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Public Key Infrastructure
(PKI)
Jerad Bates
University of Maryland, Baltimore County
December 2007

Overview
 Introduction
 Building Blocks
 Certificates
 Organization
 Conclusions

Introduction
In the beginning there were shared secret keys
 Early cryptographic systems had to use the
same key for encryption and decryption
 To establish an encrypted channel both users
needed to find out this key in some secure
fashion
 Limited – Users could meet and exchange the key
 Flexible – Users could use a key server

Introduction
Key Exchange – User to User
 This exchange eliminates a communication channel that could be
attacked
 Limited - Users must meet all other users
 In a system with n users, number of meetings is on the order of O(n2)
 Users must recognize each other or show proper identification

Introduction
Key Exchange – Key Server
 Each user has set to up a key with the Key Server
 Key Server creates and transmits secure session keys to users
 Flexible – Users need only have a prior established key with the Key
Server
 For a system with n users only (n) meetings must occur
 Key Server takes care of the initial validation of user’s identities
KA,KS KB,KS

Building Blocks
 Cryptographic tools
 Putting them together
 Names
 Time
 A secure communication session

Building Blocks
Cryptographic Tools
 Symmetric Key Cryptography
 Encryption: SEK(M) = C
 Decryption: SDK(C) = M
 Secure as long as only communicating users know K
 Having K lets one read C
 Fast to calculate
 Public Key Cryptography
 Encryption: PEK+(M) = C
 Decryption: PDK-(C) = M
 Secure as long K- is only known by the receiver
 Having K- lets one read C, but having K+ does not
 Slow to calculate

Building Blocks
Cryptographic Tools
 Digital Signatures
 Sign: PEK-(H(M)) = S
 Verify: PDK+(S) = H(M)
 Reliable as long as only the signer knows K-
 Having K- allows one to sign, having K+ only allows
one to verify the signature
 Slow to calculate
 K’s + and - could just be a user’s public and private
keys

Building Blocks
Putting Them Together
 Symmetric cryptography is used for
majority of communications
 Public Key cryptography is used for
exchanging Symmetric keys
 Digital Signatures are used to validate
Public Keys

Building Blocks
Names
 A name in PKI must be unique to a user
 Assigning these names presents similar
difficulties as found in other areas of
Distributed Systems
 Without proper and well thought out
naming PKI is pretty much useless

Building Blocks
Time
 A PKI must know the current time
 Much of a PKI’s security relies on having
an accurate clock
 For the most part, time does not need to
be known extremely reliably and being off
by a minute will usually not be an issue

Building Blocks
A Secure Communications Session
 Alice and Bob wish to set up a secure
communications channel
 They use Public Key Cryptography to exchange a
Symmetric key
 Alice: Private PK = K-A, Public PK = K+A
 Bob: Private PK = K-B, Public PK = K+B
 Time T and random Symmetric Key KS
 Simplified example:
1: Alice -> Bob: PEK+B(Alice, T, K+A, PEK-A(T, KS))
2: Bob -> Alice: PEK+A(T, KS)
3: Alice <-> Bob: SEKS(Mi)

Certificates
 What they are
 How they are issued
 How they are distributed
 How they are revoked

Certificates
What they are
 The issue with building a secure session is that it
assumes that both Alice and Bob know each others
public keys
 We need some way for them to learn this besides
meeting each other (otherwise we are in the same
predicament as with Symmetric Key exchange meetings)
 We could use a similar strategy to the Key Server but
can we do better?
This is where Certificates come in…

Certificates
What they are
 A Certificate is a combination of a user’s public key,
unique name, Certificate start and expiration dates, and
possibly other information
 This Certificate is then digitally signed, by some Trusted
3rd Party, with the signature being attached to the rest of
the Certificate
 This Signed Certificate is commonly referred to as just
the user’s Certificate
 The Certificate for a user Bob, signed by signer Tim, in
essence states
“I Tim certify that this Public Key belongs to Bob”

Certificates
How they are issued
 The users of a PKI must place their trust in a 3rd
Party to carefully verify a user’s identity before
signing his or her public key
 Each user generates their own Public-Private Key
pair and Certificate
 A user then verifies them self to the 3rd Party
and shows his or her Certificate’s content. At this
point the third party will sign the Certificate.

Certificates
How they are distributed
 Users are free to distribute their signed
Certificates over any medium, public or private,
without concern
 Other users may acquire this Certificate from
any source and check the 3rd Party’s signature
for tampering
 If the signature is good then the other users
know that the 3rd Party affirms that the
Certificate belongs to the user who is listed in
the Certificate

Certificates
How they are Revoked
 Periodically Certificates may become compromised,
requiring a Certificate Revocation
 A Certificate Revocation message is simply a message
signed by K-i (the private version of the Certificate’s K+i)
saying that the Certificate is revoked
 A PKI will have a database of revoked Certificates (a
Certificate Revocation List, CRL) that users may access
periodically for the latest list of revoked Certificates
 An alternative to certificate revoking is to set the
expiration time to very shortly after the issue time. Thus
every key in this system is revoked so rapidly that we do
not need to worry what may happen to the compromised
key

Organization
 What is “Trust”?
 How do we organize a PKI to disseminate
trust?

