Cryptographic Protocols Lecture 13

1
Cryptographic ProtocolsLecture 13

• Key Exchange

2
Reading Assignment

• Reading assignments for current lecture
• Required
• Pfleeger 2.8
• Reading assignments for next class
• Required
• Pfleeger Ch 4.5 - Authentication

3
Asymmetric-Key Exchange

• Without server
• Broadcasting
• Publicly available directory
• With server
• Public key distribution center
• Certificates

4
Public announcement
KE-J.S.
KE-J.S.
KE-J.S.
KE-J.S.
John Smith
KE-J.S.
KE-J.S.
Bad Uncontrolled distribution ? easy to
forge
5
Publicly available directory
Better but not Good enough ? Directory could Be
compromised
Public Key Directory
KE-J.S.
KE-M.R..
John Smith
Mary Rose
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Public-key authority
Public-Key Authority
1. Request Time1
4. Request Time2
2. EKD-AuthKE-RRequestTime1
5. EKD-AuthKE-SRequestTime2
3. EKE-R(ID-SN1)
Sender
Recipient
6. EKE-S(N1N2)
7. EKE-R(N2)
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Public-key certificates
Certificate Authority
KE-R
KE-S
C-SEKD-CAuthTime1,ID-S,KE-S
CREKD-CAuthTime2,ID-R,KE-R
1. C-S
Sender
Recipient
2. C-R
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Certificates

• Guarantees the validity of the information
• Establishing trust
• Public key and user identity are bound together,
  then signed by someone trusted
• Need digital signature

9
Digital Signature

• Need the same effect as a real signature
• Un-forgeable
• Authentic
• Non-alterable
• Not reusable

10
Digital signature

• Direct digital signature public-key cryptography
  based
• Arbitrated digital signature
• Conventional encryption
• Arbiter sees message
• Arbiter does not see message
• Public-key based
• Arbiter does not see message

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Digital Signatures in RSA
Insecure channel
Sign
Verify
Plaintext
Signed plaintext
Plaintext
Encryption Alg.
Decryption Alg.
Recipient
Sender
Ss public key
Ss private key
(need reliable channel)
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Non-repudiation

• Requires notarized signature, involving a third
  party
• Large system hierarchies of notarization

13
Voting System

• Goal to establish the intent of the voter, and
  transfer that intent to the vote counter
• Assumptions
• Vote is open and everyone can monitor it
• Requirements
• Anonymous
• Scalable (speed, efficiency)
• Auditable
• Accurate
• Need to focus on accuracy and availability

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TEST 1

• This lecture is the last to be covered
• Need
• All lecture material (including hash functions)
• Chapters 1, 2
• Chapter 10.1, 10.2, 10.3 (characteristics and
  RSA), 10.5

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Next class
Identification Authentication