SSL Explained

Cryptography Demystified

Bill Childers

http://wildbill.nulldevice.net/presentations/sslpreso

Agenda

• Why encryption?
• How SSL works
• Digital Certificates
• Practical issues
No Math!

Agenda: Why Encryption?

• PANIC! The risks
• Cryptography objectives
• Everyday uses for cryptography
• Practical issues

"Imagine code arriving over the Internet. It presents its credentials and says, 'I can prove I don't eat children for breakfast; I rarely eat children for lunch.' You know: all these things you'd like to know about a program if you're going to invite it into your home."

PANIC!

Stuff we need to do on the Internet

E-Commerce

Communication

Remote working

Nasty stuff out there

Identity Theft

SPAM

Viruses

Phishing

Cryptography Objectives

Privacy

Authentication

Non-repudiation

Integrity (Message)

Confidence (ie, trust)

Everyday uses for cryptography

• SSL Server
• E-Mail
• IPSec (VPN)

We will look at SSL as an example

DO try this at home!

Agenda: SSL

How SSL works

• A Special Intermission
• What is SSL?
• Connecting to a secure web site
• SSL handshaking

A Special Intermission

Whereupon I try to explain a security concept without math, or diagrams, or fumbling for words. Warning, here be dragons. Who's got the beer?

What is SSL?

• Secure Sockets Layer
• Developed by Netscape
• Standardized by IETF as Transport Layer Security (TLS)

Secure Web Example

Secure Web (continued)

• SSL allows us to encrypt stuff for privacy (ie, credit card number)
• Also checks who we are encrypting to for authentication

Is it really Wells Fargo when you do your on-line banking, or is it just a web site made to look like it? (phishing)

SSL Handshaking

SSL Summary

• https://server.domain/
• Browser authenticates the server by using its certificate
• Browser and server negotiate cryptography methods
• Traffic between browser and server is private

PANIC - Where are we?

• Privacy
• Authentication
• Non-Repudiation
• Integrity
• Confidence

Agenda - Digital Certificates

Digital Certificates

• Anatomy
• Digital signatures

Anatomy of a Digital Certificate

Also Important (but not shown):

• Public Key
• Signature

Issued To: Server FQDN

• Fully Qualified Domain Name
• Common Name (CN)
• Browser checks that FQDN matches CN

Issued To: Continued

• IF the URL does not match the CN of the cert, warn the user

Issued By: Certificate Authority

• Certificate Authority (CA) issues and signs certs
• Browser checks who signed the cert
• Looks up in a list of pre-loaded/trusted CAs

Issued By: Continued

• IF the CA is not known, alert the user

Public Key

• Part of a unique key pair
• Private key stays on the server
• Anything encrypted with the public key can only be decrypted with the private key
• Therefore, anything passed from browser to server via encryption is private

Validity

• Certs are issued for a finite period
• Often 1-2 years for a SSL server cert
• If the webserver presents an expired cert the browser will warn the user

Digital Signature

• Constructed by encrypting the cert contents with the CA's private key
• Does not make anything private
• Anyone who has the CA's public key can decrypt and thereby validate the signature
• The CA's public key can be found on the CA cert
• CA certs are widely distributed
• Hopefully the cert is genuine!
• At some point we need to take a leap of faith and choose who to trust

PANIC - Where are we?

• Privacy
• Authentication
• Non-Repudiation
• Integrity
• Confidence

Digital Certificate Summary

• Digital signature on the cert makes it possible to prove who issued it

Agenda: Practical Issues

• Faster encryption
• Smaller signatures
• Trust

Practical Challenges

• Encrypting/decrypting using public/private keys is very slow
• For the same reason, it is not practical to encrypt a whole document to sign it
• If only the server has a private key, then anything sent to the client will not be encrypted
• We need privacy in both directions

Faster Encryption

• Public keys are long, eg 2048 bits
• Public key encryption is very expensive (CPU intensive)
• Much faster secret key encryption is available - similar to ancient methods
• Encrypt just the secret key with the public key
• Can now use fast secret key encryption for data encryption both ways

Smaller Signatures

• Not necessary to encrypt the whole document with the private key
• Create a hash - like a checksum, but more sophisticated
• Encrypt just the hash
• Quicker and smaller

Smaller Signatures - Continued

Verifying a Signature

PANIC - Where are we?

• Privacy
• Authentication
• Non-Repudiation
• Integrity
• Confidence

Google Homework

• Public Key Algorithms (RSA, DSA, Elliptic-Curve)
• Secret Key Algorithms (DES, 3DES, IDEA, Blowfish, RC4)
• Hash Algorithms (MD5, SHA1, SHA2)
• Standards (TLS, S/MIME, PKCS)

Final Summary

• Encryption Technology is fundamental to the Internet
• Technologies such as SSL provide practical solutions
• Signed digital certificates are the currency of encryption
• Certificates provide a level of trust, but we must still choose who to trust

Who do we ultimately trust?

• Browser comes with pre-trusted certificates
• Ultimately we trust whoever shipped the browser
• Who ships the browser?
• PANIC!

Questions and answers

"When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl." --Kevin McCurley's Thought for the day, June 24, 1997

Where to find me:

• Linux Journal (Magazine, Web, IRC)
• Twitter: WildBill
• Facebook: wildbillchilders