Public key cryptography & RSA

Question 1

What is public key cryptography?

Answer 1

The use of two different but related keys for encryption and decryption. One key serves as a public key while another serves a private key. Either key can be used to encrypt or decrypt, but the same key cannot be used to perform both encryption and decryption at the same time.

Question 2

What are the advantages of public key cryptography?

Answer 2

1. Increased security, since the secret key is kept secret. There is never a need to transmit the secret key when communicating over the internet unlike in secret key cryptography.
2. Can be used for digital signatures as well in addition to encryption.

Question 3

What are the disadvantages of public key cryptography?

Answer 3

1. Encryption/decryption speed is slow as compared to private key cryptography

Question 4

How do you generate a private key in RSA?

Answer 4

1. A user selects two prime numbers p and q.
2. Calculate N, where N = p*q
3. Calculate Φ(N), where Φ(N) = (p-1)(q-1)
4. Select public key e, where 1< e < Φ(N), where e is a prime number and where e does not have common factors with Φ(N), N.
5. To find secret key d, solve for d in the equation
   e*d MOD Φ(N) = 1

Question 5

Briefly describe RSA.

Answer 5

RSA is a public key cryptographic system which derives its security from the difficulty of factoring large numbers. The mathematical foundation is based on number theory and modular exponentiation.

Question 6

How to use RSA for authentication/integrity/non-repudiation (sign and verify)?

Answer 6

Sender encrypts message with sender’s private key, recipient decrypts message with sender’s public key

Question 7

How to use RSA for confidentiality?

Answer 7

Sender encrypts message with recipient’s public key, recipient decrypts message with recipient’s private key.