Encryption

1
Encryption

• Encryption Transforms Message so that
  Interceptor Cannot Read it
• Plaintext (original message)
• Not necessarily text Can be graphics, etc.

Plaintext
Encryption
Ciphertext
Decryption
Plaintext
Method Key
Method Key
Transmitted
Original Message
Original Message
2
Encryption

• Encryption Transforms Message so that
  Interceptor Cannot Read it
• Ciphertext (transformed) for transmission
• Stream of ones and zeros for transmission

Plaintext
Encryption
Ciphertext
Decryption
Plaintext
Method Key
Method Key
Transmitted
Original Message
Original Message
3
Encryption

• Encryption Transforms Message so that
  Interceptor Cannot Read it
• Receiver decrypts ciphertext back to plaintext

Plaintext
Encryption
Ciphertext
Decryption
Plaintext
Method Key
Method Key
Transmitted
Original Message
Original Message
4
Encryption

• Encryption Requires a Method and a Key
• Encryption method is the specific transformation
  process
• Key is a string of bits used in the method
• Get different ciphertext with different key

Plaintext
Encryption
Ciphertext
Decryption
Plaintext
Method Key
Method Key
Transmitted
Original Message
Original Message
5
Encryption

• Encryption Requires a Method and a Key
• Method cannot be kept secret
• Key must be kept secret

Plaintext
Encryption
Ciphertext
Decryption
Plaintext
Method Key
Method Key
Transmitted
Original Message
Original Message
6
Encryption Key Length

• Key can be guessed by exhaustive search
• Try all possible keys
• See which one decrypts the message

7
Encryption Key Length

• Long keys make exhaustive search difficult
• Key is a string of bits (11000100010101)
• If length is n bits, 2n tries may be needed
• On average, need half this many
• If key length is 8 bits, only 256 tries maximum

8
Encryption Key Length

• Long keys make exhaustive search difficult
• Weak security Today lt 100 bits
• Strong security Today gt 100 bits
• Need for length will grow over time
• Laws may limit export of strong security, sending
  strongly encrypted messages internationally

9
Methods and Algorithms

• Encryption Method Categories
• Two general ways of doing encryption
• Symmetric key versus public key encryption
• Encryption Method Algorithms
• Specific ways of doing encryption
• With symmetric key encryption DES, 3DES, AES,
  IDEA, Blowfish, and RC5 algorithms
• With public key encryption RSA, elliptical curve
  cryptosystem (ECC), El Gamal

10
Symmetric Key Encryption

• Both sides use a single key to encrypt decrypt
• When A send to B
• A encrypts with the key, B decrypts with the key
• When B sends to A
• B encrypts with the key, A decrypts with the key

B
A
Symmetric Key
11
Symmetric Key Encryption

• Symmetric key encryption and decryption processes
  are simple enough for fast encryption/decryption
• Fast enough for long messages

B
A
Symmetric Key
12
Symmetric Key Encryption

• Problem 1 Symmetric key must be distributed
  secretly between partners or interceptors can
  read subsequent messages

Key A
A
Key B
B
13
Symmetric Key Encryption

• Problem 2 Need a different symmetric key for
  each business partner
• Or other partners could read messages
• Complicates symmetric key distribution

Key A
A
Key B
B
14
Symmetric Key Encryption

• Problem Need a different symmetric key for each
  business partner
• If there are N partners
• And if each needs to communicate with all others
• Then N(N-1)/2 keys must be distributed

Key A
A
Key B
B
15
Symmetric Key Encryption

• Data Encryption Standard (DES)
• Extremely popular symmetric key algorithm
• DES breaks the plaintext into blocks of 64 bits
• It then encrypts each block of plaintext using a
  64-bit key
• However, it is proper to say that DES uses a
  56-bit key, because 8 of the key bits are
  redundant (can be computed from the other 56)
• 56-bit key is small, giving inadequate strength
  for important transactions (OK for small ones)

16
Symmetric Key Encryption

• Triple DES (3DES)
• Applies DES three times to encrypt
• With 3 keys, get 168-bit effective key length
• Encrypt block with first key
• Decrypt (yes, decrypt) result with second key
• Encrypt result with third key Send this
  ciphertext
• Decryption
• Decrypt ciphertext with third key
• Encrypt (yes, encrypt) result with second key
• Decrypt result with third key

