2. PKI Concept
An arrangement that binds public keys with
respective user identities by means of a certificate
authority (CA).
Public Key Infrastructure based on asymmetric
cryptography. Relies on a key pair, one private and
one public
– Private key is secret
– Public key is freely available, linked to identity of
certificate owner
– Private key cannot be computed from public key
Concept is then applied into applications
4. Certification Authority
An Organization that binds public key to an
entity
Or an entity which issues digital certificates
for use by other parties.
Also called trusted third party (TTP).
5. Public Key Infrastructure
We want to use public key universally
Single CA is not sufficient to handle public
queries
Example Single DNS is not sufficient to
handle multiple clients request for IPs
6. Certificate Authority
Hierarchy
We need a hierarchical structure
Root CA
CA1 CA2
CA1 CA2 CA1
Root CA: Certify
performance of CAs in
second level
Level 1: Operate in large
geographical areas or
logical areas
Level 2: Operate in Small
geographical areas
7. Certificate Authority Hierarchy
Every body trust Root CA
Not trust intermediate CAs
Alice may obtain bobs certificate from
some authority but not trust it and
ultimately ask next higher CA to certify the
original certificate
8. X.509 Certificate Format
A protocol used for PKI
X.509 describe certificate in a structural
way
Uses Abstract Syntax Notation 1 Standard.
9. X.509 Structure
Version
Serial Number Id used by CA
Algorithm ID
Issuer Name of CA
Validity Start and end Period of Certificate
• Not Before
• Not After
Subject Entity whoes PK is being certified
Subject Public Key Info
• PK Algorithm
• Subject Public Key
Issuer Unique Identifier (Optional)
Subject Unique Identifier (Optional)
Extensions (Optional)
• ...
Certificate Signature Algorithm
Certificate Signature
10. ASN.1
Based on hierarchical structure.
A standard and flexible notation that describes
data structures for representing, encoding,
transmitting, and decoding data.
Top level uses integer values:
– 0 ITU-use
– 1 ISO use
– 2 joint ITU-ISO use.
Second level depends on first level for different
standards administered by the unit.
12. Problems with PKI
System originally contains entire globe.
– Would require one root CA.
– Governments are fickle and don’t like to trust
each other.
– Alternative
– Pretty Good Privacy is a computer program
that provides cryptographic privacy and
authentication
PGP’s Web of Trust
13. PGP’s Web of Trust
Public / private keys with an attached name, email
address, and optional photo.
No centralized CA to sign keys.
– PGP users sign keys when they’ve verified the
owner’s identity, so in essence each PGP user is
acting as a CA.
– Your trust of a public key is related to how many
signing “hops” you are away from that key and how
much you trust each signer along the route.
Decentralized key distribution – users send keys.
Makes key management issues very apparent
– Web of trust depends on end users verifying and
signing large quantities of keys.
14. Trust model issues
Who to trust?
– Which certificates can be trusted
Source of Trust
– How it is established?
Limiting/controlling trust in a given
environment
16. CA Hierarchy
Tree architecture
Single Root CA
– Number of subordinate CA’s
• Etc…
– Parent certifies children
– Leaves are non-CA (end-) entities
Typically CA either certifies other CA’s or
end-entities, but not both
Everyone has Root CA PK
17. Distributed Trust Architecture
A set of independent hierarchies
Cross-certification or PKI networking
– Connect the hierarchies
Fully-meshed – all CAs are cross-certified
18. Web Model
A bunch of root CAs
pre-installed in
browsers
The set of root CAs can
be modified
Root CAs are unrelated
(no cross-certification)
– Except by “CA powers”
of browser manufacturer
– Browser manufacturer =
(implicit) Root CA –Any wrongdoing at any of these CAs
can cause serious trouble.
20. User-Centric (Anarchy)
PGP
User = her own Root CA
– Webs of trust
Good
– User fully responsible for trust
Bad
– User fully responsible for trust
– Corporate/gov/etc. like to have central control
• User-centric not friendly to centralized trust policies
