Public Key Cryptography Explained

Security Concept
Part-2
Mr.Gopal Sakarkar
Mr. Gopal Sakarkar

Public Key Cryptography
• It is used two keys for encryption and for decryption.
– a public-key, which may be known by anybody, and can be
used to encrypt messages
– a private-key, known only to the recipient, used to decrypt
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messages
• It has six ingredient
1 Plain text
2 Encryption algorithm
3 Public and private keys
4 Ciphertext
5 Decryption algorithm

Public-Key Characteristics
• Public-Key algorithms rely on two keys where:
– it is computationally infeasible to find decryption key
knowing only algorithm & encryption key
– it is computationally easy to en/decrypt messages when the
relevant (en/decrypt) key is known
– either of the two related keys can be used for encryption,
with the other used for decryption (for some algorithms)
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Public key Cryptosystem : Authentication and secrecy
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Requirement of Public key Cryptography
1. It is easy for party B to generate a pair of keys (public key PUb ,
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Private key PRb).
2. It is easy for a sender A , knowing the public key and message to
be encrypt. C=E(PUb, M)
3. It is easy for receiver B to decrypt the resulting ciphertext using
the private key . M=D(PRb,C)=D[PRb,E(PUb,M)]
4. It is infeasible for an any person , to know the public key PUb to
determine the private key PRb.
5. It is infeasible for any person to know the public key PUb and a
ciphertext C to recover the original message M.
6. Two keys can be applied in either order
M=DP[PUb, E(PRb,M)] = D[PRb,E(PUb, M)]

Exercise
• Explain the difference between
conventional and public key encryption.
• What are the different requirements for
public key cryptography .
Mr. Gopal Sakarkar

Related Links
• http://docs.sun.com/source/816-6154-10/contents.htm
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RSA
• Invented by Rivest, Shamir & Adleman of MIT in
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1977
• It is a best known & widely used public-key scheme.
• It is a block cipher algorithm in which palintext and
ciphertext integers between 0 to n-1 for some n.
• A typical size for n is 1024 bits or 309 decimal digits.

RSA Algorithm
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An Example
• Let p= 3 and q=5,
• n= 3 X 5 =15
• Q(n)= (3-1) * (5-1) = 2 x 4= 8
• Select e such that gcd(Q(n), e) =1 where, 1<e<Q(n)
• Say e=3 (any prime number)
• Calculate d , such that d e mod Q(n)=1
• 8k+1= 9, 17,25, 33, 41……..where k=1,2,3,4….
• Now check which number is divisible by 3.
• 33 is divisible by 3 .So, d=33/3=11. //9 is also divisible by 3.
• Now k1=(3,15) and K2=(11,15)
• Take plan text M =13 , where (M<n)
• Encryption C= 133 mod 15 =7
• Decryption D= 711 mod 15 =13
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Video

Exercise
• Perform encryption and decryption using
the RSA algorithm for the following
1. p=3, q=11, e=7, M=5
2. P=5,q=11, e=3 , M=9
• Explain various Asymmetric Encryption
Algorithms .
• Draw an algorithm, flowchart for
implementing the RSA Algo.
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Diffie –Hellman
Key Exchange
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in 1976
• It is used by two users to securely exchange a key that can be used for
subsequent encryption of messages.
a public-key distribution scheme
– cannot be used to exchange an arbitrary message
– rather it can establish a common key
– known only to the two participants
value of key depends on the participants (and their private and public
key information)
based on mathematical principles
security relies on the difficulty of computing discrete logarithms (similar
to factoring) – hard

Diffe-Hellman Key Exchange Algorithm
Global Public Elements
q = prime number(300 decimal, i.e. 1024 bits)
 = Integer
User A key Generation
Select private Xa , Xa < q
Calculate public Ya , Ya= Xa mod q
User B Key Generation
Select private Xb , Xb < q
Calculate public Yb , Yb= Xb mod q
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Diffe-Hellman Key Exchange Algorithm
Generation of secret key by user A
K=(Yb)Xa mod q
Generation of secret key by user B
K=(Ya)Xb mod q
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Video

