Free QCE Digital Solutions Flashcard Review
Review 20 public cards with spaced repetition. Flip a card, then grade your recall. Cards you miss return today. Your schedule stays in this browser.
Loading your review schedule…
Browse the free cards
What is symmetric encryption?
Encryption in which the same secret key is used to encrypt and decrypt. It is fast, but the key must be shared securely between sender and receiver.
What is the key-distribution problem of symmetric encryption?
Both parties need the same secret key, so it must be exchanged without interception; asymmetric methods such as RSA are commonly used to exchange it.
State the block size and effective key length of DES.
DES encrypts 64-bit blocks with a 56-bit effective key (64 bits including 8 parity bits), using 16 rounds.
Why is DES no longer considered secure?
Its 56-bit key space (about 7.2 × 1016 keys) can be searched by brute force with modern hardware in a short time.
How does Triple DES work?
It applies DES three times (encrypt–decrypt–encrypt) using two or three keys, giving a much larger effective key space while keeping DES's 64-bit block.
What is the main drawback of Triple DES compared with AES?
It is about three times slower than DES and still uses a small 64-bit block, so it has been deprecated in favour of AES.
State the block size and key lengths of AES.
AES uses a 128-bit block with a 128-, 192- or 256-bit key (10, 12 or 14 rounds).
Why is AES the most widely used symmetric algorithm?
It is a fast, openly standardised algorithm with no practical brute-force attack on its key lengths, and many processors accelerate it in hardware.
State the block size and key range of Blowfish.
Blowfish uses a 64-bit block and a variable key length from 32 to 448 bits.
What is a limitation of Blowfish?
Its 64-bit block makes it unsuitable for encrypting very large amounts of data under one key, and its key setup is slow when keys change often.
State the block size and key lengths of Twofish.
Twofish uses a 128-bit block with keys up to 256 bits; it was a finalist in the AES selection.
Name the five symmetric algorithms listed in the Digital Solutions syllabus.
DES, Triple DES, AES, Blowfish and Twofish.
What is asymmetric encryption?
Encryption using a mathematically linked key pair: a public key that can be shared and a private key kept secret. Data encrypted with one key can only be decrypted with the other.
On what mathematical problem does RSA's security depend?
The difficulty of factoring the product of two very large prime numbers.
To send a confidential message with RSA, which key encrypts it?
The recipient's public key; only the recipient's private key can decrypt it.
How is an RSA digital signature created and checked?
The sender signs (a hash of) the message with their private key; anyone can verify the signature with the sender's public key, proving origin and integrity.
Why is RSA usually combined with a symmetric algorithm?
RSA is slow, so it is used to exchange a symmetric session key (e.g. AES) securely, and the bulk data is then encrypted with that faster key.
What RSA key length is generally regarded as a current minimum?
2048 bits; longer keys such as 3072 or 4096 bits give more security at the cost of speed.
How does a Caesar cipher encrypt a letter?
It shifts the letter a fixed number of places along the alphabet: C = (P + k) MOD 26 with A = 0.
How many useful keys does a Caesar cipher have?
25 (a shift of 0 or 26 leaves the text unchanged), so it can be broken by trying every key.