Encryption: Counter (CTR) Mode Quiz

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Questions and Answers

What does the counter value represent in the Counter (CTR) mode of encryption?

  • It is a direct mapping to the plaintext being encrypted.
  • It is an invariant value for all encryption operations.
  • It is the nonce reused for encryption.
  • It is an offset to the initialization value for each block. (correct)

In the context of error propagation, how does a one-bit change in plaintext affect the CTR mode?

  • It changes one block of ciphertext and affects all plaintext blocks.
  • It affects only the corresponding ciphertext block and leaves others intact. (correct)
  • It affects the corresponding ciphertext bit and all subsequent blocks.
  • It has no effect on the ciphertext generated.

Which of the following is not a characteristic of the Counter (CTR) mode?

  • Requires a unique initialization value for each encryption session.
  • Utilizes a feedback mechanism to encrypt subsequent blocks. (correct)
  • Processes blocks independently without propagation.
  • Is sensitive to bit changes in the plaintext.

What would be the result of changing one bit in the ciphertext when decrypting using the Counter (CTR) mode?

<p>Only the corresponding plaintext block is affected, with no impact on others. (D)</p> Signup and view all the answers

When encrypting with the Counter (CTR) mode, what operation is performed on each plaintext block?

<p>The plaintext is XORed with the encrypted counter value. (C)</p> Signup and view all the answers

What happens if the condition T =?= T’ is not met?

<p>The program halts. (D)</p> Signup and view all the answers

Why won't Bob accept the modified message with C2'?

<p>C2' has an unmatched MAC value. (D)</p> Signup and view all the answers

What is a major concern associated with DES in real applications?

<p>Insufficient key size. (C)</p> Signup and view all the answers

In the context of this lab, what is the primary purpose of using DES?

<p>For experimental purposes only. (C)</p> Signup and view all the answers

What cryptographic technique is indicated as still being in use instead of DES?

<p>3DES. (C)</p> Signup and view all the answers

What does the cancellative property of XOR imply?

<p>x ⊕ y ⊕ y = x (C)</p> Signup and view all the answers

In the Electronic Codebook Mode (ECB), what is characteristic of how blocks are encrypted?

<p>Each block is encrypted independently with the same key. (C)</p> Signup and view all the answers

What is the primary advantage of using XOR for encryption and decryption?

<p>It allows for easy reversal of the process using the same key. (A)</p> Signup and view all the answers

Which of the following accurately describes the Electronic Codebook Mode?

<p>It creates an electronic code book that maps blocks to outputs. (C)</p> Signup and view all the answers

Which of these properties does NOT hold true for the operation of XOR?

<p>Distributive property holds. (C)</p> Signup and view all the answers

What happens to a block if the encryption algorithm is applied to it multiple times in ECB without changing the input?

<p>The block will always produce the same output. (C)</p> Signup and view all the answers

In which of the following modes does the same key yield the same encryption results for identical blocks?

<p>Electronic Codebook Mode (ECB) (C)</p> Signup and view all the answers

Which operation is directly related to reversing the XOR encryption process?

<p>XORing the ciphertext with the key again (C)</p> Signup and view all the answers

What does Mallory change in the cipher during the integrity attack?

<p>She changes the second cipher block. (B)</p> Signup and view all the answers

What equation does Mallory use to alter the message being sent?

<p>Pi+1 = Dk(Ci+1) ⊕ Ci (A)</p> Signup and view all the answers

Which of the following correctly represents the plaintext Mallory aimed to change?

<p>: £10000 (C)</p> Signup and view all the answers

How does Mallory calculate the modification needed for the cipher block?

<p>Using XOR operation on the plaintext values. (C)</p> Signup and view all the answers

What is the value of P3 and what does it translate to in hexadecimal?

<p>£10.00, 3A 20 A3 31 30 30 2E 30 30 (A)</p> Signup and view all the answers

What result does Mallory aim to achieve by changing the cipher?

<p>To increase the transferred amount. (C)</p> Signup and view all the answers

What is the logical outcome of changing C2 according to Mallory's method?

<p>The subsequent plaintext will reflect the new value. (C)</p> Signup and view all the answers

Which of the following statements describes the relationship between Ci, Pi+1, and P’i+1?

<p>Ci = Dk(Ci+1) ⊕ Pi+1 ⊕ P’i+1 (B)</p> Signup and view all the answers

What is the primary purpose of a Message Authentication Code (MAC)?

<p>To ensure the authenticity and integrity of a message (C)</p> Signup and view all the answers

In the context of MAC, what does 'C(k, M)' represent?

