1. 15-440 Distributed Systems Fall 2016
L-23 Security

2. Today's Lecture Internet security weaknesses
Establishing secure channels (Crypto 101)
Key distribution

3. What is “Internet Security” ?
Denial-of-Service
Password Cracking
Traffic modification
Worms & Viruses
Trojan Horse
DNS Poisoning
* This list is pretty broad, some things are actual incidents that may cause harm, while others are vulnerabilities that lead to more complex attacks that cause harm.
From such a list, we might define Internet security broadly as preventing bad things that can happen when someone is using the Internet.
Phishing
Spyware
IP Spoofing
Route Hijacks
Traffic Eavesdropping
End-host impersonation
Spam
Internet Security: Prevent bad things from happening on the internet!

4. Internet Design Decisions: (ie: how did we get here? )
Origin as a small and cooperative network ( largely trusted infrastructure)
Global Addressing (every sociopath is your next-door neighbor)
Connection-less datagram service (can’t verify source, hard to protect bandwidth)
Assumption is still true, even now when Internet is a huge collection of independent ISPs.

5. Internet Design Decisions: (ie: how did we get here? )
Anyone can connect
( ANYONE can connect)
Millions of hosts run nearly identical software
( single exploit can create epidemic)
Most Internet users know about as much as Senator Stevens aka “the tubes guy”
( God help us all…)
Assumption is still true, even now when Internet is a huge collection of independent ISPs.
And with the advent of things like Mobile Devices and Internet of Things, it does not look like things will become any easier in the future.

6. Our “Narrow” Focus Yes:
1) Creating a “secure channel” for communication
Some:
2) Protecting resources and limiting connectivity
No:
1) Preventing software vulnerabilities & malware, or “social engineering”.
For this course though, we want to focus on understanding how the Internet’s design contributes to a variety of security vulnerabilities, how these vulnerabilities can be exploited, and, most importantly, give you an idea of what tools are used to allow secure communication.

7. Secure Communication with an Untrusted Infrastructure
Bob
ISP D
ISP B
The focus on today’s lecture will be how to we set up a “secure channel” for communication.
Alice wants to talk to Bob.
Alice probably trusts ISP A, but how does she know where her traffic is going after that? Or who else might see it, or modify it before it reaches bob?
ISP C
ISP A
Alice

8. What do we need for a secure communication channel?
Authentication (Who am I talking to?)
Confidentiality (Is my data hidden?)
Integrity (Has my data been modified?)
Availability (Can I reach the destination?)
We will talk about the first three of these things today… availability we must tolerate failures and assume network gives us some guarantee

9. Example: Eavesdropping - Message Interception (Attack on Confidentiality)
Mallory
Bob
ISP D
ISP B
Of course, its even more complicated…. As each ISP if made of up many routers, each which independently reach a forwarding decision.
Can this infrastructure be trusted? No, mallory (or the NSA) might be on any of the routers, intercepting your communication
ISP C
ISP A
Alice

10. Example: Eavesdropping - Message Interception (Attack on Confidentiality)
Unauthorized access to information
Packet sniffers and wiretappers
Illicit copying of files and programs B A
Eavesdropper
slide derived from original by Nick Feamster

11. Eavesdropping Attack: Example
tcpdump with promiscuous network interface
On a switched network, what can you see?
What might the following traffic types reveal about communications?
Full IP packets with unencrypted data
Full IP packets with encrypted payloads
Just DNS lookups (and replies)
slide derived from original by Nick Feamster

12. Authenticity Attack - Fabrication
ISP D
ISP B
Worse yet, how does Alice even know she’s actually talking to Bob at all? This way, Mallory may not even have to compromise a router!
ISP C
ISP A
Alice
Hello, I’m
“Bob”

13. Authenticity Attack - Fabrication
Unauthorized assumption of other’s identity
Generate and distribute objects under this identity B A
Masquerader: from A
slide derived from original by Nick Feamster

14. Integrity Attack - Tampering
Stop the flow of the message
Delay and optionally modify the message
Release the message again
Alice
Bob
Perpetrator
slide derived from original by Nick Feamster

15. Attack on Availability
Destroy hardware (cutting fiber) or software
Modify software in a subtle way
Corrupt packets in transit
Blatant denial of service (DoS):
Crashing the server
Overwhelm the server (use up its resource)
Alice
Bob
slide derived from original by Nick Feamster

