Cryptography & Steganography

Cryptography &
Steganography
Presented By
Animesh Shaw(Psycho_Coder)
Digital Evidence Analyst Trainee

Discussion Goals
• Cryptography – Definition.
• History of Cryptography.
• Basic Terminologies.
• Importance of Cryptography.
• Types of Cryptography.
• Cryptography Algorithms.
• RSA (Public Key)
• DES (Symmetric)
• Hash Functions
• Digital Signatures
• Watermarking

Cryptography
Definition, History, Types

What is Cryptography ?
“Cryptography is an art of Secret writing”
Or
“Cryptography -- from the Greek for “secret writing” (Kryptos
means ‘HIdden’, graphein means ‘writing’) -- is the mathematical
“scrambling” of data into unreadable form to preserve
confidentiality. ”
Or
“Cryptography is the process of converting plaintext into
ciphertext”

Friends & Foes : Juvia, Gray,
Lyon
• Juvia and Gray wants to communicate securely.
• Lyon (Intruder) may intercept and tamper the communication.
secure
sender
secure
receiver
Channel
Data, control
messages
Data
Lyon
Juvia Gray

Base Cryptography Mechanism
Encryption
Algorithmplaintext
Ciphertext
plaintext
Lyon
Juvia
Gray
Decryption
Algorithm
Key A Key B

Confidentiality: only sender, intended receiver should
“understand” message contents.
sender encrypts message.
receiver decrypts message.
End-Point Authentication: sender, receiver want to confirm
identity of each other.
Message Integrity: sender, receiver want to ensure message
not altered (in transit, or afterwards) without detection.
..contd.

History of Crytography
• There are three eras in the history of Cryptography:
• The Manual era
• The Mechanical era
• The Modern era
• Manual era refers to Pen and Paper Cryptography and dates
back to 2000 B.C.eg : Scytale, Atbash , Caesar, Vigenere.
• Mechanical era refers to the invention of cipher machines.
E.g.: Japanese Red and Purple Machines , German Enigma.
• The modern era of cryptography refers to computers.
• There are infinite permutations of cryptography available
using computers. E.g.: Lucifer, Rijndael , RSA , ElGamal.
9

BASIC TERMINOLOGIES
Some must known Terms

Basic Terms
Cipher: the algorithm that does the encryption.
Ciphertext: the encrypted (scrambled) version of the message.
Message altered to be unreadable by anyone except the
intended recipients.
Cryptanalysis: the science of breaking cryptographic
algorithms.
Cryptanalyst: a person who breaks cryptographic codes; also
referred to as “the attacker”.

..contd.
Cryptosystem – The combination of algorithm, key, and key
management functions used to perform cryptographic
operations.
Decryption: the process of converting ciphertext back to the
original plaintext.
Encryption: scrambling a message or data using a specialized
cryptographic algorithm.
Initialization Vector – Random values used with ciphers to
ensure no patterns are created during encryption. 5

..contd.
 Key – Sequence that controls the operation and behavior of
the cryptographic algorithm.
 Keyspace – Total number of possible values of keys in a
crypto algorithm.
 Plaintext – A message in its natural format readable by an
attacker.
13

Cryptosystem Services or
Security Goals
Authentication
• Ensures that whoever supplies or accesses sensitive
data is an authorized party.
Confidentiality
• Assures that only authorized parties are able to
understand the data.

..contd.
Integrity
• Ensures that when a message is sent over a network,
the message that arrives is the same as the message
that was originally sent.
Nonrepudiation
• Ensuring that the intended recipient actually received
the message & ensuring that the sender actually sent
the message.

Need for Cryptography
• Establishing a Secure communication.
• Fulfil the security goals.
• Preservation of Authentic information.
• Secure Transaction.
• Privacy.

Attributes of Strong Encryption
• Confusion
• Change key values each round
• Performed through substitution
• Complicates plaintext/key relationship
• Diffusion
• Change location of plaintext in ciphertext
• Done through transposition
18

Encryption Systems
• Substitution Cipher
• Convert one letter to another
• Cryptoquip
• Transposition Cipher
• Change position of letter in text
• Word Jumble
• Monoalphabetic Cipher
• Caesar
20

Encryption Systems
• Polyalphabetic Cipher
• Vigenère
• Modular Mathematics
• Running Key Cipher
• One-time Pads
• Randomly generated keys
21

Categories of Cryptography
Traditional
• Cryptography in its early days
• Ex :- Caesar Cipher, Playfair, Rain fence
Symmetric
• Shared Key
• Ex :- DES, AES etc.
Public Key
• Public and Private Key.
• Ex : - RSA, ElGamal etc.