Organization
Trust
 Trust is based on real world contractual
obligations between a 3rd Party and users [2]
 This Trusted 3rd Party is referred to as a
Certificate Authority (CA)
 In other models trust is based on personal
relationships that don’t have a contractual basis
(e.g. PGP)
 Users may allow a CA to delegate their trust
 This delegation of trust is what allows us to
build large PKI’s

Organization
Trust
 If Alice trusts Root CA then she trusts Bob’s Certificate
signed by Root CA
 If Alice trusts Root CA to delegate her trust to others
then she trusts Chad’s Certificate signed by Small CA
Alice
Root CA
Small CA
Bob Chad

Organization
Organizing a PKI
 A PKI may be organized based on a
variety of models using delegation of trust
 Strict Hierarchy
 Networked
 Web Browser
 PGP

Organization
Strict Hierarchy
 All users trust Root CA
 Root CA may delegate that trust to other CA’s who in turn may be
allowed to delegate that trust
 In this way a PKI may grow without all the burden being placed on
Root CA
Alice
Root CA
Small CA
Bob Chad Dan
Smaller CA
Emily Fred

Organization
Networked
 The Networked model addresses what to
do when two or more PKIs wish to join
together or merge
 Two techniques
 Mesh
 Hub-and-Spoke
 We only need the Root CAs of each PKI to
participate in this model

Organization
Networked – Mesh
 Every Root CA signs every other Root CA’s
Certificate
 Hard to join a large numbers of CAs
Root CA3
Root CA1 Root CA2
Root CA4

Organization
Networked – Hub-and-Spoke
 The Root CAs come together to create the Super Root CA
 Each Root CA signs the Super Root CA’s certificate while the Super Root CA
signs each of theirs
 Easier to join large numbers of CAs
 Question becomes, Who gets to manage the Super Root CA?
Root CA3
Root CA1 Root CA2
Root CA4
Super Root CA

Organization
Web Browser
 A Web Browser maintains a list of trusted Root CAs
 Any Certificate signed by one of these Root CAs is trusted
 Basically a list of n Hierarchy Models
 Initial list decided on by Web Browser’s producer
alice.com bob.com chad.com dan.com
Smaller CA
emily.com fred.com
Root CA3
Root CA1 Root CA2 Root CAn
…

Organization
PGP
 Each user’s Certificate is signed by zero or more other users
 Certificate validity calculated from levels of trust assigned by signers
 Assigned levels (Chad)
 Implicit: User themselves – Chad
 Complete: Any Certificate signed by the user them self – Fred and Emily
 Intermediate Calculated Item
 Partial Trust: Any Certificate signed by a ‘Complete’ Certificate – Bob and Dan
 Calculated (Chad)
 Valid: Any Certificate signed by an ‘Implicit’ or ‘Complete’ level Certificates – Chad, Fred, Emily, Dan,
and Bob
 Marginally Valid: Any Certificate signed by two or more ‘Partial’ trust Certificates – Gary
 Invalid: Any Certificate signed by a ‘Marginally Valid’ or no one - Alice
Alice
Bob
Chad
Dan
Emily
Fred
Gary

Conclusions
 A PKI allows us to take the concept of a Key Server and apply it to
Public Keys
 It allows greater flexibility then a Key Server in that users do not
need to communicate with the Root CA every time a Session Key is
needed
 There are a vast variety of models for disseminating trust in a PKI
 Even though PKIs look like an amazing idea, in practice there are
numerous problems implementing them on a large scale
 Who does everyone trust?
 What format do people use?
 Security of the multitude of programs that rely on PKIs

Sources
[1] Adams, Carlisle, and Steve Lloyd. Understanding
PKI: Concepts, Standards, and Deployment
Considerations. Second ed. Boston, MA: Addison-
Wesley, 2003.
[2] Ferguson, Neils, and Bruce Schneier. Practical
Cryptography. Indianapolis, IN: Wiley, Inc., 2003.
[3] Stinson, Douglas R. Cryptography: Theory and
Practice. 3rd ed. Boca Raton, FL: Chapman &
Hall/CRC, 2006.
[4] Tanenbaum, Andrew S., and Maarten V. Steen.
Distributed Systems: Principles and Paradigms. 2nd
ed. Upper Saddle River, NJ: Pearson Prentice Hall,
2007.

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