Public Key Infrastructure

Alex Bardas

What is Cryptography ?

• Cryptography is a mathematical method of protecting information– Cryptography is part of, but not equal to,

security

• In modern computing, crypto is used to remediate deficiencies in the cyber space

Cryptographic Primitives

• Four Cryptographic Primitives:

1. Cryptographic Hash

2. Symmetric Encryption

3. Asymmetric Encryption

4. Digital Signatures

Cryptographic Hash

• If the message content is changed, the hash will be different (provides integrity guarantee)

• Knowing the hash does not reveal the input message

Hashing is NOT encryption!

Examples: SHA-1

Text Message(variable length)

Cryptographic Hash Function

“Unique” Fixed-length String (Hash or Digest)

Image source: http://en.wikipedia.org/wiki/File:Cryptographic_Hash_Function.svg

Cryptographic Hash Example

Encryption vs. Hashing

Image source: http://www.unixwiz.net/techtips/iguide-crypto-hashes.html

Symmetric Encryption (Secret-key Encryption)

• Encryption and decryption use the same key• Examples: AES

Clear Text Message Encryption Algorithm

Encrypted Message

Shared Key

Encrypted Message Decryption Algorithm

Clear Text Message

Shared Key

1.

2.

Asymmetric Encryption (Public-key Encryption)

• Every party has a pair of keys: <Kpub , Kpriv>

• Encryption and decryption use different keys• It is hard to infer private key from the public key

Examples: RSA, El-Gamal

Public Key: announced to

everyone

Private Key: known to the owner only

Asymmetric Encryption (Public-key Encryption)

Clear Text Message Encryption Algorithm

Encrypted Message

Public Key

Encrypted Message Decryption Algorithm

Clear Text Message

Private Key

1.

2.

Digital Signature

• Based on asymmetric crypto - Examples: RSA, DSA, El-Gamal

• Properties of a Digital Signature:1. Verification of the validity of a digital signature

needs only the public key

2. Only the owner of the corresponding private key can produce a valid signature

There is also MAC (Message Authentication Code) – signing using a shared key (based on symmetric cryptography)

Digital Signature

Message Signing Algorithm

Signed Message

Private Key

Signed Message Verification Algorithm

Signature is valid

Public Key

1.

2.

A digitally signed Email Message

Image source: http://www.wintellect.com/cs/blogs/pmehner/archive/2009/10/10/howto-obtain-and-configure-a-free-certificate-for-digitally-signing-your-outlook-2007-email.aspx

Public Crypto Challenge

Alice has Bob’s Public

KeyBob has Alice’s

Public KeyI am out of

luck today

- What if Alice and Bob cannot meet and exchange public keys ?- What if Alice and Bob don’t know each other ?- How to do they know that the public key that they are using belongs to the other

legitimate party and not to a malicious third party ?

Man-In-The-Middle

Alice thinks she has

Bob’s Public Key Bob thinks

he has Alice’s

Public Key

1. Eve has Bob’s and Alice legitimate public keys2. “Somehow” Alice and Bob have Eve’s public keys3. It’s Eve’s lucky day

How to Distribute Public Keys ?

• Ad-Hoc public key distribution (distribute at will)– Alice and Bob exchange public keys in a reliable

way• Public directory (similar to the telephone

directory)– Use a read-only directory (hard to modify/forge in

a large scale)• Published on paper

Public Key Distribution

• We want to distribute public keys in electronic form, NOT on paper

• How to verify the authenticity of the digital directory?Use digital signature

Certification Authority (CA)

• Alice and Bob don’t know each other but they both trust Cindy (Certification Authority)

• Alice and Bob have Cindy’s public key

• Cindy certifies Alice and Bob’s public keys => Digital Certificates

Digital Certificates

Cindy’s (CA) Digital Signature

What does the certificate tell us?• This public key belongs to Alice. • Alice is not a CA (Certification Authority)

Public Key Infrastructure

• What if Alice and Bob do not have a common friend?

– Cindy cannot be everywhere, Bob knows her but Alice doesn’t

• We have to find a trustworthy person that knows Cindy and Alice– Carl knows Cindy but doesn’t know Alice directly– Carl knows John and John knows Alice

• Certification chain

Multiple Certification Authorities (CAs)

R

L11 L12

L21 L22L23 L24

L31 L32 L33 L34 L35 L36 L37 L38

CA hierarchy

Alice Bob

John

Carl

Cindy

How are we getting the CA keys?

• Web Browsers are coming with an important number of root CA keys

• Other CA’s or single digital certificates can be added by the user (can be risky)

Valid Certificate (signed by VeriSign)

Valid Certificate Example

Certificates Warnings in Different Web Browsers

Sources of Information

• CIS751 Basic Crypto & PKI slide sets by Xinming (Simon) Ou – Kansas State University