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Tomáš Foltýnek foltynek@pef.mendelu.cz Faculty of Business and Economics An Introduction to Cryptology & Steganography Mgr. Tomas Foltynek, Ph.D. Department of Informatics Faculty of Business and Economics Mendel University in Brno Czech republic

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An Introduction to Cryptology & Steganography Introduction Secure communication problem is as old as mankind –people want privacy (=nobody reads their messages) –people are curious (=want to read others’ messages) More communication => more need of security We often hear about –Hackers, crackers, attackers, –Viruses, worms, Trojan horses –E-banking breakthroughs –Breaking ciphers and secret codes –Hidden messages, watermarks How are these topics related?

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An Introduction to Cryptology & Steganography Content What is cryptology Symmetric cryptology –Basic principles Asymmetric cryptology –Digital signature Steganography

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An Introduction to Cryptology & Steganography What is cryptology Science of cryptography and cryptanalysis Cryptography –science of secret codes, enabling the confidentiality of communication through an insecure channel –e.i. how to make a message uncomprehensible for unauthorised persons Cryptanalysis –theory of (in)security analysis of cryptographic systems –e.i. how to break ciphers and read secret messages Also includes Steganography & Steganalysis –how to hide a message –how to find a hidden message Word origin from Greek: crypto = hidden

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An Introduction to Cryptology & Steganography The Paradigms of Cryptography Confidentiality –the content of a message remains secret –information should’n leak to third party Data integrity –to avoid any malicious data manipulation insertion, deletion, substitution Authentication –identification of the author –signature authentication, access control, etc.

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An Introduction to Cryptology & Steganography Other Goals of Cryptography Authorisation –confirmation about data origin Non-repudiation –nobody can deny previous action Practical notions –Anonymity, electronic payment, electronic votes, zero-knowledge protocol,…

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An Introduction to Cryptology & Steganography Cryptographic methods Transposition –change the position of characters –Characters remain the same Substitution –position of characters remain the same –characters in the message are changed

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An Introduction to Cryptology & Steganography Transposition The position of letters is changed Example: IWSAYNMNAERG – TAMNADAYYAAO IAIGOBTEE – NKNDMYHSA TAAADNHRLVDHMOMYNW – HTMIETEEIEWOYUAKO BTEAEFNAELE – YHNMOANBLE Solution: It was many and many a year ago In a kingdom by the sea That a maiden there lived whom you may know By the name of Annabel Lee

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An Introduction to Cryptology & Steganography Scytale (Sparta) First military cipher in history Leather tape wound on a pole of given thickness

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An Introduction to Cryptology & Steganography Substitution The letters are changed Codes –binary code –Morse code Ciphers –Alphabet shifting (Caesar cipher) –Polyalfabetic substitution (Vigenère cipher)

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An Introduction to Cryptology & Steganography Denotion of Communication Parties A – Alice – sender of the message B – Bob – recipient of the message E – Eve – eavesdropper – tries to break the message E – encoding/enciphering algorithm D – decoding/deciphering algorithm M – message (plaintext) K – key C – ciphertext C = E(M, K); M = D(C, K)

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An Introduction to Cryptology & Steganography General encryption process Sender applies encryption algorithm to a plain text S/he gains a cipher text, sends it to the recipient Recipient applies decryption algorithm to the cipher text S/he gains the plain text again

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An Introduction to Cryptology & Steganography Division of Cryptography Symmetric cryptography –both sender and recipient have the same key –deciphering is an inversion of enciphering Asymmetric cryptography –sender and recipient have different keys –mathematic relation –algorithms are generally different –useful for both encryption and digital signature

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An Introduction to Cryptology & Steganography Modular arithmetics Arithmetics on a cyclic set 2 + 3 = 5 (mod 7) 5 + 4 = 2 (mod 7) 5 · 4 = 6 (mod 7) –because 20/7 = 2, remainder 6 11 · 9 = 1 (mod 7) –because 99/7 = 14, remainder 1 3 5 = 5 (mod7)

