1. Chapter 4Using Encryption in Cryptographic Protocols & Practices (Part C)

2. Outline • Protocols • Use of protocols on everyday tasks • Distribution of keys, Certificates, Digital signatures, Key escrow, Mental poker, Electronic voting, oblivious transfer, contract signing, certified mail • Considerations of encryption practices • Other security-enhancing methods • Block chaining, one-way encryption, cryptographic sealing, time stamps • Other modes of using DES csci5233 computer security & integrity (Chap. 4)

3. Considerations of encryption practices • Review of Shannon’s criteria: p.160 • Amount of secrecy should be appropriate to the degree of confidentiality of the data being preserved. • Key management issues: generation, updating, distribution, supersession (replacement of old keys) • The implications of lost keys: e.g., in contract signing csci5233 computer security & integrity (Chap. 4)

4. Considerations of encryption practices • Two issues related to the complexity to perform an encryption: Initial delay The delay before encryption commences depends on the type of encryption – block or stream – and the size of the block. Many common cryptographic algorithms (RSA, Merkle-Hellman, El Gamal, DES, Skipjack) are block ciphers. Encryption speed Public key algorithms are significantly slower than symmetric ones. [Lampson 92] reports that symmetric encryption performs 1,000 to 5,000 times faster than public key cryptography. See Table 4-1 (p.162). csci5233 computer security & integrity (Chap. 4)

5. Considerations of encryption practices • Propagation of errors Network errors are often handled by the network itself (e.g., retransmission). Tamper-protection mechanism: An error resulted from an interceptor’s attempted modification should be apparent in the ciphertext. • Size of ciphertext Should size(ciphertext)  size (plaintext) ? csci5233 computer security & integrity (Chap. 4)

6. The ‘Block Replay’ Problem • Security weakness  broken secrecy • There are other types of attacks. • Example: block replay Encrypted blocks from one transmission are sent in a 2nd transmission, with possible alteration of some fields. • Analysis A block replay is possibly an insider’s attack. Information that may have been exposed: the format of the transmissions, the frequency with which encryption keys are changed, the frequency of transmissions, … Block ciphers, which treat each block of plaintext independently, are prone to block replay. Solution: block chaining csci5233 computer security & integrity (Chap. 4)

7. Block Chaining • Arithmetic basis: (a XOR b) XOR a = b • The encryption of a block is preceded by an XOR with the previous block’s ciphertext. Cj= E( Cj-1 XOR Bj), for all j > 1. • See Fig. 4-22, p.165 • Deciphering of a block is followed by an XOR with the previous block’s ciphertext : Given Cj-1, D(Cj) XOR Cj-1 = D( E( Cj-1 XOR Bj) ) XOR Cj-1 = Bj See Fig. 4-24(b), p.170. csci5233 computer security & integrity (Chap. 4)

8. Characteristics of Block Chaining • Identical plaintext blocks transmitted separately or within the same transmission do not necessarily produce the same ciphertext. (Fig. 4-23, p.166) • Prevention of ‘block replay’ • Initial chaining value may be used to further strengthen this characteristic.  To prevent standard message headers from being encrypted the same way csci5233 computer security & integrity (Chap. 4)

9. Other methods • One-way encryption • Functions that are much easier to compute than their inverses • Examples: stored, encrypted passwords • Cryptographic sealing • Encryption can be used to seal a file, a record, a data value, or a single byte. • A checksum is calculated based on the content of the file. Any change to the file by an interceptor would be detected because the calculated checksum would not match the stored checksum value. • Authentication via encryption • Time stamps to prevent forgery csci5233 computer security & integrity (Chap. 4)

10. Modes of using DES • Electronic Code Book (ECB) mode The DES presented in Chapter 3 64-bit plaintext  64-bit DES encryption • Cipher Block Chain (CBC) mode Review: Fig. 4-22, 23, 24 Removes the flaw of duplicate encryption of repeated plaintext • Cipher Feedback (CFB) mode Works on one character (8 bits) at a time Needed for remote user interaction, in which a user must transmit each character as it is entered at the terminal Operates on a 64-bit queue Fig. 4-25 (p.171) Removes the delay from waiting to encrypt a block of text at once csci5233 computer security & integrity (Chap. 4)

11. Modes of using DES • Double encryption keys Q: Would using two 56-bit keys achieve the effect of a 112-bit key? [Merkle and Hellman 81] No! It would only take only 257 tries, rather than the 2112 as expected, to break it. • Triple DES See http://www.rsasecurity.com/rsalabs/faq/3-2-6.html • DES-EDE: C = Ek3( Dk2( Ek1(M) ) ) Decryption - M = Dk1( Ek2( Dk3(M) ) ) • DES-EEE: C = Ek3( Ek2( Ek1(M) ) ) Decryption ? • Backward compatibility mode: K1 = K2 = K3 csci5233 computer security & integrity (Chap. 4)

12. Summary • Uses of encryptions • Encryption protocols • Encryption practices • Methods for other types of attacks other than breaking the secrecy • Next: Chapter 5 (software security) + VM: Ch 1 csci5233 computer security & integrity (Chap. 4)