7 - Digital Signatures

Question 1

5 requirements for a digital signature

Answer 1

• Author Authentication
• Non-repudiation (signer cannot deny)
• Integrity (no changes)
• replay prevention (confirm time)
• Anyone can check

Question 2

General idea of signatures

Answer 2

• Message
• Apply hash function to get message digest
• “Decrypt” using Bob’s private key
• Which obtains Bob’s signature
• Send to Alice
• Alice receives both
• Apply same hash to recv message
• “Encrypt” signature with Bob’s public key
• Both message digests should equal.

Question 3

Digital Sig RSA example

Pub (e=3,n=33)
Private (d=7,q=3,q=11)

Message m
Hash h 13

Answer 3

Pub key (3,33)
Priv key (7,3,11)
message m
hash = 13

Bob sig:
s = h^7 mod 33 = 13^7 mod 33 = 7
publish m with sig 7

Alice does:
s^3 mod 33 = 7^3 mod 33 = 13
message digest of m

if equal then accepted.

Question 4

Are digital signatures legally accepted?

Answer 4

Yes. eIDAS, EU reg 2016