Class: Rex::Proto::Kerberos::Crypto::AesBlockCipherBase

Inherits:
BlockCipherBase show all
Includes:
GssNewEncryptionType, Utils
Defined in:
lib/rex/proto/kerberos/crypto/aes_block_cipher_base.rb

Overview

Base class for RFC3962 AES encryption classes

Direct Known Subclasses

Aes128CtsSha1, Aes256CtsSha1

Constant Summary collapse

BLOCK_SIZE = 
16
PADDING_SIZE = 
1
MAC_SIZE = 
12
HASH_FUNCTION = 
'SHA1'

Constants included from GssNewEncryptionType

GssNewEncryptionType::GSS_ACCEPTOR_SUBKEY, GssNewEncryptionType::GSS_HEADER_LEN, GssNewEncryptionType::GSS_SEALED, GssNewEncryptionType::GSS_SENT_BY_ACCEPTOR, GssNewEncryptionType::TOK_ID_GSS_WRAP

Instance Method Summary collapse

Methods included from GssNewEncryptionType

#calculate_encrypted_length, #gss_unwrap, #gss_wrap

Methods included from Utils

#xor_bytes, #xor_strings

Methods inherited from BlockCipherBase

#add_ones_complement, #calculate_encrypted_length, #checksum, #decrypt, #encrypt, #gss_unwrap, #gss_wrap, #rotate_right

Instance Method Details

#decrypt_basic(ciphertext, key) ⇒ Object

Raises:



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# File 'lib/rex/proto/kerberos/crypto/aes_block_cipher_base.rb', line 59

def decrypt_basic(ciphertext, key)
  raise Rex::Proto::Kerberos::Model::Error::KerberosError, 'Ciphertext too short' if ciphertext.length < BLOCK_SIZE

  cipher = OpenSSL::Cipher.new(self.class::DECRYPT_CIPHER_NAME)
  if key.length != cipher.key_len
    raise Rex::Proto::Kerberos::Model::Error::KerberosError, "Decryption key length must be #{cipher.key_len} for #{self.class::ENCRYPT_CIPHER_NAME}"
  end

  cipher.decrypt
  cipher.key = key
  cipher.padding = 0

  if ciphertext.length == BLOCK_SIZE
    return cipher.update(ciphertext) + cipher.final
  end

  # Split the ciphertext into blocks.  The last block may be partial.
  block_chunks = ciphertext.unpack('C*').each_slice(BLOCK_SIZE).to_a
  last_block_length = block_chunks[-1].length

  # CBC-decrypt all but the last two blocks.
  prev_chunk = [0] * BLOCK_SIZE
  plaintext_arr = []
  block_chunks.slice(0..-3).each do |chunk|
    decrypted = cipher.update(chunk.pack('C*')) + cipher.final
    decrypted_arr = decrypted.unpack('C*')
    plaintext_arr += xor_bytes(decrypted_arr, prev_chunk)
    prev_chunk = chunk
  end

  # Decrypt the second-to-last cipher block.  The left side of
  # the decrypted block will be the final block of plaintext
  # xor'd with the final partial cipher block; the right side
  # will be the omitted bytes of ciphertext from the final
  # block.
  decrypted = cipher.update(block_chunks[-2].pack('C*')) + cipher.final
  decrypted_arr = decrypted.unpack('C*')
  last_plaintext_arr = xor_bytes(decrypted_arr[0, last_block_length], block_chunks[-1])
  omitted_arr = decrypted_arr[last_block_length, decrypted.length]

  # Decrypt the final cipher block plus the omitted bytes to get
  # the second-to-last plaintext block.

  decrypted = cipher.update((block_chunks[-1] + omitted_arr).pack('C*'))
  decrypted_arr = decrypted.unpack('C*')
  plaintext_arr += xor_bytes(decrypted_arr, prev_chunk)
  (plaintext_arr + last_plaintext_arr).pack('C*')
end

#encrypt_basic(plaintext, key) ⇒ Object

Raises:



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# File 'lib/rex/proto/kerberos/crypto/aes_block_cipher_base.rb', line 34

def encrypt_basic(plaintext, key)
  raise Rex::Proto::Kerberos::Model::Error::KerberosError, 'Ciphertext too short' if plaintext.length < BLOCK_SIZE

  cipher = OpenSSL::Cipher.new(self.class::ENCRYPT_CIPHER_NAME)
  if key.length != cipher.key_len
    raise Rex::Proto::Kerberos::Model::Error::KerberosError, "Encryption key length must be #{cipher.key_len} for #{self.class::ENCRYPT_CIPHER_NAME}"
  end

  cipher.encrypt
  cipher.key = key
  cipher.padding = 0

  padded = pad_with_zeroes(plaintext, BLOCK_SIZE)
  ciphertext = cipher.update(padded) + cipher.final
  if plaintext.length > BLOCK_SIZE
    # Swap the last two ciphertext blocks and truncate the
    # final block to match the plaintext length.
    last_block_length = plaintext.length % BLOCK_SIZE
    last_block_length = BLOCK_SIZE if last_block_length == 0
    ciphertext = ciphertext[0, ciphertext.length - 32] + ciphertext[-BLOCK_SIZE, BLOCK_SIZE] + ciphertext[-32, last_block_length]
  end

  ciphertext
end

#header_byte_count ⇒ Object

The number of bytes in the encrypted plaintext that precede the actual plaintext



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# File 'lib/rex/proto/kerberos/crypto/aes_block_cipher_base.rb', line 111

def header_byte_count
  BLOCK_SIZE
end

#string_to_key(password, salt, params: nil) ⇒ String

Derive an encryption key based on a password and salt for the given cipher type

Parameters:

  • password (String) —

    The password to use as the basis for key generation

  • salt (String) —

    A salt (usually based on domain and username)

  • params (String) (defaults to: nil) —

    When unpacked, the number of iterations used during key generation

Returns:

  • (String) —

    The derived key



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# File 'lib/rex/proto/kerberos/crypto/aes_block_cipher_base.rb', line 26

def string_to_key(password, salt, params: nil)
  params = "\x00\x00\x10\x00" if params == nil
  iterations = params.unpack('N')[0]
  seed = OpenSSL::KDF.pbkdf2_hmac(password, salt: salt, iterations: iterations, length: self.class::SEED_SIZE, hash: HASH_FUNCTION)
  tkey = random_to_key(seed)
  derive(tkey, 'kerberos'.encode('utf-8'))
end

#trailing_byte_count ⇒ Object

The number of bytes in the encrypted plaintext that follow the actual plaintext



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# File 'lib/rex/proto/kerberos/crypto/aes_block_cipher_base.rb', line 118

def trailing_byte_count
  MAC_SIZE
end