<p>The result of applying a MAC function using the secret key on the input message (B)</p> Signup and view all the answers

What is a potential limitation of using symmetric encryption alone, such as DES or AES?

<p>It does not protect against message alteration (B)</p> Signup and view all the answers

Which of the following best describes the relationship between the MAC and the input message?

<p>The MAC ensures that the message remains unchanged during transit (D)</p> Signup and view all the answers

When Bob receives a message M and its MAC, what can he conclude?

<p>The message must have been created by Alice if the MAC is valid (D)</p> Signup and view all the answers

What is the role of a secret key in the MAC process?

<p>It ensures that only the intended recipient can verify the MAC (B)</p> Signup and view all the answers

What additional feature might enhance the reliability of a MAC?

<p>Including a sequence number within the message (B)</p> Signup and view all the answers

Why is it important for Bob to verify the MAC received with the message?

<p>To confirm the sender's identity and message integrity (D)</p> Signup and view all the answers

What is the primary purpose of using a CBC-MAC function?

<p>To provide a message authentication code after optional processing (C)</p> Signup and view all the answers

In the Encrypt-then-MAC process, what does the tag (T) represent?

<p>The message authentication code output (A)</p> Signup and view all the answers

What is the role of the second secret key (K2) in the Encrypt-then-MAC scheme?

<p>To provide a MAC during encryption (A)</p> Signup and view all the answers

What happens during the decryption phase of the Encrypt-then-MAC process?

<p>The tag is compared with the MAC of the decrypted data (D)</p> Signup and view all the answers

Which characteristic is NOT provided by authenticated encryption as defined in the content?

<p>Data compression (D)</p> Signup and view all the answers

What is the significance of using optional processing in the CBC-MAC function?

<p>It enhances security against cryptanalysis (B)</p> Signup and view all the answers

How is the ciphertext (C) structured in the Encrypt-then-MAC scheme?

<p>C is formed by concatenating the encrypted data and the tag (A)</p> Signup and view all the answers

What does the process of MAC involve in this context?

<p>Generating a fixed-size output indicating data authenticity (C)</p> Signup and view all the answers

Flashcards

Bitstring

A sequence of bits representing data, typically treated as a single unit. It can be any length and contains information.

XOR (Exclusive OR)

An operation that combines two bitstrings of equal length, resulting in a new bitstring of the same length. It produces a 1 only when the input bits are different and a 0 otherwise.

XOR Associative Property

The order in which you apply XOR to multiple bitstrings does not affect the final result. You can group them in any way.

XOR Commutative Property

You can swap the order of the two bitstrings you are XORing without changing the final result. Think of it like changing the order of the switches.

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XOR Cancellative Property

XORing a bitstring with itself twice results in the original bitstring. Imagine flipping the switches twice, you get back to the original state.

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Modes of Operation

A block cipher can be used in different ways to encrypt or decrypt a sequence of blocks.

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Electronic Codebook Mode (ECB)

The simplest mode of operation, where each block is encrypted independently. It uses the key to generate a large substitution table.

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Cipher Block Chaining Mode (CBC)

A mode of operation where each block is encrypted using the XOR of the previous ciphertext block and the key. This creates a chain-like effect, making it more secure than ECB.

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Ciphertext Block Chaining (CBC) attack

A technique in cryptography where an attacker modifies a ciphertext block to change the corresponding plaintext block during decryption.

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Decryption

The process of converting ciphertext back to plaintext using a secret key.

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Plaintext

The original message before encryption.

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Ciphertext

The encrypted message.

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Secret key

The secret key used for both encryption and decryption.

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Cipher Block Chaining (CBC)

A block cipher mode where each ciphertext block depends on the previous ciphertext block.

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Encryption

The process of converting plaintext into ciphertext using a secret key.

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Block

A piece of data that is a fixed length.

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Electronic Codebook (ECB)

A mode of operation where each block is encrypted independently, using the key to generate a substitution table.

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Counter (CTR) Mode

A mode of operation where each block is encrypted using a unique counter value XORed with the plaintext.

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Cipher Feedback (CFB)

A mode of operation where a feedback mechanism is used to incorporate the ciphertext into the encryption process.

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Propagating Cipher Block Chaining (PCBC)

A type of symmetric encryption where a block of data is encrypted based on the previous block's ciphertext.

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Message Authentication Code (MAC)

A technique that uses a cryptographic function to create a short fixed-size value (MAC) that represents the integrity of a message.

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Confidentiality and Integrity

Confidentiality aims to keep information hidden, while integrity ensures the message is unaltered.

