1. Introduction to Cryptography

2. What is cryptography?
Practice and study of techniques for secure communication in the presence of third parties called adversaries
Modern cryptography includes
data confidentiality
data integrity
authentication
etc.
Cryptography prior to the modern age was effectively synonymous with encryption
conversion of information from a readable state to apparent nonsense

3. Encryption and Decryption

4. Substitution Cipher
One simple example of encryption/decryption

5. Cryptosystem (Cipher)

6. Types of Cryptosystems
Secret-key cryptosystem
Symmetric system
Same key is used for both encryption and decryption
Key has to be kept secret
Public-key cryptosystem
Asymmetric system
A pair of keys
Public key
Private key
Private key needs to be kept secret

7. Secret-key (Symmetric) Cryptosystems

8. Key Distribution Problem
A lot of keys among users
Each pair of uses needs a unique key
How to send a key to the other
Key needs to be plain text

9. Public-key (Asymmetric) Cryptosystem

10. Key distribution in public-key cryptosystem
Alice says to Bob
“Hi Bob, could you send me your public key?”
Bob replies back to Alice
“Here you go. The attached is my public key.”
Alice receives the public key of Bob and starts sending messages to Bob
Messages are encrypted by Bob’s public key
Bob uses the corresponding private key to decrypt the messages
Any concerns?
How can Alice make sure that the received key is indeed the public key of Bob?
How can Alice make sure that the received key is indeed the public key of Bob?

11. Certificate Authority
A certificate authority or certification authority (CA) is an entity that issues digital certificates
A digital certificate certifies the ownership of a public key by the named subject of the certificate
A CA acts as a trusted third party—trusted both by the subject (owner) of the certificate and by the party relying upon the certificate
Format of these certificates is specified by the X.509 standard
Top 5 CAs as of May 2018
IdenTrust, Comodo, DigiCert, GoDaddy, GlobalSign

12. Procedure of obtaining a public key certificate

13. How to obtain the public keys of CAs
Public keys are embedded in root certificates
Root certificates are contained in the release of operating systems (and their updates) and the trustable web browsers (and their updates)