This presentation tries to answer the basic question "what is cryptography?" We use a simplified approach to present complex concepts. Our goal is to reach as many people as possible since we think cryptography is a topic that everyone should be familiar with. This presentation defines cryptography in simple terms and uses real world example to show how symmetric key cryptography works. Also introduces the concepts of classic and modern cryptography.

1. 12/5/2015
AppCrypto Team

2. What is cryptography?
It is the science and
practice of securing our
communications in the
presence of adversaries.
Not clear? Let me
explain further.

3. Meet Alice and Bob.
They are friends. They
borrow cash from each
other frequently.
BobAlice
They meet at some place
and exchange cash.
BobAlice
As time passed by , they
found it difficult to
meet physically to
exchange money.

4. Alice: Eve is willing to
help us exchange money.
They were thinking hard
about how they can
exchange money without
meeting.
Bob
Alice
Bob: I know Eve is
trustworthy, but I don’t
want her to know how
much money we
exchange.

5. BobAlice
How can we help
them?
Eve
Let’s use a chest with
a key to open.
Make two keys – give
one to Alice and other
to Bob.
Alice Bob

6. Eve
Alice
Bob
(1) Alice puts the
money in the
safe.
(2) Alice locks the
chest with her key
and hand over to
Eve.
(3) Eve takes the
locked chest to
Bob.
(4)Bob uses his key
to open the chest
and get the money.

7. This is a real world
example of
cryptography.
Alice and Bob were able to
hide their message (money)
from third parties.
In other words, we
preserve the
confidentiality of
messages.
This technique in the digital
world is called encryption.
Guru

8. There are two types of
cryptography.
Classic
Cryptography
Modern
Cryptography
Encryption Encryption
Authentication
Digital
Signatures
Interactive
Proofs
Oblivious
Computation
This is not an
exhaustive list.
Guru

9. Eve
Alice
Bob
Message = Money
Encryption
(Message, Key) =>
Ciphertext
Ciphertext = Money
locked in the chest
Decryption (Cipher
text, Key) =>
Message
Also called
paintext

10. paintext
ciphertext
Key = shift
by 13
A toy substitution cipher
Guru