1. Encryption Encryption: Transforms Message so that Interceptor Cannot Read it –Plaintext (original message) Not necessarily text; Can be graphics, etc. PlaintextEncryptionCiphertextDecryptionPlaintext Method Key Method Key TransmittedOriginal Message Original Message

2. Encryption Encryption: Transforms Message so that Interceptor Cannot Read it –Ciphertext (transformed) for transmission Stream of ones and zeros for transmission PlaintextEncryptionCiphertextDecryptionPlaintext Method Key Method Key TransmittedOriginal Message Original Message

3. Encryption Encryption: Transforms Message so that Interceptor Cannot Read it –Receiver decrypts ciphertext back to plaintext PlaintextEncryptionCiphertextDecryptionPlaintext Method Key Method Key TransmittedOriginal Message Original Message

4. Encryption Encryption Requires a Method and a Key –Encryption method is the specific transformation process –Key is a string of bits used in the method –Get different ciphertext with different key PlaintextEncryptionCiphertextDecryptionPlaintext Method Key Method Key TransmittedOriginal Message Original Message

5. Encryption Encryption Requires a Method and a Key –Method cannot be kept secret –Key must be kept secret PlaintextEncryptionCiphertextDecryptionPlaintext Method Key Method Key TransmittedOriginal Message Original Message

6. Encryption: Key Length Key can be “guessed” by exhaustive search –Try all possible keys –See which one decrypts the message

7. Encryption: Key Length Long keys make exhaustive search difficult –Key is a string of bits (11000100010101) –If length is n bits, 2 n tries may be needed –On average, need half this many –If key length is 8 bits, only 256 tries maximum

8. Encryption: Key Length Long keys make exhaustive search difficult –Weak security: Today < 100 bits –Strong security: Today > 100 bits –Need for length will grow over time –Laws may limit export of strong security, sending strongly encrypted messages internationally

9. Methods and Algorithms Encryption Method Categories –Two general ways of doing encryption –Symmetric key versus public key encryption Encryption Method Algorithms –Specific ways of doing encryption –With symmetric key encryption: DES, 3DES, AES, IDEA, Blowfish, and RC5 algorithms –With public key encryption: RSA, elliptical curve cryptosystem (ECC), El Gamal

10. Symmetric Key Encryption Both sides use a single key to encrypt & decrypt –When A send to B A encrypts with the key, B decrypts with the key –When B sends to A B encrypts with the key, A decrypts with the key A B Symmetric Key

11. Symmetric Key Encryption Symmetric key encryption and decryption processes are simple enough for fast encryption/decryption –Fast enough for long messages A B Symmetric Key

12. Symmetric Key Encryption Problem 1: Symmetric key must be distributed secretly between partners or interceptors can read subsequent messages Key A Key B A B

13. Symmetric Key Encryption Problem 2: Need a different symmetric key for each business partner –Or other partners could read messages –Complicates symmetric key distribution Key A Key B A B

14. Symmetric Key Encryption Problem: Need a different symmetric key for each business partner –If there are N partners –And if each needs to communicate with all others –Then N*(N-1)/2 keys must be distributed Key A Key B A B

15. Symmetric Key Encryption Data Encryption Standard (DES) –Extremely popular symmetric key algorithm –DES breaks the plaintext into blocks of 64 bits –It then encrypts each block of plaintext using a 64-bit key –However, it is proper to say that DES uses a 56-bit key, because 8 of the key bits are redundant (can be computed from the other 56) –56-bit key is small, giving inadequate strength for important transactions (OK for small ones)

16. Symmetric Key Encryption Triple DES (3DES) –Applies DES three times to encrypt –With 3 keys, get 168-bit effective key length Encrypt block with first key Decrypt (yes, decrypt) result with second key Encrypt result with third key; Send this ciphertext –Decryption Decrypt ciphertext with third key Encrypt (yes, encrypt) result with second key Decrypt result with third key

