# Cryptography/Notes

'*Cryptography is a Greek word which means "Secret Writing"*

**Important Uses of Cryptography**
1. Protect confidentiality of message (Achieved with Encryption Algorithms)
2. Provide identity for authentication (Achieved with Asymmetric Encryption Algorithms, digital signature algorithms)
3. Verify information to check message integrity (Achieved with hash functions)

To Validate information cryptographic function are used.

Cryptographic functions are called as **hash functions**

**e.g. of Hash Functions**
1. MD5 (Message Digest 5)
2. SHA (Secure Hash Algorithm)

**Types of Cryptography**
1. Secret key cryptography (Both parties know the same secret key. Uses **symmetric encryption**)
2. Public key cryptography (Both parties have 2 different keys,**Public key for encryption** and **Private key for decryption**.
In Reversible public key algorithms, Data encrypted with private key can be decrypted with public key.Uses **asymmetric encryption**)

Encryption algorithms are called **Ciphers**
Information in encrypted form is called **Ciphertext**

**2 Types of Ciphers**
1. Stream Ciphers (Process data byte at a time)
2. Block Ciphers (Process data in blocks 8 bytes at a time. Pad packets smaller than block)

**e.g. of Symmetric Encryption Algorithms**

1. DES (Data Encryption Standard - Block) 2. 3DES (Triple strength Data Encryption Standard - Block) 3. RC2 (Rivest Cipher 2 - Block) 4. RC4 (Rivest Cipher 4 - Stream)

**e.g. of Asymmetric Encryption Algorithms**

1. RSA - Rivest Shamir Adleman 2. DSA - Digital Signature Algorithm - Can only be used for digital signatures.