1. Security through Encryption
James P. Skon

2. Encryption

3. Overview History What is encryption/decryption? Symmetrical encryption
Public-key encryption
Current encryption standards
Encryption

4. Caesar cipher
Encryption

5. German Enigma
Encryption

6. Cryptography Is Is not A tremendous tool
The basis for many security mechanisms
Is not
The solution to all security problems
Reliable unless implemented properly
Reliable unless used properly
Something you should try to invent yourself unless
you spend a lot of time becoming an expert
you subject your design to outside review
Encryption

7. Basic Cryptographic Concepts
Encryption scheme:
functions to encrypt, decrypt data
Symmetric encryption
Block, stream ciphers, same key for both sides
Hash function, MAC
Map any input to short hash; ideally, no collisions
MAC (keyed hash) used for message integrity
Public-key cryptography
PK encryption: public key does not reveal key-1
Signatures: sign data, verify signature
Encryption

8. Unsecure message Plain Text ah-ha!! Encryption Dear Sally,
The secret ...
ah-ha!!
Encryption

9. network transactions Assume attackers can control the network
We will talk about how they do this later
Attackers can intercept packets, tamper with or suppress them, and inject arbitrary packets
Encryption

10. Encryption Ciphertext Plain Text Encryption Algorithm
Ad44fgt6&55tgt
Ciphertext
Ee8e0w8e*WEdlGlr98&
d(D9ED- ….
Encryption
Algorithm
Plain Text
Dear Sally,
The secret ...
Encryption

11. Decryption Plain Text Ciphertext Decryption Algorithm Dear Sally,
Ad44fgt6&55tgt
Plain Text
Decryption
Algorithm
Dear Sally,
The secret ...
Ciphertext
Ee8e0w8e*WEdlGlr98&
d(D9ED- ….
Encryption

12. Secure Message Ciphertext Plain Text Plain Text ????? Encryption
Ee8e0w8e*WEdlGlr98&d(D9ED
fssdfsdf sfd sd-
Dear Sally,
The secret ...
Dear Sally,
The secret ...
Plain Text
Plain Text
?????
Encryption

13. Encryption Destination Source insecure key Plaintext Plaintext key
Algorithm
Encrypted
message
Algorithm
Plaintext
key
Encryption

14. Cryptanalysis breaking the code with brute force computation attack
Types
Ciphertext-only attack
Known-plaintext attack
Chosen-plaintext attack
Adaptive-chosen-plaintext attack
Others...
Encryption

15. Symmetric Key Encryption
Both parties must have the same key
Often encryption and decryption done by the exact same algorithm
Typically the algorithm is publicly known (only the key is secret)
Called the “secret key” method, since secrecy of key is essential
Encryption

16. One-time pad
Method
Each character from the plaintext is encrypted by a modular addition with a character from a secret random key (or pad) of the same length as the plaintex
Key must be length of message
Must be random, never reused
Encryption

17. One Time Pad Method Message: "THE BRITISH ARE COMING”
Key: DKJFOISJOGIJPAPDIGN
A B C D E F G H I J K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z
Encryption
(T(19)+D(03)=22) MOD 26 = 22 = W (H(07)+K(10)=17) MOD 26 = 17 = R (E(04)+J(09)=13) MOD 26 = 13 = N (B(01)+F(05)=06) MOD 26 = 06 = G (R(17)+O(14)=31) MOD 26 = 05 = F
Encryption

18. One Time Pad THEBRITISHARECOMING DKJFOISJOGIJPAPDIGN
Result: WRNGFQLRGNIATCDPQTT
Decryption
(W(22)-D(03)= ) MOD 26 = 19 = T (R(17)-K(10)= ) MOD 26 = 07 = H (N(13)-J(09)= ) MOD 26 = 04 = E (G(06)-F(05)= ) MOD 26 = 01 = B (F(05)-O(14)= ) MOD 26 = 17 = R
Encryption

19. Stream ciphers
Generate a long key with a pseudo random number generator.
Encryption

20. Block Cipher
DES, AES
Build a key for each block from the start key, and the previous block
Encryption

21. Strong Symmetric Key Encryption
Weak encryption is vulnerable to cryptanalysis
Strong encryption is an algorithm for which cryptanalysis is intractable (possible, but takes too long)
Strength of an algorithm is related to key length (algorithm complexity)
Encryption

22. Symmetric Key Encryption - Algorithms
DES (Data Encryption Standard)
56-bit key
Adopted in 1977 by the National Bureau of Standards.
Originally proposed key length of 128-bits.
Powerful computers can now break DES.
Double and Triple DES - use two or three keys for greater safety
Encryption

23. Major Systems Data Encryption Standard (DES) 1978 56-bit keys
very hard to break a key
requires 255 steps on average
never been broken in genera, but can be broken for a specific key given enough CPU power.
Usually implemented in hardware
Encryption

24. Major Systems Advanced Encryption Standard
adopted by the U.S. government and is now used worldwide
Algorithm well known, generally trusted
Attacks: Brute Force Cryptoanalysis
In 2011 a key-recovery attack created which is about 4 times faster then brute force.
Encryption

25. Symmetric Key Problems
How do we safely exchange keys?
How do we authenticate the source (I.e. who really sent this message)
Encryption

26. Encryption secret-key Sender and receiver must have an identical key
Key is used for both encryption and decryption
Problems:
Secret keys must be exchanged in private.
Each side must trust each other with their secret key.
Called the key management problem.
Encryption

27. Encryption public-key Solution to key management problem.
Each persons has a pair of keys, a public key and a private key
Public key is published in trusted directory
Private key known only to owner.
Encryption

28. Encryption public-key
Anyone can use public key to encrypt data to be sent to the owner of the public key.
ONLY the owner of the private key can decrypt the message.
Thus security is assured across unsecure media.
Encryption

29. Public-key Encryption
Destination
Source
insecure
Private key
Plaintext
Algorithm
Encrypted
message
Algorithm
Plaintext
Public key
Encryption

30. Authentication
process whereby the receiver of a digital message can be confident of the identity of
the sender or
the integrity of the message
Encryption

31. Authentication
Public-key Encryption provides concept of digital signature
General operation: Bob sends to Mary
Mary’s
Public key
Mary’s
Private key
Message
decryption
algorithm
Message
encryption
algorithm
encrypted
message
Bob’s
Public key
A mathematical
relationship that
will hold ONLY if
Bob’s private key
was used.
function
Digital
signature
Bob’s
Private key
function
Encryption

32. Advantages of Public Key Systems
Private keys need not be transmitted
Digital signature possible
Messages can be proven authentic to a third party (such as judge) allowing for legal agreements via network
Encryption

33. Disadvantages of Public Key Systems
Speed
symmetrical (secret) key algorithms much, much faster
Public key very computationally intensive
Encryption

34. Disadvantages of Public Key Systems
Solution to speed problem:
For a transfer, generate a random session key
Send the session key using a public key method
Use the session key with a symmetrical key algorithm for actual data transfer.
Encryption

35. Crypto Policy Perspectives
Issues
Law Enforcement
National Security
Right to Privacy
Encryption