Traditional
Cryptography
Cryptography in its early stages.

Caesar Cipher
• Caesar cipher is named after the Roman military and political
leader Gaius Julius Caesar (100 BC – 44 BC).1 Caesar used this
relatively simple form of ciphering to encode military
messages.
• Every character C in the message M is replaced by (C+3)
Scheme
A B C D E …..
D E F G H …..
Example :-
Plaintext = “DAD”
Ciphertext = “GDG”

Rail Fence Cipher
• Plaintext is written in successive ‘rails’ diagonally.
• No. of rails is predefined, say 3.
• After the message exhausts on rails the message is read row-
wise and it becomes the cipher text.
For example, if we have 3 "rails" and a message of 'WE ARE
DISCOVERED. FLEE AT ONCE', the cipher writes out:
W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D .S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .
Cipher Text : WECRL TEERD SOEEF EAOCA IVDEN

Kamasutra Cipher
• The Kamasutra cipher is one of the earliest known substitution
methods.
• The purpose was to teach women how to hide secret
messages from prying eyes.
Principle
The key is the permutation of the alphabet. The plaintext and
the ciphertext alphabet are the same. The alphabet is divided in
two halves to pair the letters:
F Y M Q G V O P D J R A K
C I E U B X T S Z W N L H
The letter “F” becomes the letter “C” and “B” is replaced by “G”.
The word "EXAMPLE" would be encoded by: "MVLESAM".

Enigma
• Used by the Germans
during World War II
• Replaced letters as
they were typed
• Substitutions were
computed using a key
and a set of switches
or rotors.
27

Symmetric Key Scheme
• Same key for encryption and decryption
• Key distribution problem
• Cleartext with Key makes Ciphertext
• Ciphertext with Key makes Cleartext
29
Winning Lotto #s: aWDHOP#@-w9
aWDHOP#@-w9 Winning Lotto #s:

..contd.
• Advantages
• Symmetric algorithms are fast
• They are difficult to break if a large key size is
used
• Only one key needed
30

..contd.
• Disadvantages
• Symmetric keys must remain secret
• Difficult to deliver keys (key distribution)
• Symmetric algorithms don’t support
authenticity or nonrepudiation
• You can’t know for sure who sent the message,
since two people have the same key
31

Symmetric Cryptography
Algorithms
• Types of symmetric algorithms
• Stream ciphers
• Operate on plaintext one bit at a time
• Block ciphers
• Operate on blocks of plaintext
32

Symmetric Algorithms
• DES
• Modes: ECB, CBC, CFB, OFB, CM
• 3DES
• AES
• IDEA
• Blowfish
33

..contd.
• RC4
• RC5
• CAST
• SAFER
• Twofish
34

Key Distribution
• Symmetric schemes require both parties to share a
common secret key
• issue is how to securely distribute this key
• often secure system failure due to a break in the key
distribution scheme

Key Distribution methods
• Given parties A and B have various key distribution
alternatives:
1. A can select key and physically deliver to B
2. third party can select & physically deliver key to A &
B
3. if A & B have communicated previously can use
previous key to encrypt a new key
4. if A & B have secure communications with a third
party C, C can relay key between A & B
Not suitable
for large
systems
Initial
distribution?

