1. CS2911 Week 8, Class 1 Today Week 8, Lab period Muddiest Point
31 July 2018
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. Packet Sniffer (e.g. Wireshark)
CS2911
31 July 2018
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

3. 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

4. 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. + +

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 (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

6. 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. +

7. 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

8. Man-in-the-middle attack
CS2911
31 July 2018
Man-in-the-middle attack
Alice
Here comes Trudy again!
Bob
Trudy
Dr. Josiah Yoder

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

10. 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

11. Change what Alice sends
CS2911
31 July 2018
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
Try to get Bob's keys
Take down Alice's server …
Confidentiality
Data integrity
Avoid replay attacks
Session integrity (??)
Authentication
Avoid phishing attacks
Avoid chosen-plaintext attack
Avoid denial of service attacks …
Types of attacks / types of security
Dr. Josiah Yoder

12. Example Man-in-the-middle attack
Bob
Alice
Trudy
where is
alice.com? at
My credit card:
Order complete!

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

14. Example Man-in-the-middle attack (1)
CS2911
31 July 2018
Example Man-in-the-middle attack (1)
Bob
Alice
Trudy
where is
alice.com?
What is Alice's public key? + KT
Dr. Josiah Yoder

15. Example Man-in-the-middle attack (2)
CS2911
31 July 2018
Example Man-in-the-middle attack (2)
Bob
Alice
Trudy
Please share session key
Please share session key -
Looks good
to me!
KA(KS) -
KT(KS)
Thanks for the session key, Alice!
Dr. Josiah Yoder

16. Example Man-in-the-middle attack (3)
CS2911
31 July 2018
Example Man-in-the-middle attack (3)
Bob
Alice
Trudy
KS(Are you Alice?)
KS(Are you Alice?)
KS(Yes I am!)
Ks(Yes I am!)
KS (GET /perfume.html HTTP/1.1)
KS(GET /perfume.html HTTP/1.1)
KS(Buy )
KS(Buy )
Dr. Josiah Yoder

17. Example Man-in-the-middle attack (4)
CS2911
31 July 2018
Example Man-in-the-middle attack (4)
Bob
Alice
Trudy
KS(Here's my credit card)
KS(Here's my credit card)
KS(order complete)
KS(order complete)
Thanks for the credit card info, Bob!
Dr. Josiah Yoder

18. Our current focus: 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

19. RSA: an important property
CS2911
31 July 2018
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
8-19
Dr. Josiah Yoder

20. Why ? follows directly from modular arithmetic:
CS2911
31 July 2018
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
8-20
Dr. Josiah Yoder

21. 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 + - = ?

22. Caveat
"Textbook" RSA as we discuss has several "easy" vulnerabilities, both for encryption and signing
Real RSA does a variety of other things, such as adding special padding onto the message.
As always, don't write your own algorithm for real data!
CS2911 Dr. Yoder

23. Hashing A cryptographic hash is an irreversible hash function
If you know h = H(m),
it's really hard to find another message m'
so that h = H(m'), too
(A simple sum does not work – see example in Lab 9 for an example of how easy this is to mess with)
CS2911 Dr. Yoder

24. Authentication
Alice wants to provide sender authentication & message integrity
H( ) .
KA( ) - +
H(m )
KA(H(m)) m KA
Internet
compare
Alice digitally signs message with her private key
sends both message (in the clear) and digital signature
Modified from the slides accompanying Kurose and Ross, 7th Ed.
Security
8-24

25. Confidentiality & Authentication
Alice wants to provide secrecy (confidentiality), sender authentication, & message integrity.
H( ) .
KA( ) - +
KA(H(m)) m KA
KS( )
KB( )
KB(KS ) KS KB
Internet
Alice uses three keys: her private key, Bob’s public key, newly created symmetric key
Modified from the slides accompanying Kurose and Ross, 7th Ed.
Security
8-25

26. But how does Bob get Alice's public key?KA + KA - KA +
KA(H(m)) -
H( ) .
KA( ) . -
KA(H(m)) -
KA( ) . +
H(m ) m - +
compare
Internet
H( ) . m
H(m ) m
Modified from the slides accompanying Kurose and Ross, 7th Ed.
Security
8-26

27. How can Bob tell that KT really comes from Trudy?+
How can Bob tell that KT really comes from Trudy? KA + KT +
Alice
Bob KA - KT -
Trudy

28. Let's look at Bob's order again, this time using signing to protect the data
CS2911 Dr. Yoder

29. Example Man-in-the-middle attack (2)
CS2911
31 July 2018
Example Man-in-the-middle attack (2)
Bob
Alice
Trudy
where is
alice.com?
What is Alice's public key? + KT
Dr. Josiah Yoder

30. Example Man-in-the-middle attack (3)
CS2911
31 July 2018
Example Man-in-the-middle attack (3)
Bob
Alice
Trudy
Are you Alice?
Are you Alice?
It must be from Alice!
It's signed with
her private key! -
KT(Yes I am!) -
KA(Yes I am!)
GET /perfume.html HTTP/1.1
GET /perfume.html HTTP/1.1 -
KA(Buy ) -
KT(Buy )
Dr. Josiah Yoder

