Cryptography

Cryptography

Cryptography is the science of protecting information by converting readable data (plaintext) into unreadable data (ciphertext).

Only authorized users can convert the ciphertext back into readable information using a key.

Example:

Plain Text
Plaintext:
HELLO
 
Encryption
↓
 
Ciphertext:
X7#P9K@
 
Decryption
↓
 
HELLO

Cryptography helps provide:

Confidentiality
Integrity
Authentication
Non-repudiation
Key Cryptography Concepts
Plaintext

Readable data.

Plain Text
MyPassword123
Ciphertext

Encrypted, unreadable data.

Plain Text
A9$8kL@!x
Encryption

Converts plaintext into ciphertext.

Decryption

Converts ciphertext back into plaintext.

Two Main Types of Encryption
1. Symmetric Encryption

Symmetric encryption uses the same key for encryption and decryption.

Plain Text
Plaintext
|
Encrypt with Key A
|
Ciphertext
|
Decrypt with Key A
|
Plaintext
 
Example

Alice and Bob share a secret key:

Plain Text
Secret Key = ABC123

Alice encrypts with ABC123.

Bob decrypts with ABC123.

Advantages of Symmetric Encryption

✅ Very fast

✅ Efficient

✅ Good for encrypting large amounts of data

✅ Uses less processing power

Disadvantages

❌ The secret key must be shared securely

❌ Difficult to manage many keys

❌ If the key is stolen, attackers can decrypt data

Common Symmetric Algorithms
AES (Advanced Encryption Standard)

Most important encryption algorithm to know.

Common key sizes:

Plain Text
AES-128
AES-192
AES-256

Used in:

VPNs
Wi-Fi (WPA2/WPA3)
BitLocker
HTTPS
DES (Legacy)
Plain Text
DES

Older standard and no longer considered secure.

3DES (Triple DES)

Improved DES but largely replaced by AES.

2. Asymmetric Encryption

Asymmetric encryption uses two keys.

Plain Text
Public Key
Private Key
``

One key encrypts and the other decrypts.

How It Works
Public Key

Can be shared with everyone.

Private Key

Kept secret.

Example:

Plain Text
Bob's Public Key

Anyone can use it to encrypt data.

Plain Text
Bob's Private Key

Only Bob can decrypt the data.

Example
Plain Text
Alice
|
Encrypt with Bob's Public Key
|
Internet
|
Bob
|
Decrypt with Bob's Private Key
Advantages of Asymmetric Encryption

✅ Easier key distribution

✅ Supports digital signatures

✅ Supports authentication

✅ Secure communication without sharing secret keys first

Disadvantages

❌ Slower than symmetric encryption

❌ Requires more CPU power

❌ Less efficient for large amounts of data

Common Asymmetric Algorithms
RSA

Most common exam algorithm.

Used for:

SSL/TLS
Certificates
Secure communications
ECC (Elliptic Curve Cryptography)

Provides strong security with smaller keys.

Commonly used in:

Mobile devices
Modern VPNs
Diffie-Hellman (DH)

Used for secure key exchange.

Allows two parties to establish a shared secret key.

Symmetric vs Asymmetric
Feature	Symmetric	AsymmetricKeys Used	One Key	Two Keys
Speed	Fast	Slower
Encryption	Yes	Yes
Digital Signatures	No	Yes
Key Distribution	Difficult	Easier
Typical Use	Bulk Data Encryption	Key Exchange & Authentication
Why Both Are Used Together

Most modern systems use both.

Example: HTTPS

Step 1

Asymmetric encryption:

Plain Text
RSA or ECC

used to securely exchange keys.

Step 2

Symmetric encryption:

Plain Text
AES

used for the actual data transfer.

This provides:

✅ Strong security

✅ High performance

Digital Signatures

Digital signatures provide:

Authentication
Integrity
Non-repudiation

Example:

Plain Text
Private Key
↓
Sign Message
``

Recipients use the public key to verify the signature.

Hashing (Related but Different)

Hashing is not encryption.

A hash is a one-way mathematical fingerprint.

Example:

Plain Text
Password
↓
Hash

Common hashing algorithms:

SHA-256
SHA-384
SHA-512

Used for:

Password storage
File integrity verification
Security+ Exam Tips
Symmetric Algorithms
Plain Text
AES
DES
3DES

Think:

Plain Text
One Key
Fast
Asymmetric Algorithms
Plain Text
RSA
ECC
Diffie-Hellman

Think:

Plain Text
Two Keys
Slower
What Provides Confidentiality?

✅ Encryption

What Provides Integrity?

✅ Hashing

✅ Digital Signatures

Quick Memory Trick
Plain Text
Symmetric
=
Same Key
 
Asymmetric
=
A Pair of Keys
Simple Definition

Cryptography is the practice of securing information through encryption. Symmetric encryption uses one shared key for both encryption and decryption and is fast, while asymmetric encryption uses a public/private key pair, enabling secure communication, authentication, and digital signatures.

Cryptography

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