1. CS2911 Week 8, Class 1 Today Week 8, Lab period Muddiest Point
15 June 2019
CS2911 Week 8, Class 1
Today
Muddiest Point
Questions on Videos
Encryption
Week 8, Lab period
Quiz 4: Topics include TCP, SMTP, IMAP, POP
17q ,9-24
CS2911
Dr. Yoder
Dr. Josiah Yoder

2. This slide deck is the version presented in Section 021 on Friday of Week 7.
For the latest version, see the presentation that does not end with -021.pptx
CS2911 Dr. Yoder

3. Packet Sniffer (e.g. Wireshark)
CS2911
15 June 2019
Packet Sniffer (e.g. Wireshark)
Alice
This is the problem we've been considering so far.
This is a review of how we've solved it.
Bob
Trudy
Dr. Josiah Yoder

4. How to achieve confidentiality? Approach 1: Symmetric keys
Alice wants to send confidential message, m, to Bob. KS
KS( ) .
KS(m )
KS( ) . m
KS(m ) m
Internet KS
Bob
Alice
Problem: How to get Ks
to Bob without Trudy
intercepting?
Trudy
Alice:
encrypts message with KS (for efficiency)
sends KS(m) to Bob
Modified from the slides accompanying Kurose and Ross, 7th Ed.
Bob:
uses KS to decrypt KS(m) to recover m

5. How to achieve confidentiality? Approach 2: Public/private keys
Alice wants to send confidential message, m, to Bob. KB +
Internet
KB( ) . + m
KB(m) +
KB( ) . -
Bob
Alice KB + m
KB(m ) + KB -
Trudy
Bob:
Send public key KB to Alice +
Alice:
encrypts message with Bob's public key KB
sends KB(m) to Bob
Modified from the slides accompanying Kurose and Ross, 7th Ed. + +

6. How to achieve confidentiality? Approach 2: Public/private keys
Alice wants to send confidential message, m, to Bob. KB +
Internet
KB( ) . + m
KB(m) +
KB( ) . -
Bob
Alice KB + m
KB(m ) + KB -
Trudy
Bob (final step):
Use private key KB to decrypt KB(m) (because KB(KB(m)) = m) - + - +
Modified from the slides accompanying Kurose and Ross, 7th Ed.
Problems: Inefficient for large messages
and suffers from same problems as non-chained
cipher-blocks

7. How to achieve confidentiality? Approach 3: Use both!
KS( ) .
KB( ) + -
KS(m )
KB(KS ) m KS KB
Internet
Bob
Alice
Trudy
Alice:
generates random symmetric private key, KS
encrypts message with KS (for efficiency)
also encrypts KS with Bob’s public key
sends both KS(m) and KB(KS) to Bob
Modified from the slides accompanying Kurose and Ross, 7th Ed. +

8. How to achieve confidentiality? Approach 3: Use both!
KS( ) .
KB( ) + -
KS(m )
KB(KS ) m KS KB
Internet
Bob
Alice
Trudy
Bob:
uses his private key to decrypt and recover KS
uses KS to decrypt KS(m) to recover m
Modified from the slides accompanying Kurose and Ross, 7th Ed.
Since the symmetric key Ks is designed for cipher-block chaining, it works well on a large
amount of data

9. Man-in-the-middle attack
CS2911
15 June 2019
Man-in-the-middle attack
Alice
Here comes Trudy again!
Bob
Trudy
Dr. Josiah Yoder

10. Man-in-the-middle attack
Alice
Bob
Trudy

11. Trudy can edit ALL the packets
Not just application payload
Even TCP headers
Even IP headers
Even DNS packets
Even encrypted packets!
Every packet Alice sends
Every packet Bob sends
CS2911 Dr. Yoder

12. Change what Alice sends
CS2911
15 June 2019
Trudy can…
Alice and Bob want…
See what Alice sends
Change what Alice sends
Repeat what Alice sends
Reorder what Alice sends
Pretend to be Alice
Ask for Bob's password …
Confidentiality
Data integrity
Avoid replay attacks
Session integrity (??)
Authentication
Avoid phishing attacks …
Types of attacks / types of security
Try to get Bob's keys - Avoid chosen-plaintext attack
Take down Alice's server - Avoid denial of service attacks
Dr. Josiah Yoder

