Steganography and
History of Cryptography
Dr. Ron Rymon
Efi Arazi School of Computer Science
IDC, Herzliya. 2010/11
Pre-Requisites: None
1
Overview
 Steganography
 History of Cryptography
 Modern Cryptography
 General Model of Modern Cryptography
2
Steganography
Main source: “The Code Book” / Singh
3
Steganography
 In Greek
– Steganos = covered
– Graphein = to write
 Steganography is about hiding messages
 Historically, secret messages were often hidden (or
memorized)
 Today, steganography is used primarily to protect digital
rights
– “watermarking” copyright notices
– “fingerprinting” a serial ID
4
History of Steganography
(Physically Hiding)
 Runners were memorizing messages
– Sometimes killed after delivering the message
 Demaratus tells Athens of Persia’s attack plans
– Writes the secret message on a tablet, and covers it with wax
 Greek Histaiaeus encouraged Aristagoras of Miletus to
revolt against the Persian King.
– Writes message on the shaved head of the messenger, and sends
him after his hair grew
 Chinese silk balls
– Message is written on silk, turned into wax-covered ball that was
swallowed by the messenger…
 Invisible ink-jet technology
– Ink that is too small for human eye (Univ of Buffalo, 2000)
5
History of Steganography (cont.)
 Invisible Ink
– Certain organic fluids (milk, fruit juice) are transparent when dried but
the deposit can be charred and is then visible
– Romans used to write between the lines
– A mixture of alum and vinegar may be used to write on hardboiled eggs,
so that can only be read once shell is broken
6
History of Steganography (cont.)
 Microdots
– WW2 Germany - documents shrunk to the size
of a dot, and embedded within innocent letters
– DNA microdot, embedding synthetically formed
DNA sequence (secret) into a normal DNA
strand, then posting as microdot
– Inkjet dots, smaller than human eye can see
– Microdots with barcode-like information
 Easter eggs
– Programmers embed in software
• See http://www.eeggs.com
– Claims that Beatles embedded secret messages
in their music
7
Hiding a message within a text
 An actual message from a German spy
– read second letter in each word
“Apparently, neutral’s protest is thoroughly
discounted and ignored. Isman hard hit.
Blockade issue affect pretext for embargo on by
products, ejecting suets and vegetable oils.”
“Pershing Sails from NY June 1”
8
Hiding a message within a text (more)
 Shift some words by one point/pixel.
– Shifted words (or their first letters) make the sentence
 Use different fonts
– Letter by letter or word by word (Francis Bacon
Cipher)
 Lexical steganography uses the redundancy of the
English language
– “I feel well” and “I feel fine” seem the same, but one
may be used to encode “SOS”
 Chaffing and winnowing
– Riddle text with extra parts that the receiver will know
how to remove (e.g., those that don’t “authenticate”)
9
Modern Steganography
 Hiding one message within another (“container”)
 Most containers are rich media
– Images, audio, video are very redundant, can be tweaked without affecting
human eye/ear
– US argued that Bin Laden implanted instructions within taped interviews
 Copyright notices embedded in digital art
– Prove ownership
– Serial number embedded to prevent replication
– Seek infringements on the web using spiders
 Digital cameras EXIF tags
– Not secretive, but hidden from the eye
– Embed info such as camera type, date, shutter speed, focal length,..
 Similarly, possible to embed messages in invisible parts of html pages
10
Hiding a Message in an Image
 Example: use 1-2 Least Significant Bits (LSB) in each pixel
– human eye wont notice the difference
– message can be compressed to reduce number of bits needed
– only half the bits are likely to change on average
– prefer “containers” with a lot of variations
 Message (M1) in an Image
–
Steganography is the art and science of communicating in a way which hides
the existence of the communication. In contrast to cryptography, where the
"enemy" is allowed to detect, intercept and modify messages without being able
to violate certain security premises guaranteed by a cryptosystem, the goal of
steganography is to hide messages inside other "harmless" messages in a way
that does not allow any "enemy" to even detect that there is a second secret
message present [Markus Kuhn 1995-07-03].
 Check out Steganos (www.steganos.com), Digimarc
(www.digimarc.com)
11
Example (Steganos)
Original Picture
Embedded Picture
With embedded picture
JPG version
12
Steganalysis
 Detection: is there a hidden message?
– Develop signatures for known steganographic tools,
e.g. in LSB method, expect local homogeneity
– When content is encrypted, the message should have a
high entropy (“white noise”)
– Promising results: high detection rates
 Decoding: recover hidden message
– No significant work in this area !
