Foundations of Computer SecurityLecture 47: Modes of UsageDr. Bill YoungDepartment of Computer SciencesUniversity of Texas at AustinLecture 47: 1 Modes of UsageModes of Usage: ECBThe simplest way of using a block cipher like AES is to encrypt(with the same key) each block in the plaintext. This is a blockencryption mode called “Electronic Code Book” (ECB).Original With ECB Another ModeIdentical blocks in the plaintext yield identical blocks in theciphertext.Lecture 47: 2 Modes of UsageModes of Usage: CBCTo solve the problem of EBC, do something to “randomize” blocksbefore they’re encrypted.Cipher Block Chaining (CBC): XOR each successive plaintextblock with the previous ciphertext block and then encrypt. Aninitialization vector IV is used as a “seed” for the process.Lecture 47: 3 Modes of UsageCBC VulnerabilitiesThough much better than ECB, CBC still has some weaknesses.Observed changes: An attacker able to observe changes tociphertext over time will be able to spot the first block thatchanged.Content Leak: If an attacker can find two identical ciphertextblocks, Ciand Cj, he can derive the following relation:Ci−1⊕ Cj−1= Pi⊕ Pj,and derive information about two plaintext blocks.Numerous other block encryption modes have been devised.Lecture 47: 4 Modes of UsageKey Stream Generation ModesBlock encryption modes (like ECB and CBC) generate ciphertextthat stores the message in encrypted but recoverable form.In key stream generation modes the cipher is used more as apseudorandom number generator. The result is a key stream thatcan be used as in one-time pad. Decryption uses the same keystream.StreamInputKeyOutput bitLecture 47: 5 Modes of UsageCipher Feedback Modej --??¾?-64-bit Shift RegisterKey 64 bitBlock cipherInput8-bitOutput8-bitIn Cipher Feedback mode (CFB) each input byte is XORed with thefirst block of the previous output and fed back into the encryption.Lecture 47: 6 Modes of UsageLessonsA naive use of encryption as in Electronic Code Book leavestoo much regularity in the ciphertext.Block encryption modes such as CBC attempt to hide this bychaining blocks together in some manner.Key stream generation modes use encryption algorithms togenerate random appearing streams of bits in reproduciblefashion.Next lecture: Public Key EncryptionLecture 47: 7 Modes of
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