2.3 Symmetric and Asymmetric cryptography

Question 1

Symmetric system: needs two criterias:


Answer 1

(i) encryption and decryption are done using the same secret key

(ii) encryption and decryption functions are similar



The key must be exchanged between organizations.


Question 2

Asymmetric cryptography
 

A pair of related __ are used (one public and one private). ____ functions are utilized to generate these pairs. It is not essential for the sender to have a ___ key but the receiver needs it to decrypt the message.



Asymmetric cryptography is often used to authenticate data using digital signatures.


Answer 2

keys
One-way
private

Question 3

RSA-based signature schemes
1. Based on the assumption that the … multiplicated is very hard, but easy with additional information (so called trapdoor one-way-functions)

2. …, fast to verify

3. Not used in blockchains


Answer 3

1. factorization of large prime number
2. Long signatures and public keys
3. Not used in blockchains


Question 4

ECC-based signature schemes, such as ECDSA
:

Answer 4

1. Based on discrete logarithms

2. Short signatures and public keys

3. Used by Bitcoin and Ethereum (pre-merge)


Question 5

BLS-based signature schemes
:

Answer 5

1. Short signatures, easily verifiable

2. Aggregatable signatures (useful in blockchains as it saves space

3. Supports threshold cryptography (i.e., encrypt with public key and distribute the fragments of the private key among multiple parties)

   4.Used by Ethereum (post-merge)


Question 6

Many signatures need a good source of __ otherwise private keys can be leaked. Digital signatures can only sign a ___; thanks to ___ signing the hash of the message is enough.


Answer 6

entropy
small amount of data
collision resistance