Computer Security Lecture 3 - Symmetric Cryptography

A cryptographic algorithm that uses a 56-bit key to encrypt 64-bit blocks of data.

The initial permutation rearranges bits of the 64-bit plaintext message before the first round of encryption.

The final permutation undoes the initial permutation, restoring the original bit order.

The key schedule generates 16 subkeys (K1 to K16), each 48 bits long, from the original 56-bit key.

Each round takes a 64-bit message and a 48-bit key as input, producing a 64-bit output. It uses substitution and permutation operations on the data to encrypt it.

The mangler function is a complex operation that mixes the right half of the input with the round key. It consists of several steps, including expansion, S-box substitution, and permutation.

The left half of the output in each round is simply the right half of the input.

The right half of the output is the left half of the input XORed with the right half of the input after it has been mangled with the round key (Ki+1).

It's computationally difficult to find two different messages that produce the same hash output. Imagine trying to find two identical fingerprints - it's practically impossible.

Given a hash output, it's impossible to figure out the original message. Like trying to unscramble a scrambled egg, you can't get the original ingredients back.

It's computationally infeasible to find two different messages that hash to the same value. Think of it like finding two identical snowflakes - it's practically impossible.

A family of hash functions offering various output lengths (224, 256, 384, and 512 bits). Currently recommended for secure hashing.

The winner of the NIST hash function competition. Offering various output lengths (224, 256, 384, and 512 bits).

In DES decryption, we reverse the process of encryption by undoing each round and permutation. This means retrieving the original left and right halves (Li and Ri) from the processed halves (Li+1, Ri+1).

DES decryption utilizes the same steps as encryption, but with the key schedule applied in reverse order. The key schedule is the process of generating round keys (Ki) from the original key (K).

The mangle function, used in the encryption process, is applied only in the forward direction. In decryption, we don't explicitly undo it; instead, we use the round key (Ki+1) and the processed halves (Li+1, Ri+1) to retrieve the original halves (Li, Ri).

TripleDES is a more secure version of DES that uses multiple rounds and key schedules to enhance security. It uses a longer key (either 112 bits or 168 bits) compared to DES's 56 bits.

A function that maps data of variable length to a fixed-length output called a hash code; it does not use a key and aims to provide a unique fingerprint for the input data.

For a given input, it's nearly impossible to find another input that creates the same hash code.

One of the operations used in AES to transform data, involves shifting rows of the state matrix by different amounts, implementing a byte-level permutation.

In the AES algorithm, an operation that replaces each byte in the state matrix with a corresponding value from a fixed S-box, a lookup table.

In the AES algorithm, an operation that multiplies the state matrix with a fixed matrix to achieve diffusion, spreading changes across the data.

The last step in an AES round, it involves adding a round key (a secret key derived from the main key) to the state matrix, performing an XOR operation on the corresponding bytes.

Storing passwords in a hashed format helps protect them from being read directly, but it's still vulnerable to brute-force attacks.

A random value added to a password before hashing to make brute-force attacks more difficult. Each password has a unique salt.

Pre-computed tables containing hashes of common passwords used to crack passwords quickly. They're a threat to password hashing.

A technique that repeatedly applies a hashing function to a password and salt to further slow down brute-force attacks.

Storing passwords in encrypted form, using a key only known to the system. This prevents brute-force attacks.

While encryption is strong against brute-force, it relies on the security of the encryption key. Losing this key weakens the entire system.

A brute-force attack where a system tries to guess a password by systematically testing all possible combinations.

This involves using likely passwords, such as common words or dates, to guess a user's password.

A technique used in symmetric-key encryption, where the encryption key is the same as the decryption key.

A symmetric cipher that operates on blocks of data, typically 64 or 128 bits. Block ciphers use a complex series of substitutions and permutations.

An encryption mode that operates on individual blocks of data independently, without considering the context of neighboring blocks. This makes it vulnerable to attack techniques such as frequency analysis.

An encryption mode that uses a chaining mechanism where each block of plaintext data is XORed with the ciphertext of the previous block before encryption. This makes it more secure than ECB by introducing dependencies between blocks.

A technique used to pad data when the input to an encryption algorithm does not match the required block size. Padding assures the data is divisible by the block size and prevents ambiguity in decryption.

A process of verifying the integrity of data by using a cryptographic hash function. It ensures that the data has not been tampered with during transmission or storage.

A cryptographic algorithm that generates a tag (a short output) based on a message and a secret key. This tag is used for message authentication to verify the authenticity and integrity of the message.

A symmetric encryption scheme designed to provide both confidentiality and integrity. Authenticated encryption ensures that the data is both encrypted and authenticated simultaneously.