17. Symmetric Key Encryption Note: Encryption and Decryption often are Reversible –Usually, encrypt to get ciphertext and then decrypt to restore the plaintext –Some algorithms also can decrypt to get ciphertext and encrypt to restore plaintext –Both approaches transform the plaintext into ciphertext to give confidentiality (privacy), then unscramble the ciphertext back to the original plaintext

18. Symmetric Key Encryption Triple DES (3DES) –Can be done with 2 keys for 112-bit effective key length –Encryption Encrypt block with first key Decrypt (yes, decrypt) result with second key Encrypt result with first key again; Send this ciphertext –Decryption Decrypt received ciphertext with first key Encrypt (yes, encrypt) result with second key Decrypt result with first key again

19. Symmetric Key Encryption DES is an old algorithm –Developed in 1970s 3DES is merely a way of extending its life Advanced Encryption System (AES) –Being developed by the U.S. National Institutes for Standards and Technology –Will be much stronger with longer keys

20. Public Key Encryption There are Two General Encryption Method Categories –Symmetric key encryption (just seen) –Public key encryption (next)

21. Public Key Encryption Methods Different keys for encryption and decryption –Encryption with receiver’s public key –Decryption with receiver’s private key –Once encrypted, sender cannot decrypt the ciphertext; does not have receiver’s private key PlaintextEncryptionCiphertextDecryptionPlaintext Public KeyPrivate Key

22. Public Key Encryption Everyone has a public and private key –Keep the private key secret –Distribute the public key to everybody without security Public Key Public Key Private Key

23. Public Key Encryption Then anyone can encrypt messages to you using your public key But only you can decrypt the messages Public Key Public Key Private Key Encryption

24. Public Key Encryption Four Keys Needed for Two-Way Communication –Each side has a public and a private key –Each sends public key to other unsecurely A B B’s Pub Key B’s Priv Key A’s Priv Key A’s Pub Key

25. Public Key Encryption Four Keys Needed for Two-Way Communication –Encrypt with other party’s public key –Decrypt with own private key A B B’s Pub Key B’s Priv Key A’s Priv Key A’s Pub Key

26. Public Key Encryption Four Keys Needed for Two-Way Communication –Never refer to public and private keys without saying to whose public or private key you are referring A B B’s Pub Key B’s Priv Key A’s Priv Key A’s Pub Key

27. Public Key Encryption No need for separate secret key for each business partner –Greatly simplifies key management

28. Public Key Encryption Unfortunately, highly processing-intensive –100 times slower than symmetric key encryption –So can only encrypt small messages –Also, often can only encrypt messages about the size of the public key (typically a few thousand bits)

29. Combining Public, Symmetric Key Not competitors--Used Together in Practice –Public key is easy to distribute but can only be used for small messages –Symmetric key has key distribution problems but can be used for long messages –They have complementary strengths and weaknesses

30. Combining Public, Symmetric Key Symmetric Key Encryption and Public Key Encryption are Complementary, not Competitors Often, partners first communicate with public key encryption –Including initial authentication Public Key Authentication A B

31. Combining Public, Symmetric Key Then one sides generates a symmetric key –Encrypts symmetric key with partner’s public key, sends to partner –Now, both sides have the symmetric key Symmetric Session Key Public Key Encryption A B

32. Combining Public, Symmetric Key Afterward, both sides communicate with the symmetric key This symmetric “session key” is good only for this session--single flow of communications Symmetric Session Key A B

33. Public Key Algorithms Public Key Encryption is a Method Category –Must Use a Specific Public Key Algorithm RSA –Most widely used public key algorithm –Patented, but public domain in October 2000 Elliptical Curve Cryptosystem (ECC) –Can use smaller keys than RSA with same degree of protection

34. Public Key Encryption If know someone’s public key, there is no known way to compute their private key faster than exhaustive search If there was, public key encryption would be useless This is a general concern for public key encryption because there is no proof that there is no possible way to compute the private key rapidly if a public key is known