cybersecurity cyberlab3

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CryptologyDUH BRX UHDGB?

Slides by Raymond Borges

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Outline

• Background• Ancient Cryptography• Frequency Analysis• Modern Cryptography• Secure Communication• Tools of the trade• Conclusion

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Background

Cryptology

Cryptography Cryptanalysis

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Background

Crypto | graphy

hidden/secret writing

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The Ancient Art of Secret Messages

Cryptography - practice and study of techniques for secure communication

Cypher text Encryption -------------------- Decryption

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Background

CryptographyPrinciples and methods for:1. Transforming message into unintelligible2. Transforming message back to original form

Secret password #$c*&c!~`fde1 Secret password

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Background

Cryptanalysis Principles and methods for:• Retransforming message back to original form• Without knowledge of key

#$c*&c!~`fde1 Secret password

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Background

Encryption

Transposition Substitution

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Encryption

• Plaintext: This is what you want to encrypt• Ciphertext: The encrypted output• Enciphering or encryption: process which

converts plaintext to ciphertext• Encryption algorithm: Sequence processing

steps to transform plaintext into ciphertext• Secret key: Sets some or all parameters used

by encryption algorithm

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Practice (transposition)

Caesar CipherCiphertext: DUH BRX UHDGBSolution:c = E(k, p) = (p + k) mod 26c = E(3, p) = (p + 3) mod 26 for k=key=34 mod 26 = 4 so A=D

p = D(k, c) = (c − k) mod 26D=4 (4-3) mod 26 = 1

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Practice (substitution)

Monoalphabetic cipher• plaintext letters: a b c d e f .....• substitution letters: t h i j a b .....Key is sequence of substitution letters26 ! = 4.03291461 × 1026

Impossible to crack? No….

Polyalphabetic tougher…

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ETAOIN SHRDLU (Frequency analysis)

• ‘E’ is the most common letter in the English language,

• ‘Th’ is the most common bigram • ‘The’ most common trigram• Letter frequency English

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Frequency analysis

• Most frequently occurring trigrams ordered by decreasing frequency are:

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Polyalphabetic Ciphers

• The Vigenère cipher • First letter use key 1• Second uses key 2• Third uses key 3

To Decrypt go backwards

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Modern Cryptography

Data Encryption Standard (DES) 1976• 64 bit key (uses 56bits)• 16 stages• Block divided into two 32-bit halves• XOR• Subkeys made key schedule

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Modern Cryptography

Feistel function:1. Expansion- the 32-bit half-block is expanded

to 48 bits by duplicating half of the bits2. Key mixing — the result is combined with a

subkey using an XOR operation. 3. Substitution —block divided into eight 6-bit

pieces then uses substitution boxes4. Permutation

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Modern Cryptography

Advanced Encryption Standard (AES) 2001• Substitution-permutation network• Not Feistel• Fixed block size 128 bits • Key size of 128, 192, or 256 bits

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Modern CryptographyAdvanced Encryption Standard (AES)

1. KeyExpansion2. Initial Round

1. AddRoundKey3. Rounds

1. SubEytes2. ShiftRows3. MixColumns4. AddRoundKey

4.Final Round 1. SubBytes2. ShiftRows3. AddRoundKey

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Public-key cryptography

RSA- Ron Rivest, Adi Shamir, Leonard Adleman 1978• Uses factoring problem (large primes)• Two large primes of similar size > 100 digits

Example:Public (n = 3233, e = 17)Private key is (n = 3233, d = 2753)

n=p1*p2, e=coprime number to (p1-1)(p2-1)d =modular multiplicative inverse of e

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Public-key cryptography

Encryption Decryption c = me (mod n) m = cd (mod n)

Example:To encrypt m = 65c = 6517 (mod 3233) = 2790

To decrypt c = 2790, we calculatem = 27902753 (mod 3233) = 65

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Secure Communication

• AES 256 bit key• 15360-bit RSA keys are equivalent 256-bit

symmetric keys• 2^256 = 1.15792089 × 1077 brute-force?

Use RSA then AES for secure communicationHTTPS, SSH, SFTP, SSL and TLS and others

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Some Tools of the trade

File Encryption• Axantum• WinZip

Disk encryption• TrueCrypt

Network• Open SSH

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References

• http://www.axantum.com/axcrypt/Downloads.html

• http://dottech.org/freeware-reviews/15996/• http

://technet.microsoft.com/en-us/library/cc757819(v=ws.10).aspx

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Questions?