• users Alice & Bob who wish to swap keys:
• agree on prime q=353 and =3
• select random secret keys:
– A chooses xA=97, B chooses xB=233
• compute respective public keys:
– yA=397 mod 353 = 40 (Alice)
– yB=3233 mod 353 = 248 (Bob)
• compute shared session key as:
xA mod 353 = 24897 = 160 (Alice)
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– KAB= yB
– KAB= yA
xB mod 353 = 40233 = 160 (Bob)

Diffie –Hellman Key Exchange
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Exercise
users Alice & Bob who wish to swap keys:
agree on prime q=5 and =7
select random secret keys:
– A chooses xA= 8, B chooses xB= 13
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Exercise
Using diffie- hellman key exchange techniques
,Find A’s public key YA and B’s public key YB .
If, q=71 and = 7 , XA =5 and XB = 12
Draw an algorithm, flowchart and write C++
program to implement Diffe-Hellman Key Exchange
Algorithm
Mr. Gopal Sakarkar

For Further Reading
• http://postdiluvian.org/~seven/diffie.html
• AES links
• http://www.youtube.com/watch?v=SFXYCT9-SeM (AES)
• http://www.youtube.com/watch?v=ySq88y0e8u4&feature=related
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• Send your all PPT, Posters, IEEE papers
on
KnowledgeWealth
at
Facebook
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Digital Signature
Encryption, message authentication and digital signatures
are all tools of modern cryptography.
A signature is a technique for non-repudiation based on
the public key cryptography.
The creator of a message can attach a code, the
signature, which guarantees the source and integrity of
the message.
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Digital signature process
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Properties of Signatures
Similar to handwritten signatures, digital signatures must
fulfill the following:
Recipients must be able to verify them
 Signers must not be able to repudiate them later
In addition, digital signatures cannot be constant and must
be a function of the entire document it signs
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Types of Signatures
Direct digital signature – involves only the communicating
parties
 Assumed that receiver knows public key of sender.
 Signature may be formed by (1) encrypting entire
message with sender’s private key or (2) encrypting
hash code of message with sender’s private key.
 Further encryption of entire message + signature
with receiver’s public key or shared private key
ensures confidentiality.
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The message with sender’s private key
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The hash code of message with sender’s private key
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Types of Signatures
Arbitrated digital signature – involves a trusted third party
or arbiter
 Every signed message from sender, X, to receiver, Y,
goes to an arbiter (authority), A, first.
A subjects message + signature to number of tests to
check origin & content
 A date the message and sends it to Y with indication
that it has been verified to its satisfaction
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Digital Signature Standard
Public-key technique.
User applies the Secure Hash Algorithm (SHA) to the message to produce
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message digest.
User’s private key is applied to message digest using DSA to generate
signature.

Digital Signature Standard
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Exp:LIC Doc

DSA/DSS Key Generation
have shared global public key values (p,q,g):
– choose a large prime p with 2L-1 < p < 2L
where L= 512 to 1024 bits and is a multiple of 64
– choose q with 2159 < q < 2160
such that q is a 160 bit prime divisor of (p-1)
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– choose g = h(p-1)/q
where 1<h<p-1 and h(p-1)/q mod p > 1
users choose private key & compute public key:
– choose x<q //Private Key
– compute y = gx mod p //Public Key

DSA Signature Creation
to sign a message M the sender:
– generates a random signature key k, k<q
– k must be random, be destroyed after use,
and never be reused
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then computes signature pair:
r = (gk mod p)mod q
s = [k-1(H(M)+ xr)] mod q
sends signature (r,s) with message M

DSA Signature Verification
having received M & signature (r,s)
to verify a signature, recipient computes:
w = s-1 mod q
u1= [H(M)w ]mod q
u2= (rw)mod q
v = [(gu1 yu2)mod p ]mod q
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if v=r then signature is verified