 Prevention: destroy or remove a hidden message
– Most steganographies not robust to image alterations
– Short messages (e.g. copyright) can be encoded
redundantly and survive an alternation
13
Steganography (Summary)
 Steganography is arguably weaker than
cryptography because the information is revealed
once the message is intercepted
 On the other hand, an encrypted message that is
not hidden may attract attention, and in some cases
may itself incriminate the messenger
 In any event, steganography can be used in
conjunction with cryptography
14
History of Cryptography
Main source: “The Code Book” / Singh
15
Cryptography
 In Cryptography, the meaning of the message is
hidden, not its existence
– Kryptos = “hidden” in Greek
 Historically, and also today, encryption involves
– transposition of letters
• Sparta’s scytale is first cryptographic device (5th Century BC)
– Message written on a leather strip, which is then unwound to
scramble the message
– substitution
• Hebrew ATBASH (‫)אתבש‬
• Kama-Sutra suggests that women learn to encrypt their love
messages by substituting pre-paired letters (4th Century AD)
• Cipher – replace letters
• Code – replace words
16
Monoalphabetic Ciphers
 Caesar Shift Cipher
– Each letter substituted by shifting n=3 places
• EXAMPLE
• HADP SOH
– Only 25 such ciphers
 Jefferson wheel implementation
– Set the message across the wheels
– Select another line (in random) as cipher
 Substitution based on key phrase
– Substitution key consists of phrase’s letters (uniquely) followed by
rest of the alphabet in order
• Phrase: THIS IS ALICE AND BOB’S KEY
• Key: THISALCENDBOKY-FGJMPQRUVWXZ
– 26! (roughly 1026) monoalphabetic substitution ciphers
17
Breaking Monoalphabetic Ciphers
 The Arabs broke monoalphabetic substitution using
frequency analysis
– In English (Source: Beker & Piper)
a
8.2%
j
0.2
s
6.3
b
1.5
k
0.8
t
9.1
c
2.8
l
4.0
u
2.8
d
4.3
m
2.4
v
1.0
e
12.7
n
6.7
w
2.4
f
2.2
o
7.5
x
0.2
g
2.0
p
1.9
y
2.0
h
6.1
q
0.1
z
0.1
i
7.0
r
6.0
– Thus, letters ciphering e, t, and a are easily discovered
– Subsequently can look for the rest of the letters and letter pairs
18
Homophonic Substitution
 Homophonic substitution cipher can be used to
foil frequency analysis
– Keyed 2-digit substitution
T
H
E
K
A B C D E F G H I
J
K L M N O P Q R S T U V W X Y/Z
06
43
71
90
15
27
55
99
16
28
56
75
07
44
72
91
08
45
73
92
09
46
74
93
10
47
50
94
11
48
51
95
12
49
52
96
13
25
53
97
14
26
54
98
17
29
57
76
18
30
58
77
19
31
59
78
20
32
60
79
21
33
61
80
22
34
62
81
23
35
63
82
24
36
64
83
00
37
65
84
01
38
66
85
02
39
67
86
03
40
68
87
04
41
69
88
05
42
70
89
– Reverse frequency
A B C D E F G H I
J
K L M N O P Q R S T U V W X Y Z
06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 00 01 02 03 04 68 05
43 44 45 46 47 48 49 25 26
29 30 31 32 33
35 36 37 38
40
87
71
73 74 50
53 54
57
59 60
63 64 65 66
90
93 94
97 98
76
78 79
82 83 84
72
51
56 58
61 34
39 86 42
91
95
81 77
80 62
67 88 70
92
52
85
89
75
96
41
27
69
55
99
28
19
Vigenere Polyalphabetic Cipher
 Vigenere’s polyalphabetic cipher (19th century) generalizes
Caesar’s shift cipher
– Use keyword to select encrypting rows
Vigenere Tableau
 The Vigenere cipher is
not amenable to simple
frequency analysis
 Actually invented earlier
(16th century)
 Called “The Unbreakable
Cipher”
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A B C D E F G H I
J K L M N O P Q R S T U V W X Y Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
O
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
P
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
S
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
T
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
U
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
V
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
W
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
X
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
Y
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Z
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
S
R
S
T
U
V
W
X
Y
20
Babbage breaks Vigenere Cipher
 Babbage broke Vigenere’s Cipher (1854, Crimean war)
– Stage 1: Discover key length
• Look for repeated sequences, and measure their distance
• The key length is a factor of these distances
– Stage 2: Identify the key itself
• Compare distributions for each of the key letters with the
standard distribution, to identify the shift
 Babbage could not publish his work
– Similar techniques developed independently by Kasiski
(a Prussian officer); Kerckhoff (French cryptographer)
 Check out an applet that breaks Vigenere:
http://math.ucsd.edu/~crypto/java/EARLYCIPHERS/Vigenere.html
21
