1. Cryptography and Networks Advanced Network Security Peter Reiher August, 2014

2. Outline
Using cryptography in networks
IPSec
SSL and TLS

3. Encryption and Network Security
Relies on the kinds of encryption algorithms and protocols discussed previously
Can be applied at different places in the network stack
With different effects and costs

4. Cryptographic Effects for Networks
What useful purposes does crypto serve in networks?
What is necessary for it to achieve those purposes?
When is that easy and when is it hard?
And why?

5. Secrecy Encrypt the traffic before sending it across the network
If crypto strong and correct, attackers can’t deduce the message contents
Strong – a crypto algorithm that is not easy to break
Correct – proper implementation and use of crypto (keying, etc.)

6. Authentication
Proper crypto can give assurances of who created messages
Easiest using public key crypto
But sometimes of value with symmetric crypto, as well

7. Integrity Encrypted data cannot be easily tampered with
Without totally corrupting the decrypted version
Strong, correct encryption thus allows detection of message alteration
Sort of bundled together with the secrecy property

8. What If I Only Care About Integrity?
Encryption is computationally expensive
Generally based on how much data is encrypted
Can I get integrity via crypto without paying full cost of encrypting packets?
Yes, via digital signatures

9. Digital Signatures Essentially, encrypt a hash of the data
Rather than the full data
Attach encrypted hash to the unencrypted packet
Receiver undoes the crypto on the hash and checks its value
If message altered, hash doesn’t match

10. Deliver the actual message plus the hash
A Simple Example
An IP message
Deliver the actual message plus the hash
The IP
header
The payload
Compute a hash of the message

11. Checking the Digital Signature
Compare the hash in the message to the newly computed hash
Calculate the hash on the incoming message

12. Cryptographic Hashes
Hashes used for digital signatures require some special properties
Hard to find another text that hashes to the same value
Hard to invert
Changing one bit changes hash value
Not all hash functions are suitable
Suitable ones are called cryptographic hashes

13. Some Common Cryptographic Hashes
MD5
“Broken” in 2009, not good to use it
SHA-1
Not fully broken, but some weaknesses found
Soon, SHA-3
Designed to replace SHA-1

14. Some Details What do we hash? Just the payload? Entire packet?
Payload plus some header fields?
Where?
Where do we create and where do we check?
Can’t attackers replace the original hash?
With a proper hash of the altered data

15. What Do We Hash?
Integrity protection is applied only to the parts of the packet we hash
So if we only hash the payload, no protection for the headers
However, some header fields change
E.g., the packet TTL
If we hash those, the hash won’t match

16. What Do We Want to Protect?
Depends on our integrity goals
Are we using this mechanism to ensure correct headers?
Or do we only care about the payload?
Often, apps won’t examine headers anyway
Digital signature typically provide some authentication, too

17. Where Do We Hash? At the packet’s creation point?
And checked at its destination?
Or at some intermediate points?
Note: IP checksumming will be done at each hop, regardless of digital signatures

18. Avoiding Hash Replacement
If attacker can intercept message, he can replace the hash
Even if it’s a crypto hash, he can still calculate hash for another value
And replace the right one
Everyone knows the crypto hashing functions
Solved with keyed hashes

19. Keyed Hash Functions Crypto hash functions aren’t keyed
Anyone knowing the function and the text can produce the hash
Keyed hash functions also use a secret key
Perhaps merely concatenated to the text before hashing
Usually a bit more complicated

20. Message Authentication Codes
MACs
Basically, keyed hash functions
HMAC is most famous approach
HMAC(K, m) =
H((K  opad) | H((K  ipad) | m))
opad and ipad are constants
Safer against certain attacks than just hashing concatenation of m and K

21. MACs and Keys Safe digital signatures require MACs
Which requires signer and signature checker use proper keys
Which requires safe key distribution
Generally a hard problem
Typically no easier in the signature case

22. Where Do We Apply Encryption?
At some point we encrypt the data
It then travels across all or part of the network
To another point where we decrypt
Protection given by crypto only applies to the instants when data is encrypted

23. Link Level Encryption Source Destination
ciphertext
plaintext
ciphertext
plaintext
ciphertext
ciphertext
plaintext
ciphertext
ciphertext
plaintext
ciphertext
plaintext
ciphertext
Let’s say we want to send a message using encryption
Different keys (maybe even different ciphers) used at each hop

24. When Do We Use Link Encryption?
When only one link is untrusted
Or is much less trusted than the rest
When endpoints can’t/won’t do the crypto
Most common case is Wifi
standards include options for link encryption

25. End-to-End Encryption
Source
Destination
plaintext
ciphertext
ciphertext
ciphertext
ciphertext
plaintext
ciphertext
When would link encryption be better?
Cryptography only at the end points
Only the end points see the plaintext
Normal way network cryptography done

26. When Do We Use End-to-End Encryption?
When we don’t trust any element of the intermediate network
When we want crypto decisions to be under control of sender/receiver
Might be unsure what intermediates will do
Used to protect most sensitive traffic in the Internet

27. Where Are the Endpoints, Anyway?
If you do end-to-end encryption, where are the endpoints?
The network layer end points?
The transport layer end points?
The application layer end points?
Maybe not even end machine to end machine (e.g., VPNs)?
Has serious implications for where you do cryptography
And keying and trust issues