Computer Security Lecture 2: Symmetric Cryptography

Entropy represents the average amount of information expected from a message class and is measured in bits.

A one-time pad requires a key that is the same length as the plaintext and can never be reused.

Stream ciphers encrypt data one symbol at a time, while block ciphers operate on fixed-size blocks of plaintext.

<p>In modern cryptography, the key length is independent of the plaintext size and can be reused.</p> Signup and view all the answers

<p>A secure key must have a high entropy value, often 128, 192, or 256 bits.</p> Signup and view all the answers

<p>The nonce ensures that the same plaintext will encrypt to different ciphertexts on different occasions, preventing replay attacks.</p> Signup and view all the answers

<p>A change in plaintext affects only the corresponding block of ciphertext, with no impact on other blocks.</p> Signup and view all the answers

<p>The counter value is generated by starting with the nonce and incrementing it for each subsequent block, where ni = ni-1 + 1.</p> Signup and view all the answers

<p>Altering one bit of plaintext results in changes to that block and all subsequent blocks of ciphertext, due to the chaining effect.</p> Signup and view all the answers

<p>Decryption in CTR mode uses the same nonce and counter values as encryption, applying the same operation: Pi = ENC(Key, ni) ⊕ Ci.</p> Signup and view all the answers

<p>Output P represents the decrypted plaintext after the secure decryption of the ciphertext.</p> Signup and view all the answers

<p>Bob rejects C2' because the MAC(k'AB,C') will not match with the MAC(k'AB,C), indicating data integrity issues.</p> Signup and view all the answers

<p>The key weaknesses of DES include its relatively small key size of 56 bits and a block size of 64 bits, making it vulnerable to brute force attacks.</p> Signup and view all the answers

<p>The second cipher block C2 is altered by using the operation C2' = C2 ⊕ ': £10.00' ⊕ ': £10000'.</p> Signup and view all the answers

<p>DEC(K1, S, C') indicates that the ciphertext C' is decrypted using the key K1 to retrieve the original session data S.</p> Signup and view all the answers

<p>The plaintext is transformed by XORing with the previous ciphertext block or Initialization Vector (IV) before encryption.</p> Signup and view all the answers

<p>One bit change in ciphertext will cause a change in the corresponding plaintext block and all subsequent blocks.</p> Signup and view all the answers

<p>The first block of plaintext is decrypted by XORing the decrypted first ciphertext block with the Initialization Vector (IV).</p> Signup and view all the answers

<p>The IV ensures that identical plaintext blocks produce different ciphertexts, increasing security.</p> Signup and view all the answers

<p>Unauthorized modifications disrupt the decryption process, leading to errors in plaintext, which can be detected.</p> Signup and view all the answers

<p>An error in one ciphertext block during decryption affects the corresponding plaintext and the subsequent plaintext blocks.</p> Signup and view all the answers

<p>Data confidentiality ensures that only authorized entities can access the encrypted data using the shared key.</p> Signup and view all the answers

<p>Ciphertext maintains secrecy through the encryption of plaintext blocks combined with the previous ciphertext block.</p> Signup and view all the answers

<p>Chaining makes each ciphertext block dependent on all previous blocks, which increases overall security.</p> Signup and view all the answers

<p>Addressing unauthorized modifications is crucial to maintain data integrity, ensuring that the received data has not been tampered with.</p> Signup and view all the answers

<p>Secret key cryptography.</p> Signup and view all the answers

<p>Symmetric encryption.</p> Signup and view all the answers

<p>To authenticate a message and provide integrity.</p> Signup and view all the answers

<p>Signing key (KS) and verification key (KV).</p> Signup and view all the answers

<p>Message Authentication Code.</p> Signup and view all the answers

<p>Claude Shannon.</p> Signup and view all the answers

<p>Errors in ciphertext affect subsequent plaintext blocks.</p> Signup and view all the answers

<p>They are related but distinct; one is kept secret while the other is public.</p> Signup and view all the answers

<p>Confidentiality and integrity of the message.</p> Signup and view all the answers

<p>To accommodate plaintext that does not align with block size requirements.</p> Signup and view all the answers

<p>The MAC function constructed using CBC ensures data integrity and authenticity by producing a message authentication code based on the encrypted data.</p> Signup and view all the answers

<p>Optional processing in CBC-MAC involves decrypting with a second secret key k' and then re-encrypting with the first key k to enhance security against potential cryptanalysis.</p> Signup and view all the answers

<p>Authenticated encryption is a reversible transformation of data that produces ciphertext which cannot be altered by unauthorized entities without detection, ensuring confidentiality, integrity, and origin authentication.</p> Signup and view all the answers