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Integrity/Authenticity

This type of encryption ensures that the message hasn't been altered during transmission.

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Cryptographic Checksum

A process used to create a fixed-size output from an arbitrary-length input, like a message.

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MAC Function

A cryptographic function that takes a message and a key as input, producing a fixed-length value called a message authentication code (MAC). The MAC helps verify the data's integrity and authenticity.

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CBC Mode

A mode of operation for block ciphers where each plaintext block is XORed with the previous ciphertext block before encryption. It makes the ciphertext dependent on previous blocks, increasing security.

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Authenticated Encryption

A cryptographic technique that combines encryption and authentication to ensure both confidentiality and integrity of data.

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Encrypt-then-MAC

A specific type of authenticated encryption where the plaintext is first encrypted and then a MAC is generated for the resulting ciphertext.

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Separate Keys in Authenticated Encryption

A technique that uses two separate keys for encryption and MAC generation in authenticated encryption.

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Optional Processing in CBC-MAC

Involves decrypting with a second key (k') before encrypting again with the original key (k) in CBC MAC. This helps prevent cryptanalysis and message combining attacks.

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ENC

A component that includes encryption and MAC operations in authenticated encryption.

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MAC

A component that verifies the integrity of data using a MAC operation in authenticated encryption.

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Study Notes

Computer Security Lecture 2

  • Symmetric Cryptography (I): This lecture covers symmetric cryptography, a type of cryptography where a single key is used for both encryption and decryption. This lecture's focus is symmetric cryptography part 1.
  • Lecture Structure: The lecture will cover introduction, block ciphers, padding, modes of operation, error propagation, Message Authentication Codes (MACs), and authenticated encryption.
  • Cryptographic Primitives: The basic cryptographic primitives covered in this module are encryption and digital signatures.
  • Encryption: Encryption uses an encryption key (Ke) to transform plaintext (P) into ciphertext (C). Decryption uses the decryption key (Kd) to transform ciphertext back into plaintext. C = E(Ke, P), P = D(Kd, C).
  • Digital Signatures: Digital signatures use separate signing and verification keys. A message (m) is signed using the signing key (Ks) to produce a signature (σ). Verification key (Kv) verifies the authenticity and integrity of the message. σ = S(Ks, m), 0/1 = V(Kv, (σ, m)). If Ks = Kv this is a symmetric signature, otherwise it's an asymmetric signature.
  • Symmetric vs Asymmetric Cryptography: Symmetric cryptography uses a single key for encryption and decryption, while asymmetric cryptography uses a pair of related but distinct keys (one public and one private). Symmetric is also called secret key and Asymmetric is also called public key.
  • Information Entropy: Shannon's theory describes entropy as the average amount of information in a message. It's measured in bits. A secure cryptographic key needs high entropy (e.g., 128, 192, or 256 bits).
  • One-Time Pad: A one-time pad uses a key the same length as the plaintext. The key can't be reused and is impractical for widespread use due to key management challenges.
  • Stream Cipher vs Block Cipher: Stream ciphers work with plaintext symbols one at a time, while block ciphers process plaintext in blocks of a fixed size.
  • Block Ciphers: Block ciphers operate on fixed-size blocks of data, encrypting and decrypting each block independently. They have a key of a certain length. The same key is used for encryption and decryption, which is symmetric.
  • DES (Data Encryption Standard): A 64-bit block size cipher, this is an example of block ciphers and is relatively weaker compared to contemporary options.
  • AES (Advanced Encryption Standard): A more modern example of a 128-bit block size cipher.
  • DES Challenge: Finding a 56-bit DES key is computationally more realistically and practically difficult than it used to be.
  • 3DES (Triple DES): A technique to increase the security of DES by applying DES three times on each block, using multiple encryption/decryption keys.
  • Padding: Padding adds extra data to a message to make its length a multiple of the block size, essential for block cipher security. Multiple padding schemes exist. Zero padding is insecure and unsuitable.
  • Modes of Operation: Different modes are used for encrypting a sequence of blocks (e.g., ECB, CBC, CTR, CFB, OFB, PCBC, XTS, CCM).
  • Error Propagation: How errors in the plaintext or ciphertext affect the ciphertext or plaintext. How a change in the plaintext/ciphertext affects further outputs from ECB, CBC or CTR mode.
  • Message Authentication Codes (MACs): A MAC is a small, fixed-size block of data used to verify the integrity and authenticity of a message. Symmetric signature.
  • Authenticated Encryption: combines confidentiality (encryption) with integrity(authenticity) features/functions in one scheme. Encrypt-then-MAC is a common example.

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