DSA creates a 320 bits signature with 512-1024 bit data
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security.
smaller and faster than RSA
a digital signature scheme only
security depends on difficulty of computing discrete
logarithms
Summary

Number theory
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Group
The .is generic can be
addition, multiplication
,substraction etc.
 a set of elements or “numbers” , denoted by {G,.}
• Rules:
– associative law: (a.b).c = a.(b.c)
– Closure : if a and b belong to G then a.b also in G
– identity e: e.a = a.e = a
– inverses a-1: a.a-1 = e
• if commutative a.b = b.a
– then forms an abelian group
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Ring
• a set of “numbers” denoted by {R,+, X}
• with two operations (addition and multiplication) which form:
• an abelian group with addition operation and multiplication:
– has closure :is a and b belong to R , then ab is also in R
– is associative : a(bc)=(ab)c for all a,b,c in R
– distributive over addition: a(b+c) = ab + ac
(a+b)c = ac + bc
• if multiplication operation is commutative, it forms a commutative
ring i.e. ab = ba for all a, b in R
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Prime Factorisation
• to factor a number n is to write it as a product
of other numbers: n=a x b x c
• note that factoring a number is relatively hard
compared to multiplying the factors together to
generate the number
• the prime factorisation of a number n is when
its written as a product of primes
– eg. 91=7x13 ; 3600=24x32x52
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Modular Arithmetic
• define modulo operator “a mod n” to be
remainder when a is divided by n
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eg. 11 mod 7=4
• congruent modulo
Two integer a and b are said to be
congruent modulo n if,
a mod n = b mod n
eg. 75 mod 10 = 85 mod 10

Divisors
• say a non-zero number b divides a if for some m have
a=mb (a,b,m all integers)
• that is b divides into a with no remainder
• denote this b|a
• and say that b is a divisor of a
• eg. all of 1,2,3,4,6,8,12,24 divide 24
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Modular Arithmetic Operations
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Properties of Modular Arithmetic
(a+b) mod n = [a mod n + b mod n] mod n
<proof>
(a-b) mod n = [a mod n - b mod n] mod n
(a X b) mod n = [a mod n X b mod n] mod n
Eg. (11 + 15 ) mod 8 = [11 mod 8 + 15 mod 8] mod 8

Modulo 8 Addition Example
+ 0 1 2 3 4 5 6 7
0 0 1 2 3 4 5 6 7
1 1 2 3 4 5 6 7 0
2 2 3 4 5 6 7 0 1
3 3 4 5 6 7 0 1 2
4 4 5 6 7 0 1 2 3
5 5 6 7 0 1 2 3 4
6 6 7 0 1 2 3 4 5
7 7 0 1 2 3 4 5 6
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Exercise
1.Draw a flowchart and an algorithm and write a C ++ program for
Modulo of n Addition.
2. Proved that (a-b) mod n and (a X b) mod n .
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Greatest Common Divisor (GCD)
• GCD (a,b) of a and b is the greatest number that
divides evenly into both a and b
– eg GCD(60,24) = 12
 The positive integer c is said to be the greatest
common divisor of a and b if
1. C is a divisor of a and of b
2. Any divisor of a and b is a divisor of c
It is denoted by
gcd(a,b)= max[k, such that k/a and k/b]
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Find the gcd of 36 and 15
a/b gives a remainder of r
b/r gives a remainder of s
r/s gives a remainder of t
...
w/x gives a remainder of y
x/y gives no remainder
H/w gcd(25,10)
Mr. Gopal Sakarkar