<p>The Encrypt-then-MAC approach first encrypts the plaintext to produce ciphertext, followed by generating a MAC from the ciphertext and a starting value, which is then concatenated to the ciphertext.</p> Signup and view all the answers

<p>K1 is used for the encryption of the plaintext, while K2 is utilized to create the MAC from the ciphertext and starting value.</p> Signup and view all the answers

<p>The output is structured as C = C' || T, where C' is the ciphertext and T is the tag generated by the MAC operation.</p> Signup and view all the answers

<p>During decryption, the ciphertext is divided to retrieve C' and T, and then T' is computed using MAC; the operation halts if T does not equal T'.</p> Signup and view all the answers

<p>The starting value S acts as an initialization vector, adding randomness to the encryption process and helping prevent vulnerabilities to certain attack patterns.</p> Signup and view all the answers

A cryptographic technique in which the same key is used for both encryption and decryption.

A method of encoding data to make it unreadable without the appropriate key.

A digital signature algorithm that uses the same key for generating and verifying the signature.

A process of adding extra data to a block of plaintext to make it the right size for encryption.

A type of encryption that uses a single block cipher algorithm to encrypt multiple blocks of data.

A measure of randomness or uncertainty in a message, influencing the strength of cryptographic algorithms.

The ability of errors in encryption to propagate through multiple blocks, potentially affecting the decryption of subsequent blocks.

A method of protecting both the confidentiality and integrity of data during transmission.

A cryptographic technique that uses two mathematically related keys: a public key for encryption and private key for decryption.

A digital signature algorithm that uses separate keys for signing and verifying the signature.

The amount of information expected, on average, in a message from a particular class. It's measured in bits, representing the minimum number of bits required to encode the information.

A symmetric encryption scheme where a key of the same length as the plaintext is used. The key cannot be reused.

A symmetric encryption system that combines individual plaintext symbols with corresponding keystream symbols using an invertible function. It encrypts one symbol at a time.

A symmetric encryption system where a block of plaintext is encrypted as a unit, producing a block of ciphertext of the same size. It uses a fixed-length key.

The process of encoding plaintext into ciphertext using a key and the same key is used for decryption. Blocks of a specific size are operated on with a fixed-length key.

The repeated nature of the plaintext remains hidden in the ciphertext.

The decryption algorithm using the key 'k'.

The process of transforming ciphertext back into plaintext using the key and the decryption algorithm.

The impact an error in the plaintext has on the ciphertext, and vice versa.

A change in a single bit of plaintext results in multiple blocks of ciphertext changing.

A change in a single bit of ciphertext results in only one block of plaintext changing.

The ability to ensure only authorized entities can access protected data.

The ability to ensure only authorized entities can create data and detect any unauthorized modifications.

A scenario where an attacker could potentially modify a message in transit, potentially causing financial harm or fraud.

Ensuring that only authorized entities can access sensitive data (confidentiality) and that the data remains unaltered (integrity).

A method of encryption where each plaintext block is XORed with the output of a counter that increments with each block. This ensures that each block is encrypted independently, making it resistant to error propagation.

The initial value used for the counter in CTR mode. It's a nonce, meaning it's unique and not re-used for security reasons.

In CTR mode, the encryption of the counter value using the secret key to generate a keystream for XORing with the plaintext.

An attack where an attacker modifies a ciphertext block without knowing the secret key, aiming to alter the decrypted message.

A MAC (Message Authentication Code) verifies the integrity of a message, ensuring it hasn't been tampered with during transmission. It uses a secret key shared by both sender and receiver.

DES (Data Encryption Standard) is a block cipher outdated for modern applications due to its small key size (56 bits) and block size (64 bits), making it vulnerable to attacks.

3DES, a more secure alternative to DES, uses a combination of three DES keys, providing a 168-bit key length. However, it's still considered a legacy encryption standard.

A cryptographic system that encrypts data one symbol at a time, producing a stream of keystream symbols that are combined with plaintext symbols.

A technique for transforming data using a block cipher and a secret key, outputting a fixed-length MAC for message authentication.

An authenticated encryption technique where data is first encrypted and then a MAC is generated based on the ciphertext. This order ensures the integrity of the encrypted data.

A starting value used in encryption algorithms, similar to the initialization vector (IV), that helps generate unique ciphertexts for the same plaintext.

A cryptographic operation that encrypts data using a specific algorithm and key, creating a ciphertext.

A cryptographic operation that decrypts data using the corresponding algorithm and key, revealing the original plaintext.

A cryptographic operation that generates a message authentication code (MAC) based on data, a secret key, and a specific algorithm, ensuring authenticity and integrity.