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# Caesar cipher

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A brief description about what Caesar Cipher is.

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### Caesar cipher

1. 1. CAESAR CIPHER Cryptography & Cryptanalysis Ramadhi Irawan
2. 2. DEFINITION • Plaintext • Is a message to be communicated. • Ciphertext • A disguided version of a plaintext. • Encryption • The process of turning Plaintext into Ciphertext. • Decryption • The process of turning Ciphertext into Plaintext. • Encipher / Decipher • Are synonymous with the verb encrypt & decrypt.
3. 3. DEFINITION • Cryptology • The study of encryption and decryption. • Cryptography • The application of Cryptology.
4. 4. CAESAR CIPHER • One of the earliest known example of substitution cipher. • Said to have been used by Julius Caesar to communicate with his army (secretly). • Each character of a plaintext message is replaced by a character n position down in the alphabet. • Belongs to Substitution Cipher
5. 5. CAESAR CIPHER A B C D E … … X Y Z D E F G H … … A B C • Example: • First row denotes the plaintext • Second row denotes the ciphertext • Ciphertext is obtain by “shifting” the orginal letter by N position to the right. • In This example, it is shifted by 3 to the right. • A becomes D • B becomes E • X becomes A, and so on…
6. 6. CAESAR CIPHER A B C D E … … X Y Z D E F G H … … A B C • Suppose the following plaintext is to be encrypted: ATTACK AT DAWN • By shifting each letter by 3, to the right. Then the resulting ciphertext would be: DWWDFN DW GDZQ
7. 7. CAESAR CIPHER • One could shift other than 3 letters apart. • The offset (Number of shift) is called the “Key” • Decryption Process: • Given that the key is known, just shift back N letter to the left. • Example: • Ciphertext: WJYZWS YT GFXJ • Key used: 3 • Plaintext: RETURN TO BASE
8. 8. CAESAR CIPHER • Math behind this: • Can be represented using modular arithmetic • Assume that : • A = 0, B = 1, C = 2, …, Y = 24, Z = 25 • Encryption process can be represented as: • Such that: • X  the plaintext. • k  the number of shift (offset). • 26  There are 26 letters in the alphabet (English alphabet).
9. 9. CAESAR CIPHER • Math behind this: • Can be represented using modular arithmetic • Assume that : • A = 0, B = 1, C = 2, …, Y = 24, Z = 25 • Decryption process can be represented as: • Such that: • X  the plaintext. • k  the key (number of shift or offset). • 26  There are 26 letters in the alphabet (English alphabet).
10. 10. CAESAR CIPHER • Summary • Considerably easy to break. • Brute force attack works pretty well, due to relatively small keys (only allows 26 different keys). • Also known as monoalphabetic cipher, which the same plaintext letters are always replaced by the same ciphertext letters. • Mono – means one, indicates that each letter has a single substitute.