Previous lecture

1
Previous lecture

• Practical things about the course.
• Example of cryptosystem substitution cipher.
• Symmetric vs. asymmetric cryptography.
• RSA keys, encryption, decryption. (Proof of
  correctness not part of course.)

2
This lecture

• Block ciphers
• Modes of operations
• First assignment
• Hash functions

3
Block ciphers

• A block cipher B is an encryption function
  Ekey0,1k ? 0,1l and a decryption function
  Dkey0,1l ? 0,1k such that Dkey(Ekey(m)) m.
• The value k is called block length. Usually k
  l.
• Commonly used block ciphers include DES, 3DES and
  IDEA.

Clear (plain) text
Cipher text
Key
4
Chaining ciphers

• What happens when the clear text is longer than
  the block length k?
• Most simple solution encrypt each block
  separately.
• This mode is called ECB, Electronic Code Book

Clear text
Enc
Enc
Enc
Enc
Key
Cipher text
5
Problems with ECB

• The main problem with ECB is that an adversary
  can change order or remove blocks without
  detection.
• The solution link the encrypted blocks to each
  other.
• Most common option Cipher Block Chaining, CBC

6
Cipher Block Chaining

• A feedback is introduced to link the blocks
  together

Clear text
IV
Enc
Enc
Enc
Enc
Key
Cipher text
7
Cipher Block Chaining, cont.

• Let Ekey be the encryption function, Dkey be the
  decryption function, Pi block i of the clear text
  and Ci block i of the cipher text, i 1, 2, 3...
  
• Encryption of block i Ci Ekey(Pi ? Ci-1) where
  C0 IV (initialization vector)
• Decryption of block i Pi Ci-1 ? Dkey(Ci)
• The Initialization Vector, IV C0, must be known
  to both parties and can be sent in clear.

8
First assignment

• Implement encryption and decryption using your
  favourite block cipher (DES, 3DES, IDEA etc) for
  two modes (e.g., ECB and CBC) with a usable (not
  necessarily user-friendly!) command-line
  interface.
• Use an existing crypto library for the block
  cipher, but implement the chaining yourself!
• Examples of possible crypto libraries to use
  openssl (for C) or JSSE (for Java).
• You can get a maximum of 20 points for the exam
  from this assignment.

9
Rules for the assignment

• Choose your favourite language!
• If you pick another language than C, C, Pascal
  or Java, or another platform than UNIX/Linux or
  Windows/DOS, please contact me first!
• Solve the assignment either individually or in
  pairs.
• Hand in the solution no later than March 5th. You
  lose one point per day if you hand in late. You
  can hand in your solution
• By email to marten_at_nada.kth.se.
• On a diskette at the lecture
• As a link to a site that I can reach

10
Rules for the assignment, cont.

• Please include
• source code
• executable
• a brief description of the interface (just enough
  so that I can run it)
• contact information
• the amount of time you spent on the assignment
  (not used for grading, just to tune the
  difficulty of the assignments)

11
Rules for the assignment, cont.

• Co-operation between groups is allowed only on a
  conceptual level
• Example of things you may discuss Is it easier
  to solve the assignment in Java than C? What is a
  good format to provide the key? Is this input
  format reasonable?
• Example of things you may not discuss Please
  show me your code so I can copy part of it!
• Please state the persons you have discussed the
  solution with.
• You may be asked to explain your solution orally.

12
Hash functions

• A hash function computes a fixed length value
  from a variable length source
• Example Check sums in communication protocols
• Indices in databases
• More convenient to handle a hash of a document
  instead of the document itself
• We will consider cryptographically secure hash
  functions.

13
Hash functions, definition

• A hash function is a function f0,1 ? 0,1n.
• The size of the output, n, is a property of the
  function. Common values are 128, 160 and 256.
• Commonly used hash functions are MD5, SHA and
  SHA-1

14
Hash function examples

• f(m) first 40 bits of m
• f(m) last 40 bits of m
• f(m) XOR of the bytes of m

15
Properties of good hash functions

• Let H be a hash function
• One-way
• Given x, unfeasible to compute an v such that
  H(v) x
• Collision-free
• Unfeasible to find x1 and x2 such that H(x1)
  H(x2) and x1 ? x2