16. Example: Web access
Alice wants to connect to her bank to transfer some money...
Alice wants to know ...
that she’s really connected to her bank.
That nobody can observe her financial data
That nobody can modify her request
That nobody can steal her money!
The bank wants to know ...
That Alice is really Alice (or is authorized to act for Alice)
The same privacy things that Alice wants so they don’t get sued or fined by the government.
Authentication
Confidentiality
Integrity
(A mix)

17. Today's Lecture Internet security weaknesses Crypto 101
Key distribution

18. Cryptography As a Tool Using cryptography securely is not simple
Designing cryptographic schemes correctly is near impossible.
Today we want to give you an idea of what can be done with cryptography.
Take a security course if you think you may use it in the future (e.g )

19. Well... What tools do we have at hand? Hashing
e.g., SHA-1
Secret-key cryptography, aka symmetric key.
e.g., AES
Public-key cryptography
e.g., RSA

20. Secret Key Cryptography
Given a key k and a message m
Two functions: Encryption (E), decryption (D)
ciphertext c = E(k, m)
plaintext m = D(k, c)
Both use the same key k.
“secure” channel
Hello,Bob
Alice
Bob.com
knows K
knows K
But... how does that help with authentication?
They both have to know a pre-shared key K before they start!

21. Symmetric Key: Confidentiality
Motivating Example:
You and a friend share a key K of L random bits, and a message M also L bits long.
Scheme:
You send her the xor(M,K) and then they “decrypt” using xor(M,K) again.
Using XOR for encryption by itself not useful since it can be broken by frequency analysis, but it is a component in many encryption schemes.
Do you get the right message to your friend?
Can an adversary recover the message M?

22. Symmetric Key: Confidentiality
One-time Pad (OTP) is secure but usually impractical
Key is as long at the message
Keys cannot be reused (why?)
Keys cannot be resused often since if you use the same key with others they know how to decrypt the message. Also, using frequency analysis an attacker could guess the key.
In practice, two types of ciphers are used that require only constant key length:
Stream Ciphers:
Ex: RC4, A5
Block Ciphers:
Ex: DES, AES, Blowfish

23. Symmetric Key: Confidentiality
Stream Ciphers (ex: RC4)
PRNG
Alice:
Pseudo-Random stream of L bits
XOR
K A-B
Message of Length L bits
PRNG == Pseudorandom number generator =
Encrypted Ciphertext
Bob uses KA-B as PRNG seed, and XORs encrypted text to get the message back (just like OTP).

24. Symmetric Key: Confidentiality
Block Ciphers (ex: AES)
(fixed block size, e.g. 128 bits)
Block 1
Block 2
Block 3
Block 4
Round #1
Round #2
Round #n
The book has AES and DES descriptions. Turns out DES by itself has been shown to be succeptible to cracking, while a variant a it (called 3DES is still Ok and used).
Alice:
K A-B
Block 1
Block 2
Block 3
Block 4
Bob breaks the ciphertext into blocks, feeds it through decryption engine using KA-B to recover the message.

25. Symmetric Key: Integrity
Background: Hash Function Properties
Consistent hash(X) always yields same result
One-way given X, can’t find Y s.t. hash(Y) = X
Collision resistant given hash(W) = Z, can’t find X such that hash(X) = Z
Hash Fn
Fixed Size
Hash
Message of arbitrary length

26. Symmetric Key: Integrity
Hash Message Authentication Code (HMAC)
Step #1:
Alice creates MAC
Hash Fn
Message
MAC
This is similar to a checksum, but secure. Why? Note: the message does not have to be encrypted, unless you also desire confidentiality.
Will the MAC always check out on the receiving end? Yes, b/c receiver has key, and HASH is consistent.
Can attacker substitute in another message? No, b/c of collision resistance of HASH
Can attacker recover the key, based on the message? No, b/c of one-way nature of HASH
K A-B
Alice Transmits Message & MAC
Step #2
Step #3
Bob computes MAC with message and KA-B to verify.
MAC
Message
Why is this secure from a message integrity perspective? How do properties of a hash function help us?

