Cryptography and
Network Security
Chapter 2
Fourth Edition
by William Stallings
Lecture slides by Lawrie Brown
Chapter 2 – Classical Encryption
Techniques
Many savages at the present day regard their
names as vital parts of themselves, and
therefore take great pains to conceal their real
names, lest these should give to evil-disposed
persons a handle by which to injure their
owners.
—The Golden Bough, Sir James George Frazer
Symmetric Encryption
or conventional / private-key / single-key
sender and recipient share a common key
all classical encryption algorithms are
private-key
was only type prior to invention of public-
key in 1970’s
and by far most widely used
Some Basic Terminology
plaintext - original message
ciphertext - coded message
cipher - algorithm for transforming plaintext to ciphertext
key - info used in cipher known only to sender/receiver
encipher (encrypt) - converting plaintext to ciphertext
decipher (decrypt) - recovering ciphertext from plaintext
cryptography - study of encryption principles/methods
cryptanalysis (codebreaking) - study of principles/
methods of deciphering ciphertext without knowing key
cryptology - field of both cryptography and cryptanalysis
Symmetric Cipher Model
Requirements
two requirements for secure use of
symmetric encryption:
a strong encryption algorithm
a secret key known only to sender / receiver
mathematically have:
Y = EK(X)
X = D K( Y )
assume encryption algorithm is known
implies a secure channel to distribute key
Cryptography
characterize cryptographic system by:
type of encryption operations used
• substitution / transposition / product
number of keys used
• single-key or private / two-key or public
way in which plaintext is processed
• block / stream
Cryptanalysis
objective to recover key not just message
general approaches:
cryptanalytic attack
brute-force attack
Cryptanalytic Attacks
ciphertext only
only know algorithm & ciphertext, is statistical,
know or can identify plaintext
known plaintext
know/suspect plaintext & ciphertext
chosen plaintext
select plaintext and obtain ciphertext
chosen ciphertext
select ciphertext and obtain plaintext
chosen text
select plaintext or ciphertext to en/decrypt
More Definitions
unconditional security
no matter how much computer power or time
is available, the cipher cannot be broken
since the ciphertext provides insufficient
information to uniquely determine the
corresponding plaintext
computational security
given limited computing resources (eg time
needed for calculations is greater than age of
universe), the cipher cannot be broken
Brute Force Search
always possible to simply try every key
most basic attack, proportional to key size
assume either know / recognise plaintext
Key Size (bits) Number of Alternative Time required at 1 Time required at 106
Keys decryption/µs decryptions/µs
32 232 = 4.3 109 231 µs = 35.8 minutes 2.15 milliseconds
56 256 = 7.2 1016 255 µs = 1142 years 10.01 hours
128 2128 = 3.4 1038 2127 µs = 5.4 1024 years 5.4 1018 years
168 2168 = 3.7 1050 2167 µs = 5.9 1036 years 5.9 1030 years
26 characters 26! = 4 1026 2 1026 µs = 6.4 1012 years 6.4 106 years
(permutation)
Classical Substitution
Ciphers
where letters of plaintext are replaced by
other letters or by numbers or symbols
or if plaintext is viewed as a sequence of
bits, then substitution involves replacing
plaintext bit patterns with ciphertext bit
patterns
Caesar Cipher
earliest known substitution cipher
by Julius Caesar
first attested use in military affairs
replaces each letter by 3rd letter on
example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
Caesar Cipher
can define transformation as:
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
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
mathematically give each letter a number
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
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
then have Caesar cipher as:
c = E(p) = (p + k) mod (26)
p = D(c) = (c – k) mod (26)
Cryptanalysis of Caesar
Cipher
only have 26 possible ciphers
A maps to A,B,..Z
could simply try each in turn
a brute force search
given ciphertext, just try all shifts of letters
do need to recognize when have plaintext
eg. break ciphertext "GCUA VQ DTGCM"