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An Introduction to Cryptology & Steganography XOR operation eXclusive OR Logical OR, only one of two given expression can be true –0 0 = 0 –0 1 = 1 –1 0 = 1 –1 1 = 0 Sum modulo 2 Simple enciphering and deciphering C = M K, M = C K

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An Introduction to Cryptology & Steganography Kerckhoffs’ principle Basic cryptographic principle Dutch lingvist Auguste Kerckhoffs von Nieuwenhoff (1883) “A cryptosystem is secure even if everything about the system, except the key, is public knowledge” Security shouldn’t depend on the secrecy of algorithm, but on the secrecy of the key

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An Introduction to Cryptology & Steganography Caesar Cipher Alphabet shifted by 3 abcdefghijklmnopqrstuvwxyz DEFGHIJKLMNOPQRSTUVWXYZABC Example –veni, vidi, vici YHQL, YLGL, YLFL Algorithm: alphabet shift Key: by how many letters –25 possible keys (English)

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An Introduction to Cryptology & Steganography Classical Ciphers Monoalphabetic substitution cipher abcdefghijklmnopqrstuvwxyz JULISCAERTVWXYZBDFGHKMNOPQ Kryptanalysis via frequency analysis –found by arabic theologists Zero letters, code words, homomorfic substitution Vigenère (polyalphabetic) cipher –Key length by Kasiski examination –Frequency analysis Vernam cipher (one-time pad) –Theoretically unbreakable

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An Introduction to Cryptology & Steganography Why Was a Computer Invented? New inventions mostly come of –human laziness –wars First computer –1943 Colossus –Great Britain, Bletchley Park –Breaking German ENIGMA code

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An Introduction to Cryptology & Steganography

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Breaking the Enigma Poland – Marian Rejewski –codebooks for day key inference from repeated message key –mechanical decipherer – “bomb” Alan Turing (1912 – 1954) –Inventor of Turing machine, founder of the theory of computation –Analysed plenty of messages given structure (weather info) –New type of “bomb” guessing the key from ciphertext and supposed plaintext

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An Introduction to Cryptology & Steganography

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Computers in Cryptology Breaking ciphers = trying huge amount of possibilities –computer does this in quite short time –the end of “classical” ciphers One-way functions –computation of every input in polynomial time –computation of inverse in exponential time –P != NP problem

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An Introduction to Cryptology & Steganography Symmetric encryption algorithms DES, 3DES, AES, IDEA –Block ciphers –Many rounds consisting of transpositions, permutations, substitutions, XOR with key, etc. Security depends on the key length –Let’s consider 128 bit key –2 128 possible values –1GHz processor: 2 30 operations per second –Breaking time: 2 98 seconds –The age of the Universe: 2 60 seconds –1 more bit => breaking time doubles Problem: How to distribute the key?

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An Introduction to Cryptology & Steganography Data Encryption Standard Block symmetric cipher 1973 – 1974 Horst Fiestel 16 rounds, Fiestel funciton –expansion, XOR, substitution, permutation Better methods than brute force attack are known 3DES –good for the present

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An Introduction to Cryptology & Steganography Asymmetric Cryptography: RSA A pair of keys is needed –Public key –Private key Try to –count 13*37 –factorize 527 Security relies on infeasible factorization of p*q (p,q are primes) –The secret key is derived from a product

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An Introduction to Cryptology & Steganography Using RSA for Encryption and Digital Signature Using RSA for encryption –Sender encrypts the message with receiver’s public key (everyone can do this) –Only receiver is able to decrypt the message (s/he is the only one having private key) Using RSA for digital signature –Author encrypts the message (hash) with his own private key (only he can do this) –Anybody can examine his/her authorship by decrypting the message by author’s public key Combination (encryption and signature) –Sender encrypts the message both with receiver’s public key and his own private key –Only receiver can decrypt the message and examine authorship

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An Introduction to Cryptology & Steganography Digital signature scheme

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An Introduction to Cryptology & Steganography Verification of the Digital Signature