Scale of key distribution problem
• A network with N
hosts => N(N-1)/2
pairs
• Node-level
encryption N(N-1)/2
• Application-level
encryption
• 10 applications/node

Key distribution center (KDC)
Key distribution
center (KDC)
KDC shares a unique key (master key) with each user to distribute
secret key (session key) between a pair of users:
scale of key distribution problem reduces to N
EMK1 (Secret key)
Secret key Secret key
EMK2 (Secret key)

Key Distribution Scenario
nonce: an identifier
that differs for each request
Session key Identifier for A (ex. address)
Master key Ka Master key Kb
(avoid replay attack)
1. Verify the original request
2. Avoid replay attack

Hierarchical key control
…
KDC
…
KDC
KDC
a b

Design Principles of
DES
To achieve high degree of diffusion and confusion.
Diffusion: making each plaintext bit affect as many ciphertext bits as possible.
Confusion: making the relationship between the encryption key and the ciphertext as
complex as possible.
1

DES: The Data Encryption Standard
• Most widely used block cipher in the world.
• Adopted by NIST in 1977.
• Based on the Feistel cipher structure with 16
rounds of processing.
• Block = 64 bits
• Key = 56 bits
• What is specific to DES is the design of the F
function and how round keys are derived from
the main key. 42

Initial Permutation IP
• IP: the first step of the encryption.
• It reorders the input data bits.
• The last step of encryption is the inverse of IP.

Round Keys Generation
• Main key: 64 bits.
• 56-bits are selected and permuted using Permuted
Choice One (PC1); and then divided into two 28-bit
halves.
• In each round:
• Left-rotate each half separately by either 1 or 2 bits according to
a rotation schedule.
• Select 24-bits from each half, and permute the combined 48 bits.
• This forms a round key.

46
Permuted Choice One (PC1)
57 49 41 33 25 17 9
1 58 50 42 34 26 18
10 2 59 51 43 35 27
19 11 3 60 52 44 36
63 55 47 39 31 23 15
7 62 54 46 38 30 22
14 6 61 53 45 37 29
21 13 5 28 20 12 4

Round i
+
F
Li-1 Ri-1
ki
Li Ri
32
48
3232

49
  
The and each have 32 bits, and the round key 48 bits.
The function, on input and , produces 32 bits:
( , )
where :
(
expands 32 bits o 4
)
t
The function of DES
L R K
F R K
F R K P S E K
E
R
F



8 bits;
: shrinks it back to 32 bits;
: permutes the 32 bits.
S
P

Public-Key Cryptography
• probably most significant advance in the 3000
year history of cryptography
• uses two keys – a public key and a private key
• asymmetric since parties are not equal
• uses clever application of number theory
concepts to function
• complements rather than replaces private key
cryptography

..contd.
• public-key/two-key/asymmetric cryptography
involves the use of two keys:
• a public-key, which may be known by anybody, and
can be used to encrypt messages, and verify
signatures
• a private-key, known only to the recipient, used to
decrypt messages, and sign (create) signatures
• is asymmetric because
• those who encrypt messages or verify signatures
cannot decrypt messages or create signatures

Comparison between symmetric and
public-key encryption

Requirement for public-key
cryptography
• Diffie and Hellman (1976) proposed the system
without the algorithm for E and D. They laid out
the requirement:
• It is computationally easy to generate a pair of keys
• It is computationally easy for a sender to encrypt
• It is computationally easy for a receiver to decrypt
• It is computationally infeasible for an opponent,
knowing the public key, to determine the private key
• It is computationally infeasible for an opponent,
knowing the public key and ciphtertext, to recover the
plaintext
Y = EKU (X)b
X = DKR (Y)
b

• developed to address two key issues:
• key distribution – how to have secure communications
in general without having to trust a KDC with your key
• digital signatures – how to verify a message comes
intact from the claimed sender
• public invention due to Whitfield Diffie & Martin
Hellman at Stanford U. in 1976
• known earlier in classified community
Why Public-Key Cryptography?

Asymmetric Algorithms
• Diffie-Hellman
• RSA
• El Gamal
• Elliptic Curve Cryptography (ECC)
57

Public-Key Applications
• can classify uses into 3 categories:
• encryption/decryption (provide secrecy)
• digital signatures (provide authentication)
• key exchange (of session keys)
• some algorithms are suitable for all uses, others
are specific to one

Security of Public Key Schemes
• like private key schemes brute force exhaustive
search attack is always theoretically possible
• but keys used are too large (>512bits)
• security relies on a large enough difference in
difficulty between easy (en/decrypt) and hard
(cryptanalyse) problems
• more generally the hard problem is known, its
just made too hard to do in practise
• requires the use of very large numbers
• hence is slow compared to private key schemes