31. Example Man-in-the-middle attack (4)
CS2911
31 July 2018
Example Man-in-the-middle attack (4)
Bob
Alice
Trudy +
KT (Here's my credit card) +
KA(Here's my credit card) -
KA(order complete) -
KT(order complete)
Thanks for the credit card info, Bob!
A real exchange would of course
use a session key,
too. I've left that out
to simplify the discussion.
Dr. Josiah Yoder

32. Let's look at Bob's order again, this time using signing to protect the data To make things more realistic, let's include a (shared symmetric) session key too.
CS2911 Dr. Yoder

33. Example Man-in-the-middle attack (2)
CS2911
31 July 2018
Example Man-in-the-middle attack (2)
Bob
Alice
Trudy
where is
alice.com?
What is Alice's public key? + KT
Dr. Josiah Yoder

34. Example Man-in-the-middle attack (2)
CS2911
31 July 2018
Example Man-in-the-middle attack (2)
Bob
Alice
Trudy
Please share session key
Please share session key +
Here's my public key: KB +
Here's my public key: KT +
Looks good
to me! -
KT(KA(KS)) + -
KB(KT(KS)
Thanks for the session key, Alice!
Dr. Josiah Yoder

35. Example Man-in-the-middle attack (3)
CS2911
31 July 2018
Example Man-in-the-middle attack (3)
Bob
Alice
Trudy
KS(Are you Alice?)
KS(Are you Alice?)
KS(Yes I am!)
Ks(Yes I am!)
KS (GET /perfume.html HTTP/1.1)
KS(GET /perfume.html HTTP/1.1)
KS(Buy )
KS(Buy )
Dr. Josiah Yoder

36. Example Man-in-the-middle attack (4)
CS2911
31 July 2018
Example Man-in-the-middle attack (4)
Bob
Alice
Trudy
KS(Here's my credit card)
KS(Here's my credit card)
KS(order complete)
KS(order complete)
Thanks for the credit card info, Bob!
Dr. Josiah Yoder

37. How can Bob tell that KT really comes from Trudy?+
How can Bob tell that KT really comes from Trudy? KA + KT +
Alice
Bob KA - KT -
Trudy

38. We need to sign the key, too!
CS2911
31 July 2018
We need to sign the key, too!
Can sign key exactly same way Alice signed the message
Suppose Carol sends Bob a signed message with Alice's public key saying it really is hers.
Suppose Bob trusts Carol
Suppose Bob already has Carol's public key
Then Bob can trust that Alice's public key is real!
CS2911 Dr. Yoder
Dr. Josiah Yoder

39. We need to sign the key, too!
CS2911
31 July 2018
We need to sign the key, too!
Suppose Carol sends Bob a signed message with Alice's public key saying it really is hers.
Suppose Bob trusts Carol
Suppose Dan sends Bob a signed message with Carols' public key saying it really is hers
Suppose Bob trusts Dan
Suppose Bob has Dan's public key already
Then Bob can trust that Alice's public key is real!
CS2911 Dr. Yoder
Dr. Josiah Yoder

40. We need to sign the key, too!
CS2911
31 July 2018
We need to sign the key, too!
Suppose Carol sends Bob a signed message with Alice's public key saying it really is hers.
Suppose Bob trusts Carol
Suppose Dan sends Bob a signed message with Carols' public key saying it really is hers
Suppose Bob trusts Dan
Suppose Esther sends Bob a signed message with Dan's public key saying it really is his
Suppose Bob trusts Esther
Suppose Bob already has Esther's public key
Then Bob can trust that Alice's public key is real!
Dr. Josiah Yoder

41. Bob still needs to have SOMEONE's public key to start with!
CS2911
31 July 2018
Bob still needs to have SOMEONE's public key to start with!
This "someone" is called a root certificate authority (CA)
Everyone else in the chain is a certificate authority (CA)
The root CA's key is stored on Bob's computer by the manufacturer (or by Bob's institution)
Dr. Josiah Yoder

42. Confirming a certificate chain
CS2911
31 July 2018
Confirming a certificate chain
Suppose Bob goes to alice.com, a perfume outlet
alice.com sends Bob
Her key, signed by CA1
CA1's key, signed by CA2
CA2's key, signed by the root CA
Bob checks that all keys are properly signed.
Bob checks that the root CA is one he trusts
Now Bob knows Alice's key is real. He will use it to start a secure exchange with Alice
(Example from our textbook)
Dr. Josiah Yoder

43. What if someone is able to actually steal Alice's private key?
She would like to "revoke" her key – tell everyone that it is no longer valid
Certificate Revocation Lists (CRLs) are the classic way to do this.
Each CA offers lists of all keys it has signed, but also revoked
New ways are OCSP and OCSP stapling
User goes back to CA to ask if certificate is valid
Neither one is used 100%... compromised keys are still really bad

44. Big-picture review
Public-private key and symmetric key together make for efficient way to confidentially exchange data
Man-in-the middle attack makes key distribution difficult
Public-key infrastructure and trusted root CA keys pre-installed make communication with new sites possible, even with man-in-middle attack
Lots of trust on CA hierarchy!

45. CS2911
31 July 2018
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