Index

1. Cryptography

• cryptography is used to fulfil the following functions
  • confidentiality
    • information in a message is only accessible by those people authorised to access it
  • authentication
    • it should be possible for the receiver of a message to know who really sent the message
  • integrity
    • it should be possible for the receiver of a message to verify that the message has not been modified in transit
  • non-repudiation
    • a sender should not be able to falsely deny that they sent the message

2. Types of ciphers

• symmetric cipher
  • same key used to encrypt and decrypt information
• asymmetric cipher
  • does not use the same key to encrypt and decrypt information

• 

3. A Simple Cipher

• the Caesar cipher is named after Julius Caesar, said to be the first to use it
• in the Caesar cipher each character is substituted by another
• a technique called a mono-alphabetic cipher

• Plain text: ABCDEFGHIJKLMNOPQRSTUVWXYZ
  Ciphertext: OPQRSTUVWXYZABCDEFGHIJKLMN

  • Thus using a Caesar cipher, the message
    • "Mission Impossible" would be encoded as:
    • "Awggwcb Wadcggwpzs"
      

4. Types of cipher

• simple substitution cipher
  • mono-alphabetic, where each character in the plain text is replaced with a corresponding character of cipher-text
• homo-phonic substitution cipher
  • is like a simple substitution cipher except that a single character of plain-text can map onto several characters of cipher-text
    • A might map onto 5, 14, 147
    • used in 1401 by Duchy of Mantua
• polygram substitution
  • blocks of plain-text characters are encrypted in groups into blocks of cipher-text
    • ABA might map onto RTQ
    • used by British during WWI
• polyalphabetic substitution
  • cipher is made up of multiple mono-alphabetic ciphers
  • the cipher changes with the position of each character in the plain text
    • used in American civil war by the Union

5. Cryptanalysis and attacks

• cipher-text only attack
  • analyst has cipher-text of several messages
  • all encrypted using same encryption algorithm and key
• known plain text attack
  • analyst has cipher-text and plain-text
  • tries to deduce the keys
• chosen plain text attack
  • has all plain text attack and also can choose the particular plain-text to be encrypted
  • tries to deduce the keys
• adaptive chosen plain text attack
  • special case of chosen plain text attack
  • can modify the choice of plain-text to be encrypted based on previous results
  • tries to deduce the keys
• purchase key attacks
  • analyst threatens, blackmails, tortures or bribes someone for the keys..

6. Modern encryption algorithms

• DES was created in 1977
• plain-text divided into 64 bits in length and the key is 56 bits long
• pros: can be implemented in hardware
• cons: can be broken in 3.5 hours on a fast machine
  • watch this space for breaking DES on the new 64 bit hardware
• DES is a monoalphabetical substitution cipher using a 64 bit character
  • fundamentally a 56 bit key is too short and this algorithm is open to a brute force attack

7. Triple DES

• with triple DES, each block of plain-text is processed three times
• pros: two key version of DES is estimated to be stronger
  • the triple DES even stronger
    • triple DES uses two keys, K1 used, K2 used, then K1 used again
• cons: if key1, key2 are the same triple DES collapses to single DES
  • key management is harder

• 

8. IDEA

• should be used instead of DES or triple DES
• it uses a 128 bit key
• input is characters of 64 bits
• no currently known technique is thought to break IDEA
  • keysize makes it secure from brute force for many decades
  • Tanenbaum 1997, Computer Networks, p596

9. Public/Private key encryption

• key management is a problem with previous algorithms
  • how does someone send their key to the receiver?
• algorithms have been discovered which use two keys
  • private
  • public
• public/private key encryption is asymmetric
• each user publishes their public key
• so: P = D(Kpriv, E(Kpub, P))
  • a user can decode with a private key what someone else has encoded with the corresponding public key
• also P = D(Kpub, E(Kpriv, P))
  • a user can decode with a public key what someone else has encrypted with the corresponding private key
    • used for digital signatures

10. Combining Public/Private key encryption and signatures

• suppose that Alice wishes to send a message to Bob
• Bob wishes to know that the message really comes from Alice
• the message must only be read by Bob and Alice
• both Alice and Bob publish their public keys
  • Alice = Apub
  • Bob = Bpub
• and keep their private keys hidden
  • Alice = Apri
  • Bob = Bpri
• Alice encrypts a document, M, with her private key
  • E(Apri, M)
• she encrypts the document with Bobs public key and sends the message, C, to Bob
  • C = E(Bpub, E(Apri, M))
• only Bob can decrypt this message with his private key i = D(Bpri, C)
• Bob then decrypts this message with Alice’s public key
  • M = D(Apub, i)
    • only Alice could have sent it as Alice’s public key unlocks the message
• fortunately all this can be done transparently via software!

11. Some practical scripts: a script to log into remote server

• #!/usr/bin/expect
  
  log_user 0
  spawn pwd
  expect -re "(^.*/.*)$"
  set localdir $expect_out(1,string)
  expect eof
  
  
  log_user 0
  spawn ssh merlin
  expect "Password: "
  send "abcdefghi\r"
  expect "\\$"
  send "cd $localdir"
  interact

• fred@isode:~/GM2/gcc-3.2/gcc/gm2$ $HOME/SHELL/m
  cd /home/fred/GM2/gcc-3.2/gcc/gm2
  fred@merlin:~/GM2/gcc-3.2/gcc/gm2$

12. Remote enabling of a modem

• #!/usr/bin/expect
  
  log_user 0
  spawn pwd
  expect -re "(^.*/.*)$"
  set localdir $expect_out(1,string)
  expect eof
  
  
  log_user 0
  spawn ssh -l root merlin
  expect "Password: "
  send "abcdefghi\r"
  expect "\\$"
  send "cd $localdir"
  send "wvdial\r"
  interact

• what permissions should these scripts have?
• what security risks do they present?

Index

This document was produced using groff-1.19.