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RFC 2419 PPP DES Encryption Protocol
Submitted by Raman Mittal 20021047 Nitin Awasthi
20021049 Ravi Gupta 20021049
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Main Features

• Securing Telnet
• Encryption
• Decryption

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Telnet Security

• This RFC describes a the telnet encryption option
  as a generic method of providing data
  confidentiality services for the telnet data
  stream.

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Data Encryption Standard(DES)

• It mainly consists of two parts viz. Encryption
  and Decryption.
• The client Encrypts the data using the Data
  Encryption Standard algorithm with the help of
  some authenticating key and then sends the
  encrypted data to the server using the client
  server model.
• The server receives the encrypted data from the
  client and then with the help of DES algorithm it
  decrypts it using the same key to read the actual
  data.

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Encryption

• The purpose of encrypting packets exchanged
  between two nodes using TELNET implementations is
  to attempt to insure the privacy of communication
  conducted via the two implementations. The
  encryption process depends on the specification
  of an encryption algorithm and a shared secret
  (usually involving at least a key) between the
  sender and receiver.
• Generally, the encryptor will take a packet
  including the protocol field, apply the chosen
  encryption algorithm, place the resulting cipher
  text (and in this specification, an explicit
  sequence number) in the information field of
  another packet.

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Key Schedule

• The key schedule starts with an initial permuted
  choice 1(PC-1) prior to which the 8-bit parity
  vector i.e. the key must be removed.
• 1 bit in each 8-bit byte of the key may be used
  for error detection. The parity bits k(8,16,,64)
  ensure that each byte is of odd parity.
• Thus 56 of the 64 external key bits are loaded
  into two 28-bit shift registers. The contents of
  these registers are shifted one or two positions
  to the left acc. to shift schedule.
• These 56 key bits (C i , D i), 1ltIlt16, are
  concatenated in the ordered set and reduced down
  to 48 key bits through permuted choice 2 (PC-2).
• Thus these 48-bit keys K1,..K16 are created and
  executed for encipherment at each round in the
  order K1 through K16.

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Encryption

• Main Functions used in the implemented code-
• Encrypt It encrypts the given data by using the
  complete DES technique. For that it first takes
  the text or the data which is to be encrypted and
  the key to be used for the same. This text is
  then subjected to an initial permutation to make
  it split into two blocks L0(left block) and
  R0(right block) each consisting of 32 bits.
• The 64-bit text consists of a 32 bit block L0
  followed by 32 bit block R0.
• Then R0 is expanded from 32 bit to 48 bit and the
  resultant is denoted by E(R0).This done by using
  the getbit, setbit,shft functions.
• Now the key dependent function Tau is calculated
  by using the formula
• Tj E(Ri) XOR Kj 0ltilt15 and 1ltjlt16.

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Encryption (contd)

• This 48 bit input T1 is passed to the S-boxes
  through a non linear S-box transformation to form
  the 32 bit input.
• Each S box is a table of 4 rows and 16
  columns.Each entry in a box is a 4 bit number.
• The 6 input bits of S box specify under which
  row and column number to look for output.
• Consider an S-box input of 6 bits,labeled
  b1,b2,b3,b4,b5,b6.
• Bits b1 and b6 combine to give a number from 0 to
  3 which is the row number while other 4 bits
  define the column number.

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P box permutation

• The 32 bit output of S-box substitution is
  permuted according to a P box.This permutation
  maps each input bit to an output position ,no
  bits are used twice and no bits are ignored.This
  is called straight permutation.Finally the result
  of P box permutation is XOR ed with left half of
  initial 64 bit block.Then the left and right
  halves are switched and another round begins.

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Final Permutation

• The final permutation is the inverse of the
  initial permutation .The concatenated block
  R16L16 is used as the input to the final
  permutation.The output to this block becomes our
  cipher text which can be sent over the network.

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DECRYPTION

• The same algorithm works for both encryption and
  decryption.With DES it is possible to use the
  same function to encrypt or decrypt a block.The
  only difference is that the keys must be used in
  reverse order.Thus,given an encryption algorithm
  it is easy to use it for decryption as well.

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Result
Thus using DES we are able to achieve Security
for TELNET, by providing shell to it.
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Thank You