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An Introduction to Cryptology & Steganography. Mgr. Tomas Foltynek , Ph.D. Department of Informatics Faculty of Business and Economics Mendel University in Brno Czech republic. Introduction. Secure communication problem is as old as mankind

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an introduction to cryptology steganography

An Introduction to Cryptology & Steganography

Mgr. Tomas Foltynek, Ph.D.

Department of Informatics

Faculty of Business and Economics

Mendel University in Brno

Czech republic

introduction
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?
content
Content
  • What is cryptology
  • Symmetric cryptology
    • Basic principles
  • Asymmetric cryptology
    • Digital signature
  • Steganography
what is cryptology
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
the paradigms of cryptography
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.
other goals of cryptography
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,…
cryptographic methods
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
transposition
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

scytale sparta
Scytale (Sparta)
  • First military cipher in history
  • Leather tape wound on a pole of given thickness
substitution
Substitution
  • The letters are changed
  • Codes
    • binary code
    • Morse code
  • Ciphers
    • Alphabet shifting (Caesar cipher)
    • Polyalfabetic substitution (Vigenère cipher)
denotion of communication p arties
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)
general encryption process
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
division of cryptography
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
modular arithmetics
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
  • 35 = 5 (mod7)
xor operation
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
kerckhoffs princip le
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
caesar cipher
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)
classical ciphers
Classical Ciphers
  • Monoalphabeticsubstitutioncipher

abcdefghijklmnopqrstuvwxyz

JULISCAERTVWXYZBDFGHKMNOPQ

  • Kryptanalysisvia frequencyanalysis
    • found by arabictheologists
  • Zeroletters, codewords, homomorficsubstitution
  • Vigenère (polyalphabetic) cipher
    • Key length by Kasiski examination
    • Frequency analysis
  • Vernam cipher (one-time pad)
    • Theoretically unbreakable
why was a computer i nvented
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
breaking the enigma
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
computers in cryptology
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
symmetric encryption algorithms
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
    • 2128 possible values
    • 1GHz processor: 230 operations per second
    • Breaking time: 298 seconds
    • The age of the Universe: 260 seconds
    • 1 more bit => breaking time doubles
  • Problem: How to distribute the key?
data encryption standard
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 thepresent
asymmetric cryptography rsa
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
using rsa for encryption and digital signature
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
bypassing cryptography
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
steganography
Steganography
  • Hiding the existence of the message
  • Hiding messages to almost all file types is possible
    • Images, Music, Video, Executables, Text, …

Source: Wikipedia

why to use 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
steganography in antiquity and in the middle ages
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
steganography in the ww2
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
second letters
Second letters
  • „Apparentlyneutral's protest isthoroughlydiscounted and ignored. Isman hard hit. Blockadeissueaffects pretext for embargo on by-products, ejectingsuets and vegetableoils.“
  • Used by Germanspy
  • PERSHING SAILS FROM NY JUNE 1
digital 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
usage of 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
steganographic methods for text
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)
steganographic methods for images
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
steganographic methods for audio
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
steganography in executables
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
steganalysis
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
steganographic software
Steganographic software
  • www.stegoarchive.com
  • www.spammimic.com
  • JPHS (JPEG Hide and Seek)
s ources
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_security
    • www.stegoarchive.com
the end
The end
  • Thank you for your attention
  • Questions?