Exercise
1. Draw a flowchart and an algorithm and write a C++ program
to find the GCD of numbers.
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Euclid Algorithm
In mathematics, the Euclidean algorithm (also called Euclid's
algorithm) is an efficient method for computing the greatest
common divisor (GCD), also known as the greatest common
factor (GCF) or highest common factor (HCF).
It is named after the Greek mathematician Euclid (in BC 300)
The greatest common divisor g is the largest natural number
that divides both a and b without leaving a remainder .
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Euclidean Algorithm
• an efficient way to find the GCD(a,b)
• uses theorem that:
– GCD(a,b) = GCD(b, a mod b)
• Euclidean Algorithm to compute GCD(a,b) is:
EUCLID(a,b)
1. A = a; B = b
2. if B = 0 return ; A = gcd(a, b)
3. R = A mod B
4. A = B
5. B = R
6. goto 2
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Euler Theorem
Swiss mathematician noted both for his work in analysis and algebra,
including complex numbers and logarithms, and his introduction of
much of the basic notation in mathematics.
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Relatively Prime Numbers
• Two numbers a, b are relatively prime if have no
common divisors apart from 1
– eg. 8 & 15 are relatively prime since factors of 8 are 1,2,4,8 and of
15 are 1,3,5,15 and 1 is the only common factor.
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Euler Totient Function ø(n)
• It is define as the number of positive integer less than n
and relatively prime to n.
• Since a number less than or equal to and relatively
prime to a given number is called a totative.
• A totient function can be simply defined as the number
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of totatives of n.
• For example, there are eight totatives of 24 (1, 5, 7, 11,
13, 17, 19, and 23), so ø(24)=8

Euler Totient Function ø(n)
Eg. Determine ø(35)
Now find out list of all positive integer less than 35 that are
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relatively prime to it:
1,2,3,4,6,8,9,11,12,13,16,17,18,19,22, 23,24,26,27,29,31,32,33,34
Science there are 24 numbers so, ø(35)=24

Euler's Theorem
• Theorem : Euler’s theorem states that for every a and n ,and if
they are relatively prime then,
aø(n) ≡ 1 (mod n)
• The theorem may be used to easily reduce large powers modulo
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n.
• consider finding the last decimal digit of 7222, i.e. 7222 (mod 10).
• Note that 7 and 10 are relatively prime, and φ(10) = 4.
• So Euler's theorem yields 74 ≡ 1 (mod 10),
• and we get 7222 ≡ 74x55 + 2 ≡ (74)55x72 ≡ 155x72 ≡ 49
≡ 49 (mod 10)
= 9
Exp of Totient RSA

Euler's Theorem Cont…..
In general, when reducing a power of a modulo n (where a and n
are relatively prime), one needs to work modulo φ(n) in the
exponent of a:
if x ≡ y (mod φ(n)),
then ax ≡ ay (mod n)
Mr. Gopal Sakarkar

Story behind CRT
An old woman goes to market and a horse steps on her
basket and crashes the eggs. The rider offers to pay for
the damages and asks her how many eggs she had
brought. She does not remember the exact number, but
when she had taken them out two at a time, there was
one egg left. The same happened when she picked them
out three, four, five, and six at a time, but when she took
them seven at a time they came out even. What is the
smallest number of eggs she could have had?
Problems of this kind are all examples of what universally
became known as the Chinese Remainder Theorem.
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Chinese Remainder Theorem
• Find a number x such that it has
remainders of 0 when divided by 2, and 3
when divided by 5. i.e.
X= a mod n and
X =b mod m,
Where ,
gcd(n, m) =1
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Video

Chinese Remainder Theorem
• used to speed up modulo computations
• it working modulo to product of numbers
– eg. mod M = m1m2..mk
• Chinese Remainder Theorem lets us work in each
moduli mi separately
• since computational cost is proportional to size, this is
faster than working in the full modulus M.
• This can be useful when M is 150 digits or more.
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CRT statement
Let m1, m2, …, mk be pairwise relatively prime integers.
That is, gcd(mi, mj) = 1 for 1 i , j k.
Let aiZmi for 1i k and set M=m1m2…mk.
Then there exists a unique A Zm, such that ai A mod mi
for i = 1…k. then
A can be computed as:
k
A a c M
 
( )mod
1
i i
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i

Where 1 ( mod )& / i i i i i i c M M m M M m     for 1ik.

