Module: Rex::Proto::Kerberos::Crypto::GssNewEncryptionType

Included in:
AesBlockCipherBase
Defined in:
lib/rex/proto/kerberos/crypto/gss_new_encryption_type.rb

Overview

The RFC4121 implementation of GSS wrapping (Section 4.2.6.2) which applies to “newer encryption types” (defined as those not in section 1 of RFC4121) This mixin may be included by Encryption providers in Rex::Proto::Kerberos::Crypto

Constant Summary collapse

GSS_SENT_BY_ACCEPTOR = 
1
GSS_SEALED = 
2
GSS_ACCEPTOR_SUBKEY = 
4
GSS_HEADER_LEN =

The length of the GSS header

16
TOK_ID_GSS_WRAP = 
0x0504

Instance Method Summary collapse

Instance Method Details

#calculate_encrypted_length(plaintext_len) ⇒ Object 



91
92
93
 
# File 'lib/rex/proto/kerberos/crypto/gss_new_encryption_type.rb', line 91

def calculate_encrypted_length(plaintext_len)
  plaintext_len
end

#gss_unwrap(ciphertext, key, expected_sequence_number, is_initiator, opts = {}) ⇒ Object

Parameters:

  • options (Hash) —

    a customizable set of options

Raises:



59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
 
# File 'lib/rex/proto/kerberos/crypto/gss_new_encryption_type.rb', line 59

def gss_unwrap(ciphertext, key, expected_sequence_number, is_initiator, opts={})
  use_acceptor_subkey = opts.fetch(:use_acceptor_subkey) { true }
  # Handle wrap-around
  sequence_number &= 0xFFFFFFFFFFFFFFFF

  expected_flags = GSS_SEALED
  expected_flags |= GSS_ACCEPTOR_SUBKEY if use_acceptor_subkey

  if is_initiator
    key_usage = Rex::Proto::Kerberos::Crypto::KeyUsage::GSS_ACCEPTOR_SEAL
    expected_flags |= GSS_SENT_BY_ACCEPTOR
  else
    key_usage = Rex::Proto::Kerberos::Crypto::KeyUsage::GSS_INITIATOR_SEAL
  end
  header = ciphertext[0,GSS_HEADER_LEN]
  ciphertext = ciphertext[GSS_HEADER_LEN, ciphertext.length]

  tok_id, flags, filler, ec, rrc, snd_seq = header.unpack('nCCnnQ>')
  raise Rex::Proto::Kerberos::Model::Error::KerberosError, 'Invalid token ID' unless tok_id == TOK_ID_GSS_WRAP
  raise Rex::Proto::Kerberos::Model::Error::KerberosError, 'Invalid filler' unless filler == 0xFF
  raise Rex::Proto::Kerberos::Model::Error::KerberosError, 'Unexpected flags' unless flags == expected_flags
  raise Rex::Proto::Kerberos::Model::Error::KerberosError, "Invalid sequence number (received #{snd_seq}; expected #{expected_sequence_number})" unless expected_sequence_number == snd_seq

  # Could do some sanity checking here of those values
  ciphertext = rotate(ciphertext, -(rrc+ec))

  plaintext = self.decrypt(ciphertext, key.value, key_usage)

  plaintext = plaintext[0, plaintext.length - ec - GSS_HEADER_LEN]
  plaintext
end

#gss_wrap(plaintext, key, sequence_number, is_initiator, opts = {}) ⇒ String, Integer

Encrypt the message, wrapping it in GSS structures

Parameters:

  • options (Hash) —

    a customizable set of options

Returns:

  • (String, Integer, Integer) —

    The encrypted data, the length of its header, and the length of padding added to it prior to encryption



24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
 
# File 'lib/rex/proto/kerberos/crypto/gss_new_encryption_type.rb', line 24

def gss_wrap(plaintext, key, sequence_number, is_initiator, opts={})
  use_acceptor_subkey = opts.fetch(:use_acceptor_subkey) { true }
  # Handle wrap-around
  sequence_number &= 0xFFFFFFFFFFFFFFFF

  flags = GSS_SEALED
  flags |= GSS_ACCEPTOR_SUBKEY if use_acceptor_subkey

  if is_initiator
    key_usage = Rex::Proto::Kerberos::Crypto::KeyUsage::GSS_INITIATOR_SEAL
  else
    key_usage = Rex::Proto::Kerberos::Crypto::KeyUsage::GSS_ACCEPTOR_SEAL
    flags |= GSS_SENT_BY_ACCEPTOR
  end

  tok_id = TOK_ID_GSS_WRAP
  filler = 0xFF
  ec = calculate_ec(plaintext.length)
  rrc = calculate_rrc

  # RFC4121, Section 4.2.4
  plaintext_header = [tok_id, flags, filler, 0, 0, sequence_number].pack('nCCnnQ>')

  header = [tok_id, flags, filler, ec, rrc, sequence_number].pack('nCCnnQ>')
  # "x" chosen as the filler based on the Linux implementation of the kerberos client
  # https://salsa.debian.org/debian/krb5/-/blob/0269810b1aec6c554fb746433f045d59fd34ab3a/src/lib/gssapi/krb5/k5sealv3.c#L160
  ec_filler = "x" * ec
  plaintext = plaintext + ec_filler + plaintext_header
  ciphertext = self.encrypt(plaintext, key.value, key_usage)
  rotated = rotate(ciphertext, rrc+ec)

  result = [header + rotated, header_length + ec, ec]
end