1.2 Non-repudiation

Question 1

Explain Non-repudiation

Answer 1

• You can’t deny what you’ve said
  – There’s no taking it back
• Sign a contract
  – Your signature adds non-repudiation
  – You really did sign the contract
  – Others can see your signature
• Adds a different perspective for cryptography
  – Proof of integrity
  – Proof of origin, with high assurance of authenticity

Question 1

Explain Proof of Integrity

Answer 1

• Verify data does not change
  – The data remains accurate and consistent
• In cryptography, we use a hash
  – Represent data as a short string of text
  – A message digest, a fingerprint
• If the data changes, the hash changes
  – If the person changes, you get a different fingerprint
• Doesn’t necessarily associate data with an individual
  – Only tells you if the data has changed

Question 2

Explain the example of Hashing the Encyclopedia

Answer 2

• Gutenberg Encyclopedia, Vol 1,
  by Project Gutenberg (8.1 megabytes)
• Change one character somewhere in the file
  – The hash changes
• If the hash is different, something has changed
  – The data integrity has been compromised

Question 3

Explain Proof of Origin

Answer 3

• Prove the message was not changed
  – Integrity
• Prove the source of the message
  – Authentication
• Make sure the signature isn’t fake
  – Non-repudiation
• Sign with the private key
  – The message doesn’t need to be encrypted
  – Nobody else can sign this (obviously)
• Verify with the public key
  – Any change to the message will invalidate the signature