Part 2 of my lecture on basic cryptography. It contain symmetric and asymmetric cryptography with very brief DES, AES, RSA

1. T U T U N J U H A N A
T E L E C O M M U N I C AT I O N E N G I N E E R I N G
S C H O O L O F E L E C T R I C A L E N G I N E E R I N G & I N F O R M AT I C S
I N S T I T U T T E K N O LO G I B A N D U N G
ET4045Telecommunication Network Security
Cryptography
Part 2

2. 2
Symmetric key cryptography

3. 3
 symmetric key crypto: Bob and Alice share known same
(symmetric) key: KAB
 e.g., key is knowing substitution pattern in mono alphabetic
substitution cipher

4. DES: Data Encryption Standard
4
 US encryption standard [NIST 1993]
 56-bit symmetric key, 64-bit
plaintext input
 Block cipher with cipher block
chaining
 How secure is DES?
 “Weakest link” is size of key  brute force attack
 1993: Weiner: $1M machine, 3.5 hours
 1998: EFF’s Deep Crack: $250,000
 92 billion keys per second; 4 days on average

5. 5
 making DES more secure:
3DES: encrypt 3 times with 3 different keys
(actually encrypt, decrypt, encrypt)

6. AES: Advanced Encryption Standard
6
 New (Nov. 2001) symmetric-key NIST standard,
replacing DES
 processes data in 128 bit blocks
 128, 192, or 256 bit keys
 brute force decryption (try each key) taking 1
sec on DES, takes 149 trillion years for AES

7. Issues in Symmetric Keys Cryptography
7
The key must be agreed upon by sender and
receiver in a secure way
 Then along came Diffie & Hellman…

8. Diffie–Hellman Key Exchange
8
How Alice and Bob want to come up with the same key by talking on the phone
without giving it away to a third party listening to the conversation?
 They agree on a large prime number p and a small integer g
These numbers are not secret
 Alice picks a large random integer a, and calculates A = ga mod p
Alice tells Bob what A is.
 Bob picks a large random integer b, and calculates B = gb mod p
Bob tellsAlice what B is.
 Alice computes Ka = Ba mod p.
 Bob computes Kb = Ab mod p.
 Ka = Kb = gab mod p
 Someone spying on the phone can not get the key without knowing a and b,
which were never spoken. Figuring out a and b from A, B, g, and p is as hard as it
is to factor numbers the same size as p, hence p should be big (hundreds of
digits)
Source: www.hep.uiuc.edu/home/mats/crypto/crypto.ppt

9. 9
Public Key Cryptography
(Asymmetric Cryptography)

10. 10
 symmetric key
crypto
 requires sender,
receiver know shared
secret key
 Q: how to agree on
key in first place
(particularly if never
“met”)?
 public key cryptography
 radically different
approach [Diffie-
Hellman76, RSA78]
 sender, receiver do not
share secret key
 public encryption key
known to all
 private decryption key
known only to receiver

11. 11

12. Public key encryption algorithms
12

13. RSA: Choosing keys
13

14. RSA: Encryption, decryption
14

15. RSA example
15

16. RSA: another important property
16

17. RSA is slow
17
 Exponentiation is computationally intensive
 DES is at least 100 times faster than RSA
Solution
 At first Bob and Alice use RSA to exchange a
symmetric key, KS
 Once both have KS, they use symmetric crypto

18. 18
http://sy0201.blogspot.com/2010/06/https-process.html
RSA
(asymmetric)
Symmetric

19. Exercise
19
 Download and install openssl (https://www.openssl.org/ or
http://gnuwin32.sourceforge.net/packages/openssl.htm)
 Read http://en.wikibooks.org/wiki/Cryptography/Generate_a_keypair_using_OpenSSL for how
to generate keypair (private and public key)
 Write a small file using notepad containing your NIM number. Name your file yourNIM.txt
 Encrypt your file using my public key (download in
https://www.dropbox.com/s/jo77l5mo7hyw0fd/pubkey-tutun.pem?dl=0)
 To encrypt the file c:>openssl rsautl -encrypt -pubin -inkey pubkey-tutun.pem -in yourNIM.txt -out
yourNIM.encrypted
 Send your yourNIM.encrypted file to tutun@stei.itb.ac.id with the Subject:
ET4045#1
 Please generate your own keypair
 Send me your public key
 I will send you next assignment using your public key encrypted file
 To decrypt the file, please play with rsautl command
 Due in one week