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Lesson 2 - Encryption ITD2323 Lecture by: IZWAN SUHADAK ISHAK Assistant Lecturer Universiti Industri Selangor

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Things to learn about Concepts of encryption Cryptanalysis Symmetric encryption Assymmetric encryption Protocols and certificates Digital signatures Types of encryption algorithms

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Cryptography Secret writing – strongest tool to control against many kinds of security threats Users of cryptography usually do not create their encryption techniques – just use what’s available

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Terminology Imagine S (sender) sending a message to R (recipient) through T (transmission medium) However there’s an intruder or interceptor (O) who tries to access the message in any of these: –Block it – affects the availability –Intercept it – affecting the confidentiality –Modify it – affecting the integrity –Fabricate it – affecting the integrity as well

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…continued Encryption – the process of encoding a message (scrambling) Decryption – the process to reverse, transforming encrypted message back to original form Encode, decode, encipher, decipher are terms used in lieu of encrypt or decrypt Encode could mean translating entire word or phrases into something new Encipher could mean translating letters or symbols individually

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…continued Cryptosystem – a system for encryption and decryption Plaintext, cleartext – original form Ciphertext – encrypted (scrambled) form EncryptionDecryption PlaintextCiphertext Original Plaintext

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Encryption Algorithms Set of rules for how to encrypt plaintext and how to decrypt ciphertext Often use a device called ‘key’ (K) When C=E(K,P), it means E acts as an encryption algorithm, and K is the key. C is ciphertext; P is plaintext

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…continued When P=D(K, E(K,P)), it shows that both encryption and decryption keys are the same –This form is called ‘symmetric’ encryption When P=D(K D, E(K E,P)), it shows that encryption and decryption keys are NOT the same –This form is called ‘asymmetric’ encryption

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…continued EncryptionDecryption PlaintextCiphertext Original Plaintext KEY EncryptionDecryption PlaintextCiphertext Original Plaintext KEKEKEKE KDKDKDKD Encryption Key Decryption Key SYMMETRIC ENCRYPTION ASYMMETRIC ENCRYPTION

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…continued A key gives flexibility in using an encryption scheme Can create different encryptions by just changing the key Provides additional security Any encryption scheme that does not require a key = keyless cipher

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Some interesting terms… Cryptography – hidden writing, practice of using encryption to conceal text Cryptanalyst – studies encryption and encrypted messages, hoping to find hidden messages Cryptographer (& cryptanalyst) attempt to translate coded material to plaintext

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…continued Cryptographer works on behalf of a legitimate sender/receiver Cryptanalyst works on behalf of an unauthorized interceptor Cryptology – research into and study of encryption and decryption

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Two simples types Substitution –One letter is exchanged for another –Some call it monoalphabetic cipher or simple substitution Transposition –Order of the letters rearranged

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Caesar Cipher ABCDEFGHIJKLMNOPQRSTUVWXYZ defghijklmnopqrstuvwxyzabc Plaintext Ciphertext In this example: Shift of 3 c i = E(p i ) = p i + 3 What would the ciphertext for UNISEL? Answer: xqlvho TREATY IMPOSSIBLE? Answer: wuhdwb lpsrvvleoh

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Permutation Almost like Caesar Cipher Uses a word as the key E.g. if ‘word’ is the key: If ‘professional’ as the key: If the word has several similar alphabets, only ONE of it should be used ABCDEFGHIJKLMNOPQRSTUVWXYZ wordabcefghijklmnpqstuvxyz Plaintext Ciphertext ABCDEFGHIJKLMNOPQRSTUVWXYZ profesinalbcdghjkmqtuvwxyz Tmepty adjhqqarce

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…continued Encrypt “TREATY IMPOSSIBLE” using both permutation algorithms –Answer: –Answer: spawsy fjmlqqfola – –Answer: tmepty adjhqqarce

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…continued Both types of permutation algorithms may invoke easy access by cryptanalyst, therefore it is more desirable to have less regular rearrangement of letters A possibility is to count by three (or 5, or 7, or 9) and rearrange in that order

