Transcript L3 Classical Encryption Techniques.ppt

Data Security and Encryption
(CSE348)
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Lecture # 3
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Review
• Security concepts:
– confidentiality, integrity, availability
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Security attacks, services, mechanisms
Models for network (access) security
Classical Encryption Techniques
Symmetric Cipher Model
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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
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Symmetric Cipher Model
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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
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Brute Force Search
• Brute-force attack involves trying every possible
key until an intelligible translation of the
ciphertext into plaintext is obtained
• On average, half of all possible keys must be tried
to achieve success
• Different time is required to conduct a bruteforce attack, for various common key sizes
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Brute Force Search
• Data Encryption Standard(DES) is 56
• Advanced Encryption Standard (AES) is 128
• Triple-DES is 168
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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
Keys
Time required at 1
decryption/µs
Time required at 106
decryptions/µs
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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! = 4  1026
2  1026 µs = 6.4  1012 years
26 characters
(permutation)
6.4  106 years
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Brute Force Search
• Users of an encryption algorithm can strive for is an
algorithm that meets one or both of the following
criteria:
• The cost of breaking the cipher exceeds the value of
the encrypted information
• The time required to break the cipher exceeds the
useful lifetime of the information
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Brute Force Search
• An encryption scheme is said to be computationally
secure
• if either of the foregoing two criteria are met
• Unfortunately, it is very difficult to estimate the
amount of effort required to cryptanalyze ciphertext
successfully
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Brute Force Search
• For each key size, the results are shown assuming
that it takes 1 μs to perform a single decryption
• which is a reasonable order of magnitude for today’s
machines
• With the use of massively parallel organizations of
microprocessors, it may be possible to achieve
processing rates many orders of magnitude greater
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Brute Force Search
• The final column of Table considers the results for a
system that can process 1 million keys per
microsecond
• And this performance level, DES can no longer be
considered computationally secure.
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Classical Substitution Ciphers
• In this section and the next, we examine a
sampling of what might be called classical
encryption techniques
• A study of these techniques enables us to
illustrate the basic approaches to symmetric
encryption used today
• and the types of cryptanalytic attacks that must
be anticipated
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Classical Substitution Ciphers
• The two basic building blocks of all encryption
technique are substitution and transposition
• We examine these next. Finally, we discuss a
system that combine both substitution and
transposition.
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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
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Caesar Cipher
• Substitution ciphers form the first of the
fundamental building blocks
• Core idea is to replace one basic unit
(letter/byte) with another
• Whilst the early Greeks described several
substitution ciphers
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Caesar Cipher
• First attested use in military affairs of one was
by Julius Caesar
• Still call any cipher using a simple letter shift a
caesar cipher, not just those with shift 3.
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Caesar Cipher
• earliest known substitution cipher
• replaces each letter by 3rd letter on
• example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
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Caesar Cipher
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
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• e f g H
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Caesar Cipher
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
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Caesar Cipher
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
• t u v W
• h i j K
• e f g H
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Caesar Cipher
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
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Caesar Cipher
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
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p
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D
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B
(again start from a)
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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(k, p) = (p + k) mod (26)
p = D(k, c) = (c – k) mod (26)
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Caesar Cipher
• This mathematical description uses modulo
(clock) arithmetic.
• Here, when you reach Z you go back to A and
start again.
• Mod 26 implies that when you reach 26, you
use 0 instead (ie the letter after Z, or 25 + 1
goes to A or 0).
• Example: howdy (7,14,22,3,24) encrypted
using key f (ie a shift of 5) is MTBID
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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
• Example: howdy (7,14,22,3,24) encrypted
using key f (ie a shift of 5) is MTBID
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Caesar Cipher
• 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
• Example: howdy (7,14,22,3,24) encrypted
using key f (ie a shift of 5) is MTBID
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7 8 9 10 11
14 15 16 17 18
22 23 24 25 0
3 4 5 6 7
24 25 0 1 2
(12,19,1,8,3)
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19
1
8
3
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Cryptanalysis of Caesar Cipher
• With a caesar cipher, there are only 26
possible keys
• of which only 25 are of any use, since mapping
A to A etc doesn't really obscure the message
• Note this basic rule of cryptanalysis "check to
ensure the cipher operator hasn't goofed and
sent a plaintext message by mistake"!
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Cryptanalysis of Caesar Cipher
• Can try each of the keys (shifts) in turn, until
can recognise the original message.
• Do need to be able to recognise when have an
original message (ie is it English or whatever)
• Usually easy for humans, hard for computers
• Though if using say compressed data could be
much harder.
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Cryptanalysis of Caesar Cipher
• Example "GCUA VQ DTGCM" when broken
gives "easy to break", with a shift of 2 (key C)
• efG
• abC
• st U
• yz A
• t uV
• opQ
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Cryptanalysis of Caesar Cipher
• Example "GCUA VQ DTGCM" when broken
gives "easy to break", with a shift of 2 (key C)
• bcD
• r s T
• ef G
• abC
• k lM
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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"
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Summary
• have considered:
– classical cipher techniques and terminology
– Brute Force
• Cryptanalysis of Brute Force
– Caesar Cipher
• Cryptanalysis of Caesar Cipher
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