13. Example Man-in-the-middle attack
CS2911
15 June 2019
Example Man-in-the-middle attack
Bob
Alice
Trudy
where is
alice.com? at
My credit card:
Order complete!
Dr. Josiah Yoder

14. OK, so Bob will probably be using encryption… that will slow Trudy down… … or will it?
CS2911 Dr. Yoder

15. Example Man-in-the-middle attack (1)
CS2911
15 June 2019
Example Man-in-the-middle attack (1)
Bob
Alice
Trudy + +
Bob’s public key is KB
Bob’s public key is KT
This time Trudy really does give Bob Alice’s IP address.
Dr. Josiah Yoder

16. Example Man-in-the-middle attack (2)
CS2911
15 June 2019
Example Man-in-the-middle attack (2)
Bob
Alice
Trudy
Please share session key
Please share session key +
Looks good
to me!
KT(KSA) +
KB(KST)
Thanks for the session key, Alice!
Dr. Josiah Yoder

17. Example Man-in-the-middle attack (3)
CS2911
15 June 2019
Example Man-in-the-middle attack (3)
Bob
Alice
Trudy
KST(Are you Alice?)
KSA(Are you Alice?)
KST(Yes I am!)
KSA(Yes I am!)
KST (GET /perfume.html HTTP/1.1)
KSA(GET /perfume.html HTTP/1.1)
KSA(Buy )
KST(Buy )
Dr. Josiah Yoder

18. Example Man-in-the-middle attack (4)
CS2911
15 June 2019
Example Man-in-the-middle attack (4)
Bob
Alice
Trudy
KST(Here's my credit card)
KSA(Here's my credit card)
KSA(order complete)
KSA(order complete)
Thanks for the credit card info, Bob!
Dr. Josiah Yoder

19. CS2911
15 June 2019
When we lose authentication, we lose data integrity and confidentiality as well.
CS2911 Dr. Yoder
Dr. Josiah Yoder

20. CS2911
15 June 2019
Authentication
How can Bob know it is really Alice that sent him a message?
Alice needs to sign the message
Public-key cryptography can be used for this
CS2911 Dr. Yoder
Dr. Josiah Yoder

21. RSA: an important property
CS2911
15 June 2019
RSA: an important property
The following property will be very useful:
K (K (m)) = m B - +
K (K (m)) =
use public key first, followed by private key
use private key first, followed by public key
result is the same!
Modified from the slides accompanying Kurose and Ross, 7th Ed.
Security
8-21
Dr. Josiah Yoder

22. Why ? follows directly from modular arithmetic:
CS2911
15 June 2019
K (K (m)) = m B - +
K (K (m)) =
Why ?
follows directly from modular arithmetic:
(me mod n)d mod n = med mod n
= mde mod n
= (md mod n)e mod n
= m (as shown in RSA video)
Modified from the slides accompanying Kurose and Ross, 7th Ed.
Security
8-22
Dr. Josiah Yoder

23. Key point
If I receive
only Bob could have signed it, because only Bob has Bob's private key KB
I can recover the message like this
if m makes sense, it must be a message from Bob
c =
(K (m)) B - - m
K (K (m)) B + - = ?

24. Suppose, for a moment, that Bob has Alice’s public key already
CS2911 Dr. Yoder

25. [In class, we sketched on the board how Alice and Bob can use already-shared keys to sign and encrypt new messages] [To obtain both confidentiality and data integrity]
CS2911 Dr. Yoder

26. CS2911
15 June 2019
Acknowledgement
The content of this video is based in part on lecture slides from a very good textbook, and used with the author’s permission: Computer Networking: A Top-Down Approach, 7th edition, by Jim Kurose and Keith Ross Publisher: Pearson, 2017
Dr. Josiah Yoder