17
Symmetric Key Encryption

• Note Encryption and Decryption often are
  Reversible
• Usually, encrypt to get ciphertext and then
  decrypt to restore the plaintext
• Some algorithms also can decrypt to get
  ciphertext and encrypt to restore plaintext
• Both approaches transform the plaintext into
  ciphertext to give confidentiality (privacy),
  then unscramble the ciphertext back to the
  original plaintext

18
Symmetric Key Encryption

• Triple DES (3DES)
• Can be done with 2 keys for 112-bit effective key
  length
• Encryption
• Encrypt block with first key
• Decrypt (yes, decrypt) result with second key
• Encrypt result with first key again Send this
  ciphertext
• Decryption
• Decrypt received ciphertext with first key
• Encrypt (yes, encrypt) result with second key
• Decrypt result with first key again

19
Symmetric Key Encryption

• DES is an old algorithm
• Developed in 1970s
• 3DES is merely a way of extending its life
• Advanced Encryption System (AES)
• Being developed by the U.S. National Institutes
  for Standards and Technology
• Will be much stronger with longer keys

20
Public Key Encryption

• There are Two General Encryption Method
  Categories
• Symmetric key encryption (just seen)
• Public key encryption (next)

21
Public Key Encryption Methods

• Different keys for encryption and decryption
• Encryption with receivers public key
• Decryption with receivers private key
• Once encrypted, sender cannot decrypt the
  ciphertext does not have receivers private key

Plaintext
Encryption
Ciphertext
Decryption
Plaintext
Public Key
Private Key
22
Public Key Encryption

• Everyone has a public and private key
• Keep the private key secret
• Distribute the public key to everybody without
  security

Public Key
Private Key
Public Key
23
Public Key Encryption

• Then anyone can encrypt messages to you using
  your public key
• But only you can decrypt the messages

Encryption
Public Key
Private Key
Public Key
24
Public Key Encryption

• Four Keys Needed for Two-Way Communication
• Each side has a public and a private key
• Each sends public key to other unsecurely

B
A
Bs Pub Key
Bs Priv Key
As Priv Key
As Pub Key
25
Public Key Encryption

• Four Keys Needed for Two-Way Communication
• Encrypt with other partys public key
• Decrypt with own private key

B
A
Bs Pub Key
Bs Priv Key
As Priv Key
As Pub Key
26
Public Key Encryption

• Four Keys Needed for Two-Way Communication
• Never refer to public and private keys without
  saying to whose public or private key you are
  referring

B
A
Bs Pub Key
Bs Priv Key
As Priv Key
As Pub Key
27
Public Key Encryption

• No need for separate secret key for each business
  partner
• Greatly simplifies key management

28
Public Key Encryption

• Unfortunately, highly processing-intensive
• 100 times slower than symmetric key encryption
• So can only encrypt small messages
• Also, often can only encrypt messages about the
  size of the public key (typically a few thousand
  bits)

29
Combining Public, Symmetric Key

• Not competitors--Used Together in Practice
• Public key is easy to distribute but can only be
  used for small messages
• Symmetric key has key distribution problems but
  can be used for long messages
• They have complementary strengths and weaknesses

30
Combining Public, Symmetric Key

• Symmetric Key Encryption and Public Key
  Encryption are Complementary, not Competitors
• Often, partners first communicate with public key
  encryption
• Including initial authentication

B
A
Public Key Authentication
31
Combining Public, Symmetric Key

• Then one sides generates a symmetric key
• Encrypts symmetric key with partners public key,
  sends to partner
• Now, both sides have the symmetric key

Public Key Encryption
B
A
Symmetric Session Key
32
Combining Public, Symmetric Key

• Afterward, both sides communicate with the
  symmetric key
• This symmetric session key is good only for
  this session--single flow of communications

Symmetric Session Key
B
A
33
Public Key Algorithms

• Public Key Encryption is a Method Category
• Must Use a Specific Public Key Algorithm
• RSA
• Most widely used public key algorithm
• Patented, but public domain in October 2000
• Elliptical Curve Cryptosystem (ECC)
• Can use smaller keys than RSA with same degree of
  protection

34
Public Key Encryption

• If know someones public key, there is no known
  way to compute their private key faster than
  exhaustive search
• If there was, public key encryption would be
  useless
• This is a general concern for public key
  encryption because there is no proof that there
  is no possible way to compute the private key
  rapidly if a public key is known