Historical Coding
 Louis XIV’s Great Cipher (Rossignols) used one symbol
(3-digit number) per syllable (held 200 years)
 Mary Queen of Scots used a combination of cipher and
coded words
– Referred to as a nomenclature because many codes were for names
 e.g,
 US Army used Navajo language as code in WWII
22
Transposition Ciphers
 Railfence:
T
H
E
K
E
Y
5
3
1
4
2
6
TRHCEEIETGSSMAIAEASS
T
R
H
C
I
E
E
S
 Redfence (by key):
T
S
I
E
G
M
A
A
E
S
S
S
IETGIAESHCEESSMATRSS
 Columnar
– IEEIRSHSMESCSTATGSEA
T H E K E Y
5 3 1 4 2 6
T
A
T
G
H I S I S
S E C R E
M E S S A
E
23
Unbreakable Encryption
 One time pads
– Sender and receiver use a pre-arranged random stream of letters
– Encryption=addition modulo 26
M E S S A G E
• XOR when binary
– Every letter in the key used only once
T H I
S K E Y
F L A K K K C
 One time pads provide for the only perfectly secure
encryption algorithms
– All the rest are only computationally secure
– Used by Soviet spies, and also for US-USSR hotline
 Requires significant logistical effort and coordination
 Relies on randomness of key
24
Summary of Historical Crypto
 Encryption Algorithms and Keys
– Substitution : letters (bits), words
– Transposition
 Decryption Algorithms
– Reversed process
– Require knowledge of the algorithm and the key
 Cryptanalysis
– Identify algorithm
– Obtain as many plaintext-ciphertext pairs
– Use systematicity (patterns)
– Use hints (cribs)
25
Modern Cryptography
Main source: Network Security Essentials / Stallings
26
Kerckhoffs Principles
System + Keys
1.
2.
3.
4.
5.
6.
The system must be substantially, if not mathematically,
undecipherable;
The system must not require secrecy and can be stolen by the
enemy without causing trouble;
It must be easy to communicate and remember the keys without
requiring written notes, it must also be easy to change or modify
the keys with different participants;
The system ought to be compatible with telegraph
communication;
The system must be portable, and its use must not require more
than one person;
Finally, regarding
theKerckhoffs,
circumstances
in of
which
suchScience,
system1883
is
August
Journal
Military
applied, it must be easy to use and must neither require stress of
27
mind nor the knowledge of a long series of rules.
The German Enigma
 Invented as a commercial machine (Scherbius), and
failed
– Electrical typewriter-like encryption machine
– Each keystroke lights a letter
 Performing substitutions
– Letter-pairs are switched
– Pulse goes through scramblers
– Hits reflector and goes back
 Original Enigma (M3) based on commercial version
– Reconfigurable 6 swapped letter-pairs
– 3 rotating scramblers (263 orientations)
– scramblers can be configured in 6 (3!) ways
– Later, up to 5 scramblers to choose from
 Theoretical key space = a total of 1017 combinations
 Used extensively by Germans in WW2
28
Poles Crack the Enigma
 Polish obtained an Enigma from a German spy (1933)
– Hans-Thilo Schmidt sold to French intelligence
 Obtained information on its usage
– daily code book indicated rotors and orientation
– a different orientation key for each message
 Rejewski focused on the repetitions
– Message key encrypted twice in the message header
– Formalized relationships between 1st-4th ,2nd-5th, and 3rd-6th letters
• ABCDEFGHIJKLMNOPQRSTUVWXYZ
• FQHPLWOGBMVRXUYCZITNJEASDK
– Built chains
• (AFW), (BQZKVELRI), (CHGOYDP), (JMXSTNU)
– Chains depend only on scrambler orientation, not on pairs swaps
• Thus need to consider only 3! x 263 = 105456 configurations
– Built a catalog of characteristic chains for all configurations
29
Poles Crack the Enigma
 Rejewski’s algorithm to discover the day key
– First, use catalog to identify the scrambler setting and orientation
– Then, run the ciphertext through an Enigma and look at the text to
identify swapped letter pairs
 Bombe machines were constructed to mechanize the search
30
British Crack Improved Enigma
 In 1939, Germans increased Enigma security
– Navy admiral added 2 extra scramblers to choose from – 10x
arrangements (5 choose 3, times the 3! orderings)
– Hitler used a more complex version – Lorenz Cipher
– increased to 10 letter pair swaps
 British (Bletchley Park) continued where the Polish left
– Recruited best Mathematicians (Turing) and large staff (7000)
– Did not make much progress until received Bombes from Polish
 Used human weaknesses. Provided hints and cribs
–
–
–
–
Trivial message keys (key sequences, names initials)
Artificial restrictions on scramblers selection/orientation
Standard messages (weather) sent with 4th scrambler neutralized
Some German codebooks were captured
31
British Crack Improved Enigma