RSA
• by Rivest, Shamir & Adleman of MIT in 1977
• best known & widely used public-key scheme
• based on exponentiation in a finite (Galois) field
over integers modulo a prime
• nb. exponentiation takes O((log n)3) operations (easy)
• uses large integers (eg. 1024 bits)
• security due to cost of factoring large numbers
• nb. factorization takes O(e log n log log n) operations
(hard)

RSA Key Setup
• each user generates a public/private key pair by:
• selecting two large primes at random - p, q
• computing their system modulus N=p.q
• note ø(N)=(p-1)(q-1)
• selecting at random the encryption key e
• where 1<e<ø(N), gcd(e,ø(N))=1
• solve following equation to find decryption key d
• e.d=1 mod ø(N) and 0≤d≤N
• publish their public encryption key: KU={e,N}
• keep secret private decryption key: KR={d,p,q}

RSA Use
• to encrypt a message M the sender:
• obtains public key of recipient KU={e,N}
• computes: C=Me mod N, where 0≤M<N
• to decrypt the ciphertext C the owner:
• uses their private key KR={d,p,q}
• computes: M=Cd mod N
• note that the message M must be smaller than the
modulus N (block if needed)

Why RSA Works
• because of Euler's Theorem:
• aø(n)mod N = 1
• where gcd(a,N)=1
• in RSA have:
• N=p.q
• ø(N)=(p-1)(q-1)
• carefully chosen e & d to be inverses mod ø(N)
• hence e.d=1+k.ø(N) for some k
• hence :
Cd = (Me)d = M1+k.ø(N) = M1.(Mø(N))q = M1.(1)q =
M1 = M mod N

RSA Example
1. Select primes: p=17 & q=11
2. Compute n = pq =17×11=187
3. Compute ø(n)=(p–1)(q-
1)=16×10=160
4. Select e : gcd(e,160)=1; choose e=7
5. Determine d: de=1 mod 160 and d < 160
Value is d=23 since 23×7=161= 10×160+1
6. Publish public key KU={7,187}
7. Keep secret private key KR={23,17,11}

RSA Example cont
• sample RSA encryption/decryption is:
• given message M = 88 (nb. 88<187)
• encryption:
C = 887 mod 187 = 11
• decryption:
M = 1123 mod 187 = 88

Exponentiation
• can use the Square and Multiply Algorithm
• a fast, efficient algorithm for exponentiation
• concept is based on repeatedly squaring base
• and multiplying in the ones that are needed to
compute the result
• look at binary representation of exponent
• only takes O(log2 n) multiples for number n
• eg. 75 = 74.71 = 3.7 = 10 mod 11
• eg. 3129 = 3128.31 = 5.3 = 4 mod 11

RSA Key Generation
• users of RSA must:
• determine two primes at random - p, q
• select either e or d and compute the other
• primes p,q must not be easily derived from
modulus N=p.q
• means must be sufficiently large
• typically guess and use probabilistic test
• exponents e, d are inverses, so use Inverse
algorithm to compute the other

RSA Security
• three approaches to attacking RSA:
• brute force key search (infeasible given size of numbers)
• mathematical attacks (based on difficulty of computing ø(N), by
factoring modulus N)
• timing attacks (on running of decryption)

• mathematical approach takes 3 forms:
• factor N=p.q, hence find ø(N) and then d
• determine ø(N) directly and find d
• find d directly
• currently believe all equivalent to factoring
• have seen slow improvements over the years
• as of Aug-99 best is 130 decimal digits (512) bit with GNFS
• biggest improvement comes from improved
algorithm
• cf “Quadratic Sieve” to “Generalized Number Field Sieve”
• barring dramatic breakthrough 1024+ bit RSA secure
• ensure p, q of similar size and matching other constraints
Factoring Problem

Hashing Algorithms
• HAVAL
• Computes between 128 and 256 bit hash
• Between 3 and 5 rounds
• RIPEMD-160
• Developed in Europe published in 1996
• Patent-free
72

Digital Signatures
• Digital signatures can be permanently tied to the content
of the message being signed. They cannot then be
'moved' from one document to another, for any attempt
will be detectable.
• RSA and DSA are two of the most popular digital
signature schemes.

