Trust Management in Networks

1
Trust Management in Networks

• Dijiang Huang

2
What is trust?

• Entity trust
• Data trust
• Situation aware trust
• Rating and credential systems

3
Traditional Frameworks

• Access Control Lists (ACL)
• Role-based Access Control (RBAC)
• Public-key Infrastructure (PKI)
• Web of trust (PGP)

4
Simple Models for Entity Trust
kAB
1) Point-to-point
A
B
KDC
KDC
2) Key distribution center
(1) req
k
(1)
k
k
(2)
(2)
(3)
(3)
A
B
A
B
k
KTC
KTC
3) Key translation center
k
k
k
(1)
(1)
k
(2)
(2)
(3)
A
B
B
A
k
5
Initial keying requirements

• point-to-point parties must share a long-term
  secret key a priori
• centralized approach sharing long-term secret
  keys with trusted server

6
Centralized key management

• Pros
• key-storage efficiency
• Cons
• single point of failure
• performance bottleneck
• on-line server

7
KTC and symmetric certificates

• Basic protocol
• Symmetric key-certificates SCertAEKT(KAT,A)
• KTC only needs to store KT
• Avoids requirement of a secure DB of user
  secrets
• other key provided when necessary in form of
  certificates
• A?T SCertA, EKAT(B,M), SCertB

KTC
(1)
A, EKAT(B,M)
(2)
EKBT(M,A)
A
B
EKBT(M,A)
(3)
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Third parties
(a) in-line
in-line TTP
A
B
(b) on-line
on-line TTP
optional
A
B
(c) off-line
off-line TTP
optional
A
B
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Advantages of public-keys

• no need for secrecy of encryption keys (e.g., is
  public)
• on-line trusted server not required
• additional features not easily obtainable with
  symmetric techniques
• non repudiation
• true data origin authentication

10
Classification of keys

• master keys
• highest level keys, no cryptographic protection
  (physical security, out-of-band)
• key-encrypting keys (KEK)
• must be strongly protected
• data keys (session key, bulk data encryption)

11
Public Key Distribution Techniques

• Public key certificates
• ID-based systems

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Public-Key Certificates

• Public-key certificates well defined notion
• Transitive trust derived from public key of CA
• Creation of PKC
• Creation by CA itself
• Creation by the entity
• different certificates for different purposes
• verification (cryptographic check, revocation
  status, time check)
• attribute certificates

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ID-based system I
Party B
Party A
signature generation
signature verification
message
PASS/ FAIL
private key
SA
Trusted Party T
DA
SA
private key generation
IDA
ST
IDA
PT
PT
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ID-based System II
Party B
Party A
signature generation
signature verification
message
PASS/ FAIL
private key
public key
PA
SA
public key reconstruction
Trusted Party T
private key generation
RA
RA
IDA
ST
IDA
PT
PT
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ID-based systems

• No explicit public key available
• the key is constructed from public available
  information
• Asymmetric system where unique name plays the
  role of public key

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Self-certified public keys
Party B
Party A
signature generation
signature verification
message
PASS/ FAIL
private key
public key
PA
SA
PA
asymmetric key pair generation
public key reconstruction
Trusted Party T
RA
public data generator
IDA
ST
IDA
PT
PT
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Trust model multiple CAs (1)
(a) separate domains
(b) strict hierarchy
CA1
CA5
E1(1)
Er(1)
...
CA3
CA4
CA2
...
CA1
CA2
E1(2)
Es(2)
...
E1(1)
Er(1)
E1(2)
Es(2)
...
...
18
Trust model multiple CAs (2)
(c) Multiple rooted trees
CAX
CAY
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Trust model multiple CAs (3)
(d) hierarchy with reverse certificates
(e) Directed graph (digraph) trust model
CA5
CA5
CA3
CA3
CA2
CA4
CA1
CA2
CA4
CA1
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Key Escrow

• Is an arrangement in which the keys needed to
  decrypt encrypted data are held in escrow by a
  third party, so that someone else (typically
  government agencies) can obtain them to decrypt
  messages which they suspect to be relevant to
  national security.