1 2 1 1 ... ... i i i k M m m m m m       
k
A a c m
 
( )mod

c a c a c a r m
a m
     

Mr. Gopal Sakarkar
Proof:
A is a solution
– Since
for any ji
1
– Therefore,
1mod
( mod )
0mod
i
i i i i
j
m
c M M m
m
 
  

1
i i
i
1 1 2 2
...
mod
k k
i i

Properties:
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(A+B) mod M 
((a1 + b1) mod m1, …, (ak + bk)mod mk)
(A-B) mod M 
((a1 - b1) mod m1, …, (ak - bk)mod mk)
(AB) mod M 
((a1  b1) mod m1, …, (ak  bk)mod mk)
If X1= Y1mod n and X2=Y2 mod n then
X1+X2 = Y1+Y2 mod n and X1- X2 = Y1-Y2 mod n

Tutorial-1
“Study of Chinese Reminder Theorem ”
Submission: submission of tutorial 1 is on and before 21/8/2013.
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Today’s Agenda
• Message Digests
• Hash Functions
• Message Authentication
• Secure Hash Function
Mr. Gopal Sakarkar

Message digests
• A technique used to establish whether text sent over a network
has been tampered or not.
• It consists of a mathematical rule which, when applied to a
piece of text, generates a relatively short number, usually
between 128 and 512 bits.
• This number is then sent with the text to a recipient who
reapplies the mathematical rule to the text and compares the
result with the original number.
• If they are the same then there is a very high probability that
the message has not been tampered with during the sending
process; if it does differ it is virtually certain that the message
has been tampered with.
• It is not useful for active attack.
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MD4
– A one-way hash function that produces a 128-bit hash, or message digest.
– If as little as a single bit value in the file is modified, the MD4 checksum for the
file will change.
– Forgery of a file in a way that will cause MD4 to generate the same result as
that for the original file is considered extremely difficult.
MD5
– An improved, and more complex, version of MD4
– circa 1992
– 128-bit hash
– "almost broken" by Hans Dobbertin circa 1995
– Fully broken by collision attack Wang et. al. 2004
Data Encryption Standard (DES)
– Symmetric, feistel cipher
– Key size (in bits): 112 or 168
– Time to crack (assume a machine could try 255 keys per second - NIST): 4.6
billion years
Advanced Encryption Standard (AES)
– Symmetric, block cipher
– Key size (in bits): 128, 192, 256
– Time to crack (assume a machine could try 255 keys per second - NIST): 149
trillion years
Secure Hash Algorithm (SHA)
– produces a 160-bit hash, longer than MD5.
– The algorithm is slightly slower than MD5, but the larger message digest
makes it more secure against brute-force collision and inversion attacks.

For Further Reading
• http://www.faqs.org/rfcs/rfc1321.html
• http://www.java2s.com/Code/Java/Spring/MessageDigestExample.h
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tm
• http://docs.sun.com/app/docs/doc/816-4863/6mb20lvls?a=view