..contd.
• In digital signature schemes, there are two algorithms:
one for signing, in which a secret key is used to process
the message and one for verification, in which the
matching public key is used with the message to check
the validity of the signature.

Watermarking
• Traditionally, a watermark has been used to verify the
authenticity of a document.

..contd.
• Driver’s Licenses, diplomas, official letterhead were the
primary applications of watermarks .
• More recently, used to track or prevent redistribution of
TV logos.

..contd.
Purpose of using:
• Ensure authenticity of digital goods.
• Prevent unauthorized use/ensures
copyright.
• Prevent copying.
 Adding the watermark to the image itself prevents
removal by changing the format.
E.g. GIF->JPEG.

Steganography
A Smart way to hide data

Topics To Discuss
1. What is Steganography?
2. History Of Steganography
3. Technique
4. Basic Steganography Model
5. Steganography Terms
6. Types of Stegosystems
7. Types of Steganograph
8. Comparison of various Security techniques
9. Crypto-Steganography – A new approach
10. Applications
11. Comparison of various Secret Communication Techniques.
12. Steganography Tools
13. Future Scope
14. Conclusion
15. References

What is Steganography?
• Steganography is the art and science of writing hidden messages
in such a way that no one, apart from the sender and intended
recipient, suspects the existence of the message, a form of security
through obscurity.
STEGONOGRA
PHY
EXAMPLE
RANDOM TEXT
Since everyone can read,
encoding text
in neutral sentences is
doubtfully effective
SOME HIDDEN
PATTERN
Since Everyone Can Read,
Encoding Text
In Neutral Sentences Is
Doubtfully Effective
ORIGINAL MESSAGE SECRET INSIDE

History Of Steganography
• The first recorded uses of steganography can be traced
back to 440 BC when Herodotus mentions two examples
of steganography in his Histories.
• Ancient Greeks used Wax tablets as reusable writing
surfaces, sometimes used for shorthand.
• Ancient Chinese wrote messages on fine silk, which was
then crunched into a tiny ball and covered in wax.
• Special inks were important steganographic tools even
during Second World War.

Techniques
PHYSICAL TECHNIQUES:
• Hidden messages on paper written in secret inks under other
messages or on the blank parts of other messages.
• Hidden messages within wax tablets.
• Messages written on envelopes in the area covered by postage
stamps.
DIGITAL TECHNIQUES:
• Concealing data within encrypted data or within random data (an
unbreakable cipher like the one-time pad generates cipher texts
that look perfectly random if one does not have the private key).
• Concealed messages in tampered executable files, exploiting
redundancy in the targeted instruction set.
• Pictures embedded in video material (optionally played at slower or
faster speed).

Basic Steganography Model
Secret
Message
Cover
Medium
Stego
Decoder
Key Cover
Estimate
of
Message
Original
Cover
Stego
Encoder
Communicationchannel

Steganography Terms
• Carrier or Cover File - A Original message or a
file in which hidden information will be stored
inside of it .
• Stego-Medium - The medium in which the
information is hidden.
• Embedded or Payload - The information which is
to be hidden or concealed.
• Steganalysis - The process of detecting hidden
information inside a file.

Types Of Stegosystems and
Steganography
STEGOSYSTEM TYPES:
• Pure stegosystems - no key is used.
• Secret-key stegosystems - secret key is used.
• Public-key stegosystems - public key is used.
STEGANOGRAPHY TYPES:
• Text Steganography.
• Image Steganography.
• Audio Steganography.
• Video Steganography.
• Protocol Steganography.

Text Steganography
• Text steganography can be applied in the digital makeup
format such as PDF, digital watermark or information hiding
• It is more difficult to realize the information hiding based on
text. The simplest method of information hiding is to select
the cover first, adopt rules to add the phraseological or
spelling mistakes, or replace with synonymy words.
VARIOUS TEXT STEGANOGRAPHY METHODS:
• Line shifting Method
• Word shifting
• Open spaces
• Semantic methods
• Character Encoding

