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Lect. 9 : Mode of Operation

Lect. 9 : Mode of Operation. Modes of Operation – ECB Mode. P 2. P n. P 1. Electronic Code Book Mode Break a message into a sequence of plaintext blocks Each plaintext block is encrypted (or decrypted) independently The same plaintext block always produces the same ciphertext block

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Lect. 9 : Mode of Operation

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  1. Lect. 9 : Mode of Operation

  2. Modes of Operation – ECB Mode P2 Pn P1 • Electronic Code Book Mode • Break a message into a sequence of plaintext blocks • Each plaintext block is encrypted (or decrypted) independently • The same plaintext block always produces the same ciphertext block • May not be secure; e.g., a highly structured message • Typically used for secure transmission of single vales (e.g., encryption key) . . . K E E E C1 Cn C2 . . . D D D K P2 Pn P1

  3. Modes of Operation – CBC Mode P1 P2 Pn • Cipher Block Chaining Mode • Each ciphertext block is affected by previous blocks • No fixed relationship between the plaintext block and its input to the encryption function • The same plaintext block, if repeated, produces different ciphertext blocks • IV(Initializing Vector) must be known to both ends • Most widely used for block encryption IV . . . E K E E C1 C2 Cn . . . D K D D C3 = EK(P3 C2) C1 = EK(P1 IV) P3 = C2 DK(C3) P1 = IV  DK(C1) IV C4 = EK(P4 C3) C2 = EK(P2 C1) P1 P2 Pn P4 = C3 DK(C4) P2 = C1 DK(C2)

  4. E E E E E E Modes of Operation – CFB Mode • Cipher Feedback Mode • A way of using a block cipher as a stream cipher • A shift register of block size maintains the current state of the cipher operation, initially set to some IV • The value of the shift register is encrypted using key K and the leftmost j bits of the output is XORed with j-bit plaintext Pi to produce j-bit ciphertext Ci • The value of the shift register is shifted left by j bits and the Ci is fed back to the rightmost j bits of the shift register • Typically j = 8, 16, 32, 64 … • Decryption function DK is never used IV . . . K E E C1 Cn C2 P1 Pn P2 IV . . . K E E C1 Cn C2 P1 Pn P2

  5. E E E E E E Modes of Operation – OFB Mode • Output Feedback Mode • The structure is similar to that of CFB, but • CFB: Ciphertext is fed back to the shift register • OFB: Output of E is fed back to the shift register • For security reason, only the full feedback (j = block size) mode is used • No error propagation • More vulnerable to a message stream modification attack • May useful for secure transmission over noisy channel (e.g., satellite communication) IV . . . K E E P1 C2 Cn P2 Pn C1 IV . . . K E E C1 P2 Pn C2 Cn P1

  6. Invited Talks • 2010-04-01 황규범(Ahn Lab), Anti Malware • 2010-04-08 이만호 (Korea Financial Clearing House), Internet Banking Security • 2010-04-15 민성준 (NIA), Security policy • 2010-04-22 김중만(KISA/NCSC),VoIP Security • 2010-04-29 양정규(Korea Minting Co.),e-Passport & Smart Card Securtiy • 2010-05-06 지성배(KISA), Cyber Security • 2010-05-13 박재민(KTF), Mobile phone Security • 2010-05-18 박재관(Korea Smart Card), Smart Card (Closed Session)

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