Cryptography Part I Basic Terminology Cryptography means secret
Cryptography, Part I
Basic Terminology ● Cryptography means secret writing.
Basic Terminology ● ● Cryptography means secret writing. Encryption means converting plaintext into ciphertext. ○ hello → khoor
Basic Terminology ● ● ● Cryptography means secret writing. Encryption means converting plaintext into ciphertext. ○ hello → khoor Decryption means converting ciphertext back into plaintext. ○ khoor → hello
Symmetric Encryption Model ● Encryption and decryption require a key and an algorithm.
Symmetric Encryption Model ● ● Encryption and decryption require a key and an algorithm. Symmetric encryption: the same key is used for both encryption and decryption.
Symmetric Encryption Model ● ● Encryption and decryption require a key and an algorithm. Symmetric encryption: the same key is used for both encryption and decryption.
Caesar Cipher (A substitution cipher) ● First commonly used cipher (34 BC).
Caesar Cipher (A substitution cipher) ● First commonly used cipher (34 BC). ● Key: Shift the alphabet by N letters to create cipher alphabet. 3 3
Caesar Cipher (A substitution cipher) ● Key: Shift the alphabet by 3 letters to create cipher alphabet. ● Algorithm: Substitute plain letters with shifted letters. 3 Plain: abcdefghijklmnopqrstuvwxyz Alphabet Cipher defghijklmnopqrstuvwxyzabc Alphabet
Caesar Encryption 3 Algorithm: Substitute plain letters with shifted letters. Plain: abcdefghijklmnopqrstuvwxyz Alphabet Cipher Alphabet hello defghijklmnopqrstuvwxyzabc khoor
Caesar Decryption 3 Algorithm: Substitute shifted letters with plain letters. Plain: abcdefghijklmnopqrstuvwxyz Alphabet Cipher Alphabet khoor defghijklmnopqrstuvwxyzabc hello
Caesar Cipher (You Try It! …. Click on the image!) 1. By Hand: Use the Caesar cipher to encrypt your name. Use a key of 3 on the first try. Then use the script above to check your answer. 2. Brute force search. A brute force search for a Caesar key would be to test every possible key. How many keys would you have to test? Use brute force search to crack the following message. What is the key? YTNP HZCV JZF OTO TE!
Cracking Caesar Cipher ● Brute force attack: How many keys would Eve (the Eavesdropper) have to try to break Alice’s message to Bob? ? ?
Cracking Caesar Cipher ● Brute force attack: How many keys would Eve (the Eavesdropper) have to try to break Alice’s message to Bob? ● Answer: 25 keys. That’s too easy! ? ?
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