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…continued ABCDEFGHIJKLMNOPQRSTUVWXYZ adgjmpsvybehknqtwzcfilorux In this case, a+3=d, d+3=g, g+3=j Encrypt “TREATY IMPOSSIBLE” –Answer: fzmafu yktqccydhm

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Vernam Cipher Involves an arbitrarily long nonrepeating sequence of numbers combined with the plaintext Equate each alphabet with corresponding number, add to its random 2-digit, find the mod of its sum with 26 to get the ciphertext

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…continued ABCDEFGHIJKLMNOPQRSTUVWXYZ VERNAMCIPHER tahrspItxmab

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Vigenère Cipher Uses a table called “Vigenère Tableau” Table is a series of alphabets from A to Z Encryption is done from top to bottom, following the key which follows the ‘Permutation’ style key abcdefghijklmnopqrstuvwxyz A abcdefghijklmnopqrstuvwxyz B bcdefghijklmnopqrstuvwxyza C cdefghijklmnopqrstuvwxyzab X xyzabcdefghijklmnopqrstuvw Y yzabcdefghijklmnopqrstuvwx Z zabcdefghijklmnopqrstuvwxy

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Transposition Goal is confusion Encryption in which the letters of the message are rearranged; breaking established patterns

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Columnar Transposition Rearranging characters of plaintext into columns In a 5-column transposition, plaintext characters are written in rows of five and arranged one row after another: Ciphertext is written from column to column C1C1C1C1 C2C2C2C2 C3C3C3C3 C4C4C4C4 C5C5C5C5 C6C6C6C6 C7C7C7C7 C8C8C8C8 C9C9C9C9 C 10 C 11 C 12 C 13 C 12 CnCnCnCn plaintext ciphertext

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…continued THISISAMESSAGETOSHOWHOWACOLUMNARTRANSPOSITIONWORKS tssoh oaniw haaso lrsto imghw tssoh oaniw haaso lrsto imghw utpir seeoa mrook istwc nasns utpir seeoa mrook istwc nasnsPUTANXIFWORDSDONOTFILLALLCOLUMNSXXX pxrnl cnuid olost Fstal xawdf luxno oilmx

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Public Key Encryption Each user has a key that does not have to be kept secret Secret is the decryption technique, not the key itself Public key cryptosystem accomplish this goal by using two keys; one to encrypt and one to decrypt Each user has two keys: a public key and a private key

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…continued P = D(k PRIV, E(k PUB, P)) Some public key encryption algorithms have this relationship: P=D(k PUB, E(k PRIV, P))

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…continued Let’s say there’s 3 users, B, C and D All three have to send a message to A and each other Each distinct pair of users needs a key, each user would need 3 different keys; A would need a key for B, C and D each. With public key, each B, C and D can use A’ s public key to send the message, but A’s private key remains private, so C cannot decrypt message sent by B to A

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Comparison Secret key (Symmetric) Public Key (Asymmetric) Number of Keys 12 Protection of key Must be kept secret One key must be kept secret, the other can be freely exposed Best uses Cryptographic workhorse; secrecy and integrity of data – single characters to blocks of data, messages, files Key exchange, authentication Key distribution Must be out-of-hand Public key can be used to distribute other keys SpeedFast Slow; typically, 10,000 times slower than secret key

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Rivest-Shamir-Adelman (RSA) Encryption A public key system Introduced in 1978 and remains secure until now Combines results from number theory with degree of difficulty in determining the prime factors of a given number Uses two keys, d & e for decryption and encryption – either private or public key can be used in the encryption P=E(D(P))=D(E(P))

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…continued C=P e mod n P=C d mod n P=C d mod n = (P e ) d mod n = (P d ) e mod n Key choice: –Consists of pair of integer (e,n) for encryption and integer (d,n) for decryption –Start point to find value of n n should be quite large (a product of two prime numbers p and q) p and q are usually 100 digits each e is relatively prime to (p-1)*(q-1) e has no factors in common with (p-1)*(q-1) where e>(p-1) and e>(q-1)

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…continued e * d = 1 mod (p-1)*(q-1) Usually n is made public and d is kept secret

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