The document provides an overview of cryptography, detailing its historical roots and distinguishing between symmetric and asymmetric cryptography. It explains the mechanics of both types, highlighting their benefits and disadvantages, and emphasizes the importance of cryptanalysis in ensuring security. Additionally, it outlines specific tasks related to the weaknesses of symmetric cryptography and presentations on stream and block ciphers.

1. Cryptography
Keamanan Jaringan - 2019

2. Overview
• If we hear the word cryptography our first
associations might be e-mail encryption,
secure website access, smart cards for
banking applications or code breaking
during World War II, such as the famous
attack against the German Enigma
encryption machine

3. • Cryptography seems closely linked to modern electronic
communication.
However, cryptography is a rather old business, with early examples
dating back to about 2000 B.C., when non-standard “secret”
hieroglyphics were used in ancient Egypt.
For instance, there are documented cases of secret writing in ancient
Greece, namely the scytale of Sparta

4. • Overview of the field of cryptology
Cryptography is the science of secret writing with the goal of hiding the meaning of a message.
Cryptanalysis is the science and sometimes art of breaking cryptosystems.
Relation >> cryptanalysis is the only way to assure that a cryptosystem is secure

5. Symmetric Cryptography
• Symmetric cryptographic schemes are also referred to as symmetric-
key, secret-key, and single-key schemes or algorithms.

6. Symmetric Cryptography
• In this situation, symmetric cryptography offers a powerful solution:
Alice encrypts her message x using a symmetric algorithm, yielding
the ciphertext y. Bob receives the ciphertext and decrypts the
message.

7. Symmetric Cryptography
• Simple Symmetric Encryption: The Substitution Cipher
The goal of the substitution cipher is the encryption of text (as opposed
to bits in modern digital systems). The idea is very simple: We
substitute each letter of the alphabet with another one.

8. Symmetric Cryptography
• Shift Cipher (or Caesar Cipher) : We simply shift every plaintext letter by
a fixed number of positions in the alphabet.

9. Symmetric Cryptography
• The symmetric ciphers can be divided into stream ciphers and block
ciphers

10. Stream Ciphers vs. Block Ciphers
• Symmetric cryptography is split into block ciphers and stream ciphers,
which are easy to distinguish.
• Figure 2.2 depicts the operational differences between stream (Fig.
2.2a) and block (Fig. 2.2b) ciphers when we want to encrypt b bits at a
time, where b is the width of the block cipher.

11. Stream Ciphers vs. Block Ciphers
• Stream ciphers encrypt bits individually. This is achieved by adding a
bit from a key stream to a plaintext bit.
• Block ciphers encrypt an entire block of plaintext bits at a time with
the same key. This means that the encryption of any plaintext bit in a
given block depends on every other plaintext bit in the same block

12. Asymmetric Cryptography
• Asymmetric cryptography, also known as public key cryptography,
uses public and private keys to encrypt and decrypt data.
• The keys are simply large numbers that have been paired together but
are not identical (asymmetric).
• One key in the pair can be shared with everyone; it is called the public
key.
• The other key in the pair is kept secret; it is called the private key.
Either of the keys can be used to encrypt a message; the opposite key
from the one used to encrypt the message is used for decryption.

13. Asymmetric vs. Symmetric Cryptography
• The main difference between these two methods of encryption is that
asymmetric encryption algorithms makes use of two different but
related keys -- one key to encrypt the data and another key to decrypt
it -- while symmetric encryption uses the same key to perform both
the encryption and decryption functions.
• Another difference between asymmetric and symmetric encryption is
the length of the keys. In symmetric cryptography, the length of the
keys -- which is randomly selected -- are typically set at 128-bits or
256-bits, depending on the level of security that's needed.

14. Asymmetric vs. Symmetric Cryptography
Asymmetric cryptography
Symmetric cryptography

15. Benefits and disadvantages of asymmetric
cryptography
• Advantage :
• the key distribution problem is eliminated because there's no need for
exchanging keys.
• security is increased as the private keys don't ever have to be transmitted or
revealed to anyone.
• the use of digital signatures is enabled so that a recipient can verify that a
message comes from a particular sender.
• it allows for non-repudiation so the sender can't deny sending a message.

16. Benefits and disadvantages of asymmetric
cryptography
• Disadvantages :
• it's a slow process compared to symmetric crytography, so it's not appropriate
for decrypting bulk messages.
• if an individual loses his private key, he can't decrypt the messages he
receives.
• since the public keys aren't authenticated, no one really knows if a public key
belongs to the person specified. Consequently, users have to verify that their
public keys belong to them.
• if a hacker identifies a person's private key, the attacker can read all of that
individual's messages.

17. Tugas 1
• Carilah informasi mengenai kelemahan Symmetric Cryptography, dan
buatlah simulasi proses eksploitasi kelemahan yang dimaksud
tersebut!
• Buatlah presentasi powerpoint tentang contoh implementasi stream
ciphers dan block ciphers
• Tugas di presentasikan minggu depan !!