Examples of Text Steganography
• Minor changes to shapes of characters

Examples of Text Steganography

Image Steganography
• Using image files as hosts for steganographic messages takes advantage of the
limited capabilities of the human visual system
• Some of the more common method for embedding messages in image files can
be categorized into two main groups, image domain methods and transform
domain methods
Image And Transform Domain:
• Image – also known as spatial – domain techniques embed messages in the
intensity of the pixels directly, while for transform – also known as frequency –
domain, images are first transformed and then the message is embedded in the
image
• Image domain techniques encompass bit-wise methods that apply bit insertion
and noise manipulation and are sometimes characterized as “simple systems”
• Steganography in the transform domain involves the manipulation of
algorithms and image transforms

LSB [Least Significant bit]
Method
• Least significant bit (LSB) insertion is a common, simple
approach to embedding information in a cover image
• The least significant bit (in other words, the 8th bit) of some
or all of the bytes inside an image is changed to a bit of the
secret message
• When using a 24-bit image, a bit of each of the red, green and
blue color components can be used, since they are each
represented by a byte. In other words, one can store 3 bits in
each pixel. An 800 × 600 pixel image, can thus store a total
amount of 1,440,000 bits or 180,000 bytes of embedded data
• In its simplest form, LSB makes use of BMP images, since they
use lossless compression

• A grid for 3 pixels of a 24-bit image can be as follows:
(00101101 00011100 11011100)
(10100110 11000100 00001100)
(11010010 10101101 01100011)
• When the number 200, which binary representation is
11001000, is embedded into the least significant bits of this
part of the image, the resulting grid is as follows:
(00101101 00011101 11011100)
(10100110 11000101 00001100)
(11010010 10101100 01100011)

Example Of Image
Steganography

Audio Steganography
• Embedding secret messages into digital sound is known as
audio Steganography.
• Audio Steganography methods can embed messages in WAV,
AU, and even MP3 sound files.
• The properties of the human auditory system (HAS) are
exploited in the process of audio Steganography
• To embed data secretly onto digital audio file there are few
techniques introduced :
• LSB Coding
• Phase Coding
• Parity Coding
• Spread Spectrum

Flowchart Of Audio
Steganography

Example of LSB Method
• The message 'HEY' is encoded in
a 16-bit CD quality sample using
the LSB method.
• Here the secret information is
‘HEY’ and the cover file is audio
file. HEY is to be embedded
inside the audio file. First the
secret information ‘HEY’ and the
audio file are converted into bit
stream.
• The least significant column of
the audio file is replaced by the
bit stream of sectet information
‘HEY’. The resulting file after
embedding secret information
‘HEY’ is called Stego-file.

Comparison of Secret
Communication Techniques
Communica
tion
Technique
Confidenti
ality
Integrity Availability
Cryptograph
y
  
Digital
Signatures
  
Steganograp
hy
  

Combined Crypto-
Steganography
Plain Text
Stego
Image
Cipher
Text
Decrypti
on
Cipher
Text
Plain Text
Encryptio
n
Cover
Image

Applications
• Confidential communication and secret data storing
• Steganography provides us with:
• Potential capability to hide the existence of confidential
data
• Hardness of detecting the hidden (i.e., embedded) data
• Strengthening of the secrecy of the encrypted data
• Protection of data alteration
• Access control system for digital content distribution
• Media Database systems
• Usage in modern printers
• Alleged use by intelligence services

Steganography Tools
• Steganos
• S-Tools (GIF, JPEG)
• StegHide (WAV, BMP)
• Invisible Secrets (JPEG)
• JPHide
• Camouflage
• Hiderman

Future Scope
• Steganography, though is still a fairly new idea.
There are constant advancements in the computer
field, suggesting advancements in the field of
steganography as well.
• It is likely that there will soon be more efficient
and more advanced techniques for Steganalysis.
• What is scary is that such a small file of only one
or two sentences may be all that is needed to
commence a terrorist attack. In the future, it is
hoped that the technique of Steganalysis will
advance such that it will become much easier to
detect even small messages within an image.

Conclusion
• Interest in the use of steganography in our current digital
age can be attributed to both the desire of individuals to
hide communication through a medium rife with
potential listeners, or in the case of digital watermarking,
the absolute necessity of maintaining control over one’s
ownership and the integrity of data as it passes through
this medium. This increased interest is evidenced in the
sheer number of available tools to provide easy
steganographic techniques to the end user, as well as the
proliferation of research and press on the topic.

Cryptography & Steganography

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