Key Management and Distribution

1
Key Management and Distribution
2
Major Issues Involved in Symmetric Key
Distribution

• For symmetric encryption to work, the two parties
  of an exchange must share the same key and that
  key must be protected.
• Frequent key changes may be desirable to limit
  the amount of data compromised.
• The strength of a cryptographic system rests with
  the technique for solving the key distribution
  problem -- delivering a key to the two parties of
  an exchange.
• The scale of the problem depends on the number of
  communication pairs.

3
Approaches to Symmetric Key Distribution

• Let A (Alice) and B (Bob) be the two parties.
• A key can be selected by A and physically
  delivered to B.
• A third party can select the key and physically
  deliver it to A and B.
• If A and B have previously and recently used a
  key, one party can transmit the new key to the
  other, encrypted using the old key.
• If A and B each has an encrypted connection to a
  third party C,
• C can deliver a key on the encrypted links to A
  and B.

4
Symmetric Key Distribution Task
5
Symmetric Key Hierarchy

• Typically a hierarchy structure of keys is
  adopted.
• Session keys
• temporary key
• used for encryption of data between users
• for one logical session then discarded
• Master keys
• used to encrypt session keys
• shared by each user the key distribution center

6
Symmetric Key Hierarchy
7
Symmetric Key Distribution Scenario
8
Symmetric Key Distribution Issues

• Hierarchies of KDCs required for large networks,
  but must trust each other
• Session key lifetimes should be limited for
  greater security
• Use of automatic key distribution on behalf of
  users, but must trust system
• Use of decentralized key distribution
• Controlling key usage

9
Symmetric Key Distribution Using Public Keys

• Public key cryptosystems are inefficient.
• almost never used for direct data encryption
• rather used to encrypt secret keys for
  distribution

10
Simple Secret Key Distribution

• Merkle proposed this very simple scheme
• allows secure communications
• no keys before/after exist

11
Simple Secret Key Distribution (contd)

• Simple secret key distribution (contd)
• advantages
• simplicity
• no keys stored before and after the communication
• security against eavesdropping
• disadvantages
• lack of authentication mechanism between
  participants
• vulnerability to an active attack as described in
  the next slide
• leak of the secret key upon such active attacks

12
Man-in-the-Middle Attacks

• This very simple scheme is vulnerable to an
  active man-in-the-middle attack.

13
Secret Key Distribution with Confidentiality
Authentication
14
Secret Key Distribution with Confidentiality
Authentication (contd)

• Provision of protection against both active and
  passive attacks
• Assurance of both confidentiality and
  authentication in the exchange of a secret key
• Availability of public keys a priori
• Complexity

15
Public Key Distribution

• The distribution of public keys
• public announcement
• publicly available directory
• public-key authority
• public-key certificates
• The use of public-key encryption to distribute
  secret keys
• simple secret key distribution
• secret key distribution with confidentiality and
  authentication

16
Public Key Distribution (contd)

• Public announcement

17
Public Key Distribution (contd)

• Public announcement (contd)
• advantages convenience
• disadvantages forgery of such a public
  announcement by anyone

18
Public Key Distribution (contd)

• Publicly available directory

19
Public Key Distribution (contd)

• Publicly available directory (contd)
• elements of the scheme
• name, public key entry for each participant in
  the directory
• in-person or secure registration
• on-demand entry update
• periodic publication of the directory
• availability of secure electronic access from the
  directory to participants
• advantages greater degree of security

20
Public Key Distribution (contd)

• Publicly available directory (contd)
• disadvantages
• need of a trusted entity or organization
• need of additional security mechanism from the
  directory authority to participants
• vulnerability of the private key of the directory
  authority (global-scaled disaster if the private
  key of the directory authority is compromised)
• vulnerability of the directory records

21
Public Key Distribution (contd)

• Public-key authority

22
Public Key Distribution (contd)

• Public-key authority (contd)
• stronger security for public-key distribution can
  be achieved by providing tighter control over the
  distribution of public keys from the directory
• each participant can verify the identity of the
  authority
• participants can verify identities of each other
• disadvantages
• bottleneck effect of the public-key authority
• vulnerability of the directory records

23
Public Key Distribution (contd)

• Public-key certificates

24
Public Key Distribution (contd)

• Public-key certificates (contd)
• to use certificates that can be used by
  participants to exchange keys without contacting
  a public-key authority
• requirements on the scheme
• any participant can read a certificate to
  determine the name and public key of the
  certificates owner
• any participant can verify that the certificate
  originated from the certificate authority and is
  not counterfeit
• only the certificate authority can create
  update certificates
• any participant can verify the currency of the
  certificate

25
Public Key Distribution (contd)

• Public-key certificates (contd)
• advantages
• to use certificates that can be used by
  participants to exchange keys without contacting
  a public-key authority
• in a way that is as reliable as if the key were
  obtained directly from a public-key authority
• no on-line bottleneck effect
• disadvantages need of a certificate authority