Security (3)

Question 1

What is Asymmetric Cryptography?

Answer 1

• Some algorithms use a pair of keys
• If one key is used for encryption, the other is used for decryption
• You can generate a pair of keys, but one key cannot be obtained from the other in reasonable computation time
• One key is kept private, one key is made public (K^- and K⁺

Question 2

What kind of arithmetic does RSA encryption rely on?

Answer 2

Modular Arithmetic

Question 3

Why is RSA Secure?

Answer 3

• If you know the public key (n, e) and want to determine d, you would have to compute factors of n which is a hard problem

Question 4

What is the purpose of creating a signature by computing a message digest instead of encryption with a private / public key?

Answer 4

Computation of a signature by applying a private key to a long message is computationally expensive

Question 5

How is a fixed-length “fingerprint” computed?

Answer 5

Hash function H is applied to message m, giving a fixes size message digest H(m)

Question 6

How does a signed message digest work?

Answer 6

• Instead of signing entire message, sign only hash result
• Bob sends message and signed digest
• Alice received message and computes message digest
• Alice received signed message digest and computed message digest
• If signed digest and calculated digest are equal, message is considered signed

Question 7

What properties does a secure hash function have?

Answer 7

• Given x, it is computationally infeasible to find m such that H(m) = x
• Given m, it is computationally infeasible to find m’ != m such that H(m) = H(m’)

Question 8

Provide some examples of Hash Function Algorithms

Answer 8

• MD5 (compute 128-bit message digest in 4-step process)
• SHA1 (160 bit digest), SHA-256, SHA-512

Question 9

What is the main benefit of using a Message Authentication Code (MAC)?

Answer 9

• It is faster, since no encryption is necessary

Question 10

What is a Message Authentication Code (MAC)?

Answer 10

• Alternative to signed message digest
• Shared secret s used between parties
• Hash is computed on m + s
• Bob sends message m and h = H(m+s)
• Alice receives (m, h) and computes H(m+s)
• If h=H(m+s) message is considered signed

Question 11

Provide three examples of authentication encryption

Answer 11

• MAC-then-Encrypt (IPSec)
• Encrypt-then-MAC (SSL)
• Encrypt-and-MAC (SSH)

Question 12

What is a Certification Authority (CA)?

Answer 12

• Entity registers public key with CA and provides some proof of identity
• Certification Authority provides a certificate:
  
  • Binds public key to particular entity
  • Signed by CA’s private key
• CA’s public key is know (trusted) to public

Question 13

Describe how a certificate is checked

Answer 13

When Alice wants Bob’s public key:
- Bob provides a certificate
- Certificate is signed by CA
- Alice applies CA’s public key to confirm certificate’s authenticity
- Certificate contains Bob’s public key