27. Symmetric Key: Authentication
You already know how to do this!
(hint: think about how we showed integrity)
Hash Fn
I am Bob
A43FF234
Wrong!
K A-B
Alice receives the hash, computes a hash with KA-B , and she knows the sender is Bob

28. Symmetric Key: Authentication
What if Mallory overhears the hash sent by Bob, and then “replays” it later?
ISP D
ISP B
ISP C
ISP A
Hello, I’m
Bob. Here’s the hash to “prove” it
A43FF234

29. Symmetric Key: Authentication
A “Nonce”
A random bitstring used only once. Alice sends nonce to Bob as a “challenge”. Bob Replies with “fresh” MAC result.
Nonce
Bob
Alice
Nonce
Hash
B4FE64
K A-B
B4FE64
Performs same hash with KA-B and compares results

30. Symmetric Key: Authentication
A “Nonce”
A random bitstring used only once. Alice sends nonce to Bob as a “challenge”. Bob Replies with “fresh” MAC result.
Nonce
?!?!
Alice
Mallory
If Alice sends Mallory a nonce, she cannot compute the corresponding MAC without K A-B

31. Symmetric Key Crypto Review
Confidentiality: Stream & Block Ciphers
Integrity: HMAC
Authentication: HMAC and Nonce
Questions??
Are we done? Not Really:
Number of keys scales as O(n2)
How to securely share keys in the first place?

32. Asymmetric Key Crypto:
Instead of shared keys, each person has a “key pair” KB
Bob’s public key
Bob’s private key
Note: We are not going to go into the details of what KB(m) means as far as computation. We will treat it as a black box function with these properties.
KB-1
The keys are inverses, so:
KB-1 (KB (m)) = m

33. Asymmetric/Public Key Crypto:
Given a key k and a message m
Two functions: Encryption (E), decryption (D)
ciphertext c = E(KB, m)
plaintext m = D(KB-1 , c)
Encryption and decryption use different keys!
“secure” channel
Hello,Bob
Alice
Bob.com
Knows KB
Knows KB, KB-1
But how does Alice know that KB means “Bob”?

34. Asymmetric Key Crypto:
It is believed to be computationally unfeasible to derive KB-1 from KB or to find any way to get M from KB(M) other than using KB-1 .
KB can safely be made public.
Note: We will not detail the computation that KB(m) entails, but rather treat these functions as black boxes with the desired properties. (more details in the book).
Note: We are not going to go into the details of that KB(m) means as far as computation. We will treat it as a black box function with these properties.

35. Asymmetric Key: ConfidentialityKB
Bob’s public key
KB-1
Bob’s private key
encryption
algorithm
decryption
algorithm
plaintext message
ciphertext
m = KB-1 (KB (m))
KB (m)

36. Asymmetric Key: Sign & Verify
If we are given a message M, and a value S such that KB(S) = M, what can we conclude?
The message must be from Bob, because it must be the case that S = KB-1(M), and only Bob has KB-1 !
This gives us two primitives:
Sign (M) = KB-1(M) = Signature S
Verify (S, M) = test( KB(S) == M )

37. Asymmetric Key: Integrity & Authentication
We can use Sign() and Verify() in a similar manner as our HMAC in symmetric schemes.
S = Sign(M)
Message M
Integrity:
Note: there is another way to do authentication, which we will see when we look at SSL.
Receiver must only check Verify(M, S)
Nonce
Authentication:
S = Sign(Nonce)
Verify(Nonce, S)

38. Asymmetric Key Review:
Confidentiality: Encrypt with Public Key of Receiver
Integrity: Sign message with private key of the sender
Authentication: Entity being authenticated signs a nonce with private key, signature is then verified with the public key
But, these operations are computationally expensive*

39. The Great Divide Yes No Fast Slow Asymmetric Crypto: (Public key)
Example: RSA
Symmetric Crypto: (Private key)
Example: AES
Requires a pre-shared secret between communicating parties?
Yes No
The three attributes of a secure communication we mentioned can be achieved using two different “types”, or “families” of cryptography. Each family has within it many different algorithms, with slightly different properties.
Symmetric and Asymmetric cryptography differ in the assumptions they make about the “secrets”, or “keys” that two participants use to enable secure communication. Symmetric key crypto assumes that the parties have used some mechanism to set-up a “shared secret” between the two parties that can be used to secure further communication. Public key crypto, as we will see, does not make this assumption, yet can still provide strong security properties. However, the mechanism to do this uses complex math, and as a result, the time to perform asymmetric crypto operations is significantly longer than their symmetric counter-parts.
Overall speed of cryptographic operations
Fast
Slow

40. Today's Lecture Internet security weaknesses Crypto 101
Key distribution (cover on Thursday)