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Secure Modes of Operation in Cryptography

Learn about ECB, CBC, CFB, and OFB modes with examples and applications. Understand how each mode operates and its security implications in electronic code encryption. Explore invited talks on cutting-edge security topics.

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Secure Modes of Operation in Cryptography

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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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