Transcript Steganografie

Faculty
of Business
and Economics
Tomáš Foltýnek
[email protected]
Steganography. CyberWars
Tomas Foltynek
Department of Informatics
Faculty of Business and Economics
Mendel University in Brno
Czech republic
Steganography
What is steganography
• Part of cryptology
• Art/science about hiding the very existence of
message
• Word origin from Greek
– Stegos = hidden
– Graphein = write
• Hidden message doesn’t attract attention
– No need to encrypt
– Combination of steganography and cryptology
ensures discreetness and security
Steganography
Using steganograpghy
• In countries, where cryptography is
illegal
• When we want to hide the existence of
message
• Secret services – monitoring people
– printer tracking dots
• Private companies – copyright protection
– WoW – hidden information in screenshots
Steganography
Steganography v antiquity
• Salamis battle (480 B.C.)
– Persians were about to attack Greece
– Greek Damaratus hid a message under wax on empty
tables
– Greeks won
• Mesage in hair (described by Herodotos)
– Histiaios wanted to encourage Aristagor of Milet to revolt
against Persians
– Shaved messenger‘s hair, tattooed a message, waited
until hair grew backg, then sent the messenger
• Romans
– Secret inks based on fruit juice or milk
Steganography
Steganography in middle ages
• Ancient China
– message on silk in wax bullet
– messenger swallowed
• Giovanni Porta (16. century)
– special ink
– write message to egg, boil
– message penetrates the shell to
eggwhite
Steganography
Boer war
• Lord Robert Baden-Powell
– founder of scout movement
• Needed to draw a plan of boerean artillery
configuration
• For the case of capture, plan had to be
discrete
• Drawed a meadow with butterflies
• Butterfles encoded artillery objects
Steganography
Twentieth century
• WW2
– microdots
– null messages
• messages without real meaning
• carry just hidden message
• messages in radio, etc.
– Common paranoia led to ban of sending
newspaper clippings, flowers and childrens‘
drawings
• Digital steganography
– new opportunities
Steganography
N O T I C E
Upper people try catching
star kites. Do Indians ask at
far trains? Attach asterisk
to any of error file. Add
last byte.
Steganography
N O T I C E
Upper people try catching
star kites. Do Indians ask at
far trains? Attach asterisk
to any of error file. Add
last byte.
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
Steganography
Digital Steganography
• Any data can serve as a carrier
• Human senses mustn’t notice a message
•
•
•
•
•
Hiding to text
Hiding to images
Hiding to audio files
Hiding to video files
Hiding to executables
Steganography
Hiding to formatted text
• Using different fonts
• Bacon cipher
A = AAAAA
N = ABBAA
B = AAAAB
O = ABBAB
C = AAABA
P = ABBBA
– Francis Bacon (1561-1626)
D = AAABB
Q = ABBBB
– Carrier 5 times longer than
a message
E = AABAA
R = BAAAA
F = AABAB
S = BAAAB
– Carrier written by two fonts
G = AABBA
T = BAABA
– SOME TWENTY FIVE H = AABBB U + V = BAABB
I + J = ABAAA
W = BABAA
LETTERS HERE
K = ABAAB
X = BABAB
L = ABABA
Y = BABBA
M = ABABB
Z = BABBB
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
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.
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,…
Steganography
Where to use cryptography?
•
•
•
•
•
•
Internet banking
Phone calls
Paid TV
Multi-user OS
Business
Communication with the government
• Love letters 
• Quizzes, games, etc.
