Symmetric Encryption

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Bachelors Degree Computer Science (Computer Security) Flashcards on Symmetric Encryption, created by Alex Neves on 21/05/2016.
Alex Neves
Flashcards by Alex Neves, updated more than 1 year ago
Alex Neves
Created by Alex Neves almost 8 years ago
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Question Answer
What 3 things do encryption provide? - Confidentiality - Integrity - Authenticity
What are the 3 primitive encryption types? - Stream cipher - Block cipher - Hash function
What is the Caesar cipher? Letters are swapped with ones further in the Alphabet by a key K. Vulnerable to frequency analysis.
What is Kerkhoff's Pinciple? A cryptographic system must be secure even if everything about the system is known, with the exception of the secret key
State the 6 attack models - Brute-force - Ciphertext-only - Known-plaintext - Chosen-plaintext - Chosen-ciphertext - Related-key attack
Describe the only perfect cipher One-time pad. New key for every message. Key length == message length. Inpractical due to key size and generation.
Describe an implementation of the perfect cipher Modern stream cipher. XORs message with a key. Keys are produced by an infinite pseudorandom keystream.
What is a block cipher? Fixed-size plaintext to fixed-size ciphertext. More computationally expensive than stream ciphers. Vulnerable if changes aren't diffused.
Describe an SP Network Substitution-Permutation Network. Repeats rounds of S/P. S = swap values using lookup (confusion). P = shuffle values (diffusion)
How many rounds are needed to ensure security? One round susceptible to analysis attacks. More rounds causes more diffusion and makes it harder to analyse / detect patterns.
What is the Luby-Rackoff principle? 3 rounds = chosen-plaintext protection. 4 rounds = chosen-plaintext and chosen-ciphertext protection.
Describe SP Networks and Key mixing Combining keys with SP networks. Key XOR Message => SP Network => Key XOR cipher-text => SP Network etc.
What is a Feistal Cipher? Technique for chaining multiple rounds. Works with any round function.
Draw the Feistal Cipher C = R + L XOR F(R, K)
What is DES? Data Encryption Standard. 64-bit block size, 56-bit key size, 16 rounds. Feistal cipher.
What is AES? Advanced Encryption Standard. Rijndael SP Network. 128-bit block size, 128/192/256-bit key size, 10/12/14 rounds.
What is a Block Cipher Mode? Describes how a cipher is applied to messages larger than the block-size
Name 4 Block Cipher Modes - Electronic Code Book (ECB) - Cipher Block Chaining (CBC) - Counter Mode (CTR) - Galois Counter Mode (GCM)
Describe ECB Encrypt each block one after another. Vulnerable to analysis attacks (repeating patterns produce the same cipher blocks)
Describe CBC XOR output of each block with the next input before applying the cipher. First output is an IV. Can insert malicious blocks. Sequential.
Describe CTR Encrypt a counter and XOR with message block. Counter is combined with a nonce. Parallelisable.
Describe GCM Extension of CTR. Adds authenticity and integrity. Combines ciphertext with authentication tag.
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