Cryptography Lecture Notes - CPE 303

CPE 303 DATA AND DIGITAL COMMUNICATION Cryptography ENGR. WENDY O. SANTOS CRYPTOGRAPHY is originated from two Greek words, “krypto” which means hidden and “graphene” which means writing. It is the making or breaking of secret codes. It uses two processes: Encryption – the process of transformi...

CPE 303 DATA AND DIGITAL COMMUNICATION Cryptography ENGR. WENDY O. SANTOS CRYPTOGRAPHY is originated from two Greek words, “krypto” which means hidden and “graphene” which means writing. It is the making or breaking of secret codes. It uses two processes: Encryption – the process of transforming plain text into code form using a certain algorithm. Example: Math is fun (plain text) - original message PWDK LV IXQ (coded form) – Cipher text Decryption – the process of converting back the coded message into plain text. KEY – refers to the strings of information that is used to reveal the encrypted message into readable form. Methods of Cryptography 1. Shift Cipher (Ceasar Cipher) 2. Using Modulo Operator Example 1 Using Shift Cipher, encrypt the word CRYPTOGRAPHY, using the key of shift of 3 to the right. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Example 2 Using Shift Cipher, encrypt the word DAD SEND MONEY, using the key of shift of 4 to the left. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Example 3 Decrypt the cipher text, BJY DN BJJY, using the key of 5 shifts to the left. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Example 4 Encrypt the message CPE IS FUN using modulo operator. Let k = 5. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Example 5 Encrypt the message MAHAL KITA using modulo operator. Let k = 8. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Example 6 Decrypt the message RRB NX KZS using modulo operator. Let k = 5. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 ACTIVITY: 1. Using Shift Cipher, encrypt the word ENGINEERING, using the key of shift of 5 to the left. 2. Decrypt the cipher text, G R P Q A L F Q, using the key of 3 shifts to the left. 3. Encrypt the message I WILL GRADUATE using modulo operator. Let k = 8. 4. Decrypt the message SGOTMGETGZUTGZU using modulo operator. Let k = 6. thanks! -Ma’am Wendy Do you have any questions? [email protected]