_preferred_algorithms!() click to toggle source

Prepares the list of preferred algorithms, based on the options hash that was given when the object was constructed, and the ALGORITHMS constant. Also, when determining the host_key type to use, the known hosts files are examined to see if the host has ever sent a host_key before, and if so, that key type is used as the preferred type for communicating with this server.

# File lib/net/ssh/transport/algorithms.rb, line 258
def prepare_preferred_algorithms!
  options[:compression] = %w[zlib@openssh.com zlib] if options[:compression] == true

  ALGORITHMS.each do |algorithm, supported|
    algorithms[algorithm] = compose_algorithm_list(
      supported, options[algorithm] || DEFAULT_ALGORITHMS[algorithm],
      options[:append_all_supported_algorithms]
    )
  end

  # for convention, make sure our list has the same keys as the server
  # list

  algorithms[:encryption_client ] = algorithms[:encryption_server ] = algorithms[:encryption]
  algorithms[:hmac_client       ] = algorithms[:hmac_server       ] = algorithms[:hmac]
  algorithms[:compression_client] = algorithms[:compression_server] = algorithms[:compression]
  algorithms[:language_client   ] = algorithms[:language_server   ] = algorithms[:language]

  if !options.key?(:host_key)
    # make sure the host keys are specified in preference order, where any
    # existing known key for the host has preference.

    existing_keys = session.host_keys
    host_keys = existing_keys.flat_map { |key| key.respond_to?(:ssh_types) ? key.ssh_types : [key.ssh_type] }.uniq
    algorithms[:host_key].each do |name|
      host_keys << name unless host_keys.include?(name)
    end
    algorithms[:host_key] = host_keys
  end
end
proceed!() click to toggle source

After both client and server have sent their KEXINIT packets, this will do the algorithm negotiation and key exchange. Once both finish, the object leaves the pending state and the method returns.

# File lib/net/ssh/transport/algorithms.rb, line 245
def proceed!
  info { "negotiating algorithms" }
  negotiate_algorithms
  exchange_keys
  @pending = false
end
send_kexinit() click to toggle source

Sends a KEXINIT packet to the server. If a server KEXINIT has already been received, this will then invoke proceed! to proceed with the key exchange, otherwise it returns immediately (but sets the object to the pending state).

# File lib/net/ssh/transport/algorithms.rb, line 233
def send_kexinit
  info { "sending KEXINIT" }
  @pending = true
  packet = build_client_algorithm_packet
  @client_packet = packet.to_s
  session.send_message(packet)
  proceed! if @server_packet
end