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Questions and Answers
Match the following modes of operation with their key characteristics:
Electronic Codebook Mode = Simplest method for encryption Cipher Block Chaining Mode = Requires Initialization Vector (IV)
Match the following block cipher properties with their corresponding terms:
DES = Block length of 64 bits AES = Block length of 128 bits Ciphertext = Result of encryption process Plaintext = Original readable data
Match the following terms with their definitions in the context of block ciphers:
Initialization Vector (IV) = Random input to enhance security Plaintext = Data before encryption Ciphertext = Data after encryption Key = Secret value used for encryption and decryption
Match each mode with its security feature or issue:
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Match the following concepts with their corresponding operations:
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Study Notes
Cipher Block Modes of Operation
- Purpose: To apply block ciphers in various applications.
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Block length:
- DES & 3DES -> 64 bits.
- AES -> 128 bits.
- Padding: Plaintext is padded if necessary to create b-bit blocks.
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NIST standards: Five modes of operations are defined by NIST:
- Electronic Codebook (ECB)
- Cipher Block Chaining (CBC)
- Cipher Feedback (CFB)
- Output Feedback (OFB)
- Counter (CTR)
Electronic Codebook (ECB) Mode
- Encryption: Each plaintext block is encrypted using the same key.
- Ciphertext generation: Every b-bit plaintext block has a unique ciphertext.
- Drawback: Repeated plaintext blocks result in identical ciphertext blocks, leading to potential security risks in lengthy messages.
Cipher Block Chaining (CBC) Mode
- Encryption: Each plaintext block is XORed with the preceding ciphertext block before being encrypted.
- Same Key: The same key is used for every block.
- Key Feature: Different ciphertext blocks are produced for repeated plaintext blocks due to chaining.
- Initialization Vector (IV): Used to encrypt the first plaintext block, XORed with the first plaintext block. It must be shared between sender and receiver.
- Security: Maximize security by protecting both the key and IV (e.g., IV can be encrypted using ECB).
Cipher Feedback (CFB) Mode
- Purpose: Converts block ciphers into stream ciphers.
- Encryption: Works with segments of plaintext (s bits) and includes an initialization vector (IV).
- Output: Leftmost s bits of the encryption function output are XORed with the current plaintext segment to produce the ciphertext segment.
- Shift Register: Contents are shifted left by s bits, and the ciphertext segment is placed in the rightmost s bits.
- Decryption: Uses the same scheme as encryption, but XORs the received ciphertext segment with the encryption function output.
Output Feedback (OFB) Mode
- Structure: Similar to CFB, but the encryption function output is fed back as the input for encrypting the next block.
- Encryption: Operates on full blocks of plaintext and ciphertext.
- Initialization Vector (IV): Must be a nonce (unique for each encryption session).
- Advantage: Bit errors in transmission do not propagate.
- Difference from CFB: In OFB, the output of the XOR unit is fed back.
Counter (CTR) Mode
- Encryption: Uses a counter (initialized to a value and incremented for each block) that is encrypted and then XORed with each plaintext block, no chaining involved.
- Decryption: Uses the same counter sequence, XORing it with each ciphertext block to recover the corresponding plaintext block.
- Padding Not Required: Due to its structure.
- Advantage: It's more efficient than CFB and OFB and can be implemented in parallel because each block's encryption is independent.
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Description
This quiz covers the various modes of operation used in block ciphers, including ECB, CBC, CFB, OFB, and CTR. You'll learn about the structure, encryption methods, and security considerations associated with each mode. Test your understanding of how these modes are applied in different cryptographic scenarios.