 Turing built swap-independent chains (a la Rejewski)
– First British Bombe (Victory) delivered in 1940
– Search still required significant human help
 In 1942, Germans add 4th active scrambler (M4)
– Bletchley Park could not decipher M4’s messages for 10 months
 Could only break it when info was captured in u-boats
– Captured machines, rotors, weather manuals, providing cribs
 Later in the war, US Navy also constructed even faster and
more sophisticated bombes
– Japanese used PURPLE, a machine modeled after Enigma
– Pearl Harbor Attack was broken hours before the attack
 The British ULTRA – broken German, Italian and
Japanese communications were crucial to winning the war
32
A General Model of
Cryptography
Main source: Network Security Essentials / Stallings
33
Modern Encryption Principles
 An encryption scheme has 5 ingredients
– Plaintext, Encryption Algorithm, Key, Ciphertext, and
Decryption Algorithm
– Security depends on secrecy of the key, not algorithm
• Recall Kerckhoff
34
Notation
 M, or P will usually denote the plaintext message
 C will usually denote the ciphertext
 K will usually denote a key
 Ek(M)=C is the encryption function
 Dk(C)=M is the decryption function
 Dk(Ek(M))=M represents the typical flow
35
Cryptographic Protocols
 Self enforcing protocols
 Arbitrated protocols
– Trusted third party helps in real time
 Adjudicated protocols
– Trusted third party, but only if needed and after the fact
36
Attacks Against Cryptographic
Protocols (Not the Algorithms)
 Passive attacks (eavesdropping)
– Cryptanalysis
– Traffic analysis
 Active attacks
–
–
–
–
–
Impersonation
Interruption / denial
Modification of messages
Fabrication of new messages
Replay / Reflect messages
 “man-in-the-middle” is a common tactic in active attacks
37
Cryptographic Algorithms Typology
 Type of operations applies to plaintext
– Substitution and transposition
 Type of key(s)
– Symmetric : same key: Dk(Ek(M))=M
– Asymmetric, Public-Key : Dk2(Ek1(M))=M
 How plaintext is processed into ciphertext
– Which operations
– How many operations
– How the operations are combined
– Block ciphers, Stream ciphers
38
Cryptanalysis: Attacks against
Cryptographic Algorithms
 Ciphertext only
– Uses only knowledge of algorithm and ciphertext
 Known plaintext
– Uses one or more plain-ciphertext pairs
– Or, probable words: dictionary, known formats, etc.
 Chosen text
– Chosen to reveal information about the key
– Chosen plaintext and its ciphertext
• Differential chosen plaintext
• Adaptive chosen plaintext
– Chosen ciphertext and its original plaintext
• Mostly against public-keys
39
Computationally Secure
Encryption
 Encryption scheme is computationally secure if
– The cost of breaking the cipher exceeds the value of the encrypted
information; or
– The time required to break the cipher exceeds the useful lifetime of
the information
 Most schemes that we will discuss are not unbreakable in
principle, but are computationally secure
 Rely on lack of knowledge of effective algorithms for
certain hard problems, not on a proven inexistence of ones
– E.g., factorization, discrete logarithms, or square roots mod p
 Rely on very large key-space, impregnable to brute force
40
Shannon’s Theory of Secrecy
 Cryptanalysts try to modify the a priori probabilities of
alternative messages until one emerges
 A cryptographic scheme is perfectly secure if knowledge
of the ciphertext does not change the odds in favor of any
of the possible plaintexts
– i.e., the probability function remains uniform
 Shannon’s Theory: the key must be at least as large as the
message (entropy) and cannot be reused
– Message entropy = minimum number of bits needed to express all
possible messages, e.g., English entropy is 1.3 bits per letter
– Therefore, the secrecy of a cryptographic scheme depends on its
entropy, i.e. the number of key bits, or the size of the key space
 Only the one-time pad achieves perfect secrecy
41
Shannon’s Diffusion and
Confusion Principles
 In the lack of perfect security, a cryptographic algorithm
shall at least try to foil statistical attacks
– E.g., use frequency of plaintext to rule out or substantially change
the odds of possible cipher texts
 Shannon’s Cryptographic Principles:
– Diffusion: every letter of plaintext should affect many letters in
ciphertext
• Sometimes called avalanche effect
– Confusion: the relationship between plaintext and ciphertext shall
be complex
• Many substitutions and transpositions make it difficult to reverse
engineer the relationship
 Shannon’s principles are the cornerstone of block cipher
design
42
Next Classes
 First
– Conventional (Symmetric) Cryptography
 Then
– Public-Key Cryptography
43