Steganography
Cryptographic methods
• Transposition
– change the position of letters
– letters remain the same
• Substitution
– position of letters remain the same
– letters in the message are changed
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
Steganography
Scytale (Sparta)
• First military cipher in history
• Leather tape wound on a pole of given
thickness
Steganography
Substitution
• The letters are changed
• Codes
– binary code
– Morse code
• Ciphers
– Alphabet shifting (Caesar cipher)
– Polyalfabetic substitution (Vigenère
cipher)
Steganography
Let’s play a game…
• Make groups of three
– Alice
– Bob
– Eve (sitting between Alice and Bob)
• First round
–
–
–
–
–
Eve shuts her ears
Alice and Bob agree on the way of coding
Eve can hear from now on
Alice sends a message to Bob
Eve tries to understand this message
• Second round
– Eve can hear all the communication from the beginning
– Alice and Bob agree on the way of coding (Eve hears them)
– Bob has to send a message secretly to Alice
Steganography
General encryption process
• Sender applies encryption algorithm to a plain text
• S/he gains a cipher text, sends it to the receiver
• Recipient applies decryption algorithm to the
cipher text
• S/he gains the plain text again
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
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
• 35 = 5 (mod7)
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
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
Steganography
Cryptology in Antiquity
• Hebrew scholars (600 to 500 BC)
– Atbash cipher
• The battle of Salamis (480 BC)
– message hidden under wax on empty tables
• The revolat against Persians
– The message tattooed to the shaved head of a
slave, hiddeb by regrown hair
• China
– message writen on silk in a wax bullet,
messenger swallowed the bullet…
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)
Steganography
Improvements of Caesar Cipher
• Unsorted cipher alphabet
abcdefghijklmnopqrstuvwxyz
JULISCAERTVWXYZBDFGHKMNOPQ
• More than 41010 possibilities
• Monoalphabetic substitution cipher
• Kryptanalysis via frequency analysis
– found by arabic theologists
Steganography
Monoalphabetic cipher improvements
• Zero letters
– no meaning, change frequency
• Code words
• Homophonic substitution cipher
– each letter has more representations
according to its frequency
– polygram frequency analysis
Steganography
Vigenère cipher
• Polyalphabetic substitution
cipher
• 1586 Blaise de Vigenère
• Enciphering:
– Key
– Plain text
– Cipher text
WHITEWHITEWHITEWHITEWHI
diverttroopstoeastridge
ZPDXVPAZHSLZBHIWZBKMZNM
• Usage of tabula recta
– sum mod 26
• Unbroken for 300 years
Steganography
Breaking Vigenère cipher
• Charles Babbage (1791 – 1891)
– Inventor of Difference Engines
– Ciphers as a hobby
• Kasiski examintaion – guessing key length
KINGKINGKINGKINGKINGKING
thesunandthemaninthemoon
DPRYEVNTNBUKWIAOXBUKWWBT
• Guessing the key
– divide message to groups enciphered by the
same letter
– shifted alphabet – frequency analysis
Steganography
The unbreakable cipher
• Problem of Vigenère cipher: repeating
– we need a sequence of random letters
– same length as the message
• One time pad cipher
– Gilbert Vernam (1890 – 1960)
– unbreakability proved by C. Shannon
– key distribution problem, practically
useless
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
Steganography
Steganography
Steganography
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
Steganography
Steganography
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
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
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?
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
Steganography
Advanced Encryption Standard
• Block symmetric cipher
• 4 steps:
–
–
–
–
AddRoundKey
SubByte
ShiftRows
MixColumns
• NSA top secret
Steganography
Key exchange algorithm
• 1976 Diffie, Hellman, Merkle
• One-way function Yx (mod P)
– if we know the result, Y and P, it‘s infeasible to compute x
• How to generate a common value
– Alice and Bob agree on Y and P
• via untrusted channel => Y and P are publicly known
– Each of them has his/her own x
• denoted A for Alice and B for Bob
–
–
–
–
Alice counts α = YA (mod P), Bob counts β = YB (mod P)
Alice and Bob exchange α and β
Alice counts kA = βA (mod P), Bob counts kB = αB (mod P)
Since kB = kA, both of them know the value of the key
Steganography
Asymmetric cryptography: RSA
• A pair of keys is needed
• How to generate a keypair
–
–
–
–
–
choose two distinct prime numbers p,q
compute n = p·q
compute φ(n) = φ(p)·φ(q) = (p-1)·(q-1)
choose an integer e (1<e<φ(n); GCD(e,φ(n)) = 1)
determine an integer d such that d·e  1 (mod φ(n))
• The public key is the pair (n,e)
• The private key is the pair (n,d)
• It’s impossible to determine one key from another
without knowing p,q
• Try to
– count 13*37
– factorize 527
Steganography
RSA enciphering and deciphering
• Enciphering
– c = me mod n
• Deciphering
– m = cd mod n
• Proof of correctness
– cd  (me)d  me·d (mod n)
– Because e·d  1 (mod p-1) and e·d  1 (mod q1)
– Then e·d  m (mod p-1) and e·d  m (mod q-1)
– Therefore med  m1 (mod p·q) ... Euler‘s theorem
– And finally cd  m (mod n)
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
Steganography
Digital signature scheme
Steganography
Verification of the Digital Signature
Steganography
Public key certification I.
• Let’s imagine Alice wants to send a secret and signed
message to Bob
• Eve stands between them and controls the whole
communication
• Eve substitutes Alice’s public key with hers
– Bob has Eve’s public key considering it as Alice’s
• Eve substitutes Bob’s public key with her (another) key
– Alice has Eve’s public key considering it as Bob’s
• Neither Alice nor Bob know the real owner of the key
• Eve can then control and change the whole
communication considered to be secret.
Steganography
Public key certification II.
• Solution: Public key certification
• Certification authority (CA) verifies key
owner’s identity
• Certification = digitally signed message
saying “This key belongs to Alice”
• We need to trust the certification authority
• CAs are certified by the government
• CAs watch their confidentiality because of
business
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
Steganography
Steganography
• Hiding the existence of the message
• Hiding messages to almost all file types is possible
– Images, Music, Video, Executables, Text, …
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-internetsecurity-channel.htm
– en.wikipedia.org/wiki/Category:Computer_security
– www.stegoarchive.com
– Google
Steganography
The end
• Thank you for your attention
• Questions?