1. Cryptography
Lecture 12

2. Exam review

3. Historical schemes Shift, Vigenere, etc.
They are all easy to attack
They are not used anymore
The point of this material was to motivate the need for a more formal treatment

4. Perfect secrecy A more formal approach Definition of perfect secrecy
Definitions
Proofs
Definition of perfect secrecy
The one-time pad achieves this definition
Several inherent drawbacks of perfect secrecy
The one-time pad is not used

5. Private-key encryption
If we want to overcome drawbacks of perfect secrecy, we must relax the definition
Computational secrecy
EAV-security
(Computational) secrecy for encryption of one message
We now need to rely on assumptions in order to prove security

6. Private-key encryption
Pseudorandom generators/stream ciphers
Formal definition
For now, we simply assume these exist
Pseudo-one-time pad
(Provable) EAV-security based on any PRG
Message length longer than key length
Not secure when multiple messages encrypted, or against chosen-plaintext attacks

7. Private-key encryption
CPA-security
Security against chosen-plaintext attacks
Requires randomized encryption!
Pseudorandom functions/block ciphers
Formal definition
For now, we simply assume these exist (e.g., AES)
Basic encryption scheme
(Provable) CPA-security based on any PRF
2x ciphertext expansion

8. Private-key encryption
Modes of encryption
CBC-mode, CTR-mode are both CPA-secure, and have ciphertext expansion of one block
Stream-cipher modes
These are used extensively in the real world
CCA-security
Security against chosen-ciphertext attacks
This is a real-world problem (cf. padding-oracle attacks)
None of the schemes we have seen so far satisfy this notion of security

9. Message authentication codes
Integrity as an orthogonal security concern
Secrecy and integrity are different
Encryption and message authentication are different
Message authentication codes, and definition of security
Basic MAC from any PRF
Short, fixed-length messages only

10. Message authentication codes
Constructing a MAC on longer messages?
Different attacks to watch out for
(Basic) CBC-MAC
Secure for fixed-length messages
CBC-MAC
Secure for arbitrary-length messages
Used in the real world

11. Authenticated encryption
Communication with secrecy and integrity
An AE scheme is an encryption scheme that achieves both
CCA-security
Unforgeability
Encrypt and authenticate is not a sound generic construction

12. Authenticated encryption

13. Secrecy + integrity?
We have shown primitives for achieving secrecy and integrity in the private-key setting
What if we want to achieve both?

14. Authenticated encryption
An encryption scheme that achieves both secrecy and integrity
Secrecy notion: CCA-security
Integrity notion: unforgeability
Adversary cannot generate ciphertext that decrypts to a previously unencrypted message

15. Generic constructions
Generically combine an encryption scheme and a MAC
Useful when these are already available in some library
Goal: the combination should be an authenticated encryption scheme when instantiated with any CPA-secure encryption scheme and any secure MAC

16. Generic constructions?
Encrypt and authenticate
Authenticate then encrypt
Encrypt then authenticate

17. Encrypt and authenticate
c, t
k1, k2
k1, k2 m
c  Enck1(m)
t = Mack2(m)
m = Deck1(c)
Vrfyk2(m, t) = 1?

18. Problems The tag t might leak information about m!
Nothing in the definition of security for a MAC implies that it hides information about m
So the combination may not even be EAV-secure
If the MAC is deterministic (as is CBC-MAC), then the tag leaks whether the same message is encrypted twice
I.e., the combination will not be CPA-secure

19. Authenticate then encrypt
k1, k2
k1, k2
m t = Mack2(m)
c  Enck1(m | t)
m | t = Deck1(c)
Vrfyk2(m, t) = 1?

20. Problems
Padding-oracle attack still works (if possible to distinguish padding failure from MAC failure)
Other counterexamples are also possible
The combination may not be CCA-secure

21. Encrypt then authenticate
c, t
k1, k2
k1, k2 m
c  Enck1(m)
t = Mack2(c)
Vrfyk2(c, t) = 1?
m = Deck1(c)

22. Security?
If the encryption scheme is CPA-secure and the MAC is secure (with unique tags) then this is an authenticated encryption scheme
It achieves something even stronger:
Given ciphertexts corresponding to (chosen) plaintexts m1,…, mk, it is infeasible for an attacker to generate any new, valid ciphertext!

23. Authenticated encryption
Encrypt-then-authenticate (with independent keys) is the recommended generic approach for constructing authenticated encryption

24. Direct constructions
Other, more-efficient constructions have been proposed and are an active area of research and standardization
E.g., OCB, CCM, GCM
Others…
Active competition:

25. Secure sessions

26. Secure sessions?
Consider parties who wish to communicate securely over the course of a session
“Securely” = secrecy and integrity
“Session” = period of time over which the parties are willing to maintain state
Can use authenticated encryption…

27. Enck(m1)
Enck(m2) k k
Enck(m3)

28. Replay attack
Enck(m1)
Enck(m2)
Enck(m1) k k

29. Re-ordering attack
Enck(m1)
Enck(m2)
Enck(m2)
Enck(m1) k k

30. Reflection attack
Enck(m1)
Enck(m2) k k
Enck(m2)

31. Secure sessions
These attacks (and others) can be prevented using counters/sequence numbers and identifiers

32. Enck(“Bob”| m1 | 1)
Enck(“Bob” | m2 | 2) k k
Enck(“Alice” | m3 | 1)

33. Secure sessions
These attacks (and others) can be prevented using counters and identifiers
Can also use a directionality bit in place of identifiers