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An Introduction to Cryptology & Steganography Bypassing cryptography Cryptanalysis stands behind cryptography “Unbreakable” ciphers are known –Meant unbreakable in reasonable time Electromagnetic tapping –Messages are captured before encryption –Tapping can be shielded; In USA special permission from FBI is required Viruses, Trojan horses

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An Introduction to Cryptology & Steganography Steganography Hiding the existence of the message Hiding messages to almost all file types is possible –Images, Music, Video, Executables, Text, … Source: Wikipedia

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An Introduction to Cryptology & Steganography Why to use Steganography Cryptography may be illegal in some countries We want to hide a message itself –Security by obscurity Enciphered message arouse suspicion Protection of communication parties –Not only of a message

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An Introduction to Cryptology & Steganography Steganography in Antiquity and in the Middle Ages The battle of Salamis (480 BC) –message hidden under wax on empty tables The revolt against Persians –The message tattooed to the shaved head of a slave, hidden by regrown hair China –message written on silk in a wax bullet, messenger swallowed the bullet… Italy –Secret inks

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An Introduction to Cryptology & Steganography Steganography in the WW2 Hiding messages into toys, dolls, flowers… –Sending these things was later banned in UK and USA Microdots –Text reduced onto size of 1mm –Pasted instead of period Text covered by post stamp

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An Introduction to Cryptology & Steganography Second letters „Apparently neutral's protest is thoroughly discounted and ignored. Isman hard hit. Blockade issue affects pretext for embargo on by-products, ejecting suets and vegetable oils.“ Used by German spy PERSHING SAILS FROM NY JUNE 1

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An Introduction to Cryptology & Steganography Digital Steganography Hiding messages into data files Terminology: –Plaintext – message to be hid Text or a file Enciphered plaintext (ciphertext) –Covertext – carrier of a message – file we hide in –Stegotext – carrier containing a message

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An Introduction to Cryptology & Steganography Usage of Steganography WTC attacks on Sep 11 2001 –Al Quaeda – maps and instruction hidden in images on sport chats and porn sites Software watermarks –Microsoft’s patent – information about license is hidden into executable Printer steganography –Yellow tracking dots Intelligence services –Communication with illegal agents

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An Introduction to Cryptology & Steganography Steganographic methods for text Any message is a sequence of 0 and 1 –We need to encode 2 different symbols/states Plain text –White characters at the end of line –Similar characters (I-1, O-0, etc.) Formatted text –Similar fonts –Move of lines (by 1/300 inch)

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An Introduction to Cryptology & Steganography Steganographic methods for images The most often transferred multimedia –Attracts less attention LSB = Least Significant Bit –Serves for carrying the information –Image data is changed slightly –Human eye cannot recognize any change Color palette –Order of colors encodes message JPEG matrix –Less significant positions in hue matrix

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An Introduction to Cryptology & Steganography Steganographic methods for audio LSB (same as images) Parity coding –Uses parity bit as a carrier Phase movement (WAV) –DCT decomposes signal to (co)sines –Phase movement encodes message –Composition of sines to “original” sound –Totally different data Encoding to noise Echo coding

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An Introduction to Cryptology & Steganography Steganography in executables Executable = sequence of instructions Interchangable instructions –A := 3; B := 5; S := A + B; Positive / negative conditions Content of unused variables Jump over part with message

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An Introduction to Cryptology & Steganography Steganalysis Trying known methods Comparing with known originals Statistical analysis –Data from the same source Noise analysis Data may be changed before hiding –Compression, adding prefixes/suffixes

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An Introduction to Cryptology & Steganography Steganographic software www.stegoarchive.com www.spammimic.com JPHS (JPEG Hide and Seek)

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An Introduction to Cryptology & Steganography Sources Literature –Simon Singh: The Code Book –David Kahn: The Codebreakers –Serge Vaudenay: A Classical Introduction to Cryptography: Applications for Communications Security Internet –computer.howstuffworks.com/computer-internet- security-channel.htm –en.wikipedia.org/wiki/Category:Computer_securi ty –www.stegoarchive.com

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An Introduction to Cryptology & Steganography The end Thank you for your attention Questions?

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