Checksums
• A checksum or hash sum is a fixed-size data computed from
an arbitrary block of digital data for the purpose of detecting
accidental errors that may have been introduced during its
transmission or storage.
• The integrity of the data can be checked at any later time by
recomputing the checksum and comparing it with the stored
one.
• If the checksums do not match, the data was almost certainly
altered (either intentionally or unintentionally).
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Checksum Applications
• First, checksum value can be used to check data integrity when data is sent
through telecommunication networks such as Internet .
• Second, checksum value can be used to check data integrity of stored data
to see if the data has been modified or changed in any way over time.
• Third, checksum values can be used to verify data burned to CDROM,
CD-R (Compact Disc-Recordable), OR DVD, DVD-R.
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For Further Reading
• http://www.geeksengine.com/article/checksum.html
• http://www.keil.com/download/docs/54.asp
• http://computer.howstuffworks.com/encryption7.htm
• http://www.accuhash.com/what-is-checksum.html
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Message Authentication
• Message authentication is a mechanism or
service used to verify the integrity of a
message .
• Most common techniques for message
authentication are
1.Message Authentication Code (MAC)
2. Secure Hash Function.
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Message Authentication Code
• It is used to generate a fix –size block of data.
• Let A and B share a common secret key K.
• When A has to send to B , it calculate the MAC as a function of
the message and the key :
MAC= C(K,M).
• The message M pulse MAC are transmitted to the intended
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recipient.
• The received MAC is compared to the calculated MAC.
• Eg: Find out how many times r is occurred in the given
message.
• Now , here counting a occurrence of alphabet is a function i.e
C( ) and r is acting as secret key K.

Hash functions
• Reduce arbitrary message to fixed size
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– h = H(M)
• Usually assume that the hash function is public and not keyed
• Hash used to detect changes to message
• Can use in various ways with message
• Most often to create a digital signature

Hash Functions
• Take an input from a large domain and return an output
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in a smaller range.
• Easy to compute.
• Eg: Collect the alphabets , which is available at odd
position in word of the message M. i.e. h = H(M)

Basic Uses of Hash Functions
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• Use a “Keyed Hash”
1010100101010
1010101010101
1010101000101
0101001010001
1010010101010
Shared Secret
HA
100010
010101
100011
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Requirements for Hash Functions
• Can be applied to any sized message M
• Produces fixed-length output h
• Is easy to compute h=H(M) for any message M
• Given h is infeasible to find x s.t. H(x)=h
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– one-way property
• Given x is infeasible to find y s.t. H(y)=H(x)
– weak collision resistance
• Is infeasible to find any x,y s.t. H(y)=H(x)
– strong collision resistance

• For example, a simple hashing algorithm would be to
add up all digits in a number, and take the remainder
when divided by 7. Let the hashing function be f(x)
• f(13) = (1+3) % 7 = 4
• f(26) = (2+6) % 7 = 1
• f(78) = (7+8) % 7 = 1
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Digital Signature
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For Further Reading
• http://www.faqs.org/rfcs/rfc3174.html
• http://cboard.cprogramming.com/cplusplus-programming/110600-working-bits-sha-
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1-a.html
• http://www.codeproject.com/KB/recipes/csha1.aspx
• Bit-Commitment with Secure Hashes
– http://citeseer.nj.nec.com/halevi96practical.html
• SHA-1 Specification
– http://www.itl.nist.gov/fipspubs/fip180-1.htm
• MD5 Specification (rfc1321)
– http://andrew2.andrew.cmu.edu/rfc/rfc1321.html
• Keyed Hashes: HMAC
– http://www-cse.ucsd.edu/users/mihir/papers/hmac.html

Secure Hash Algorithm
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1993
– The hash function SHA-0 was issued as a federal standard by
NIST
1995
– SHA-1 published as the successor to SHA-0
2002
– SHA-2 variants
SHA-256, SHA-384, and SHA-512 published
2004
– SHA-224 published
* No known weaknesses have been found with the SHA-2 variants
(at this time)

Secure Hash Algorithm cont…
SHA-1, SHA-256, SHA-384, and SHA-512
All four of the algorithms are iterative, one-way hash functions
process a message to produce a condensed representation called a
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message digest
These algorithms enable the determination of a message’s integrity
– any change to the message will, with a very high probability,
result in a different message digest
– This property is useful in the generation and verification of digital
signatures and message authentication codes, and in the
generation of random numbers (bits).

Flavors of SHA
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SHA-0
SHA-1*
SHA-224*
SHA-256*
SHA-384*
SHA-512*
*FIPS-approved algorithm for generating a condensed representation of a message (message digest)

The Algorithm
Each algorithm can be described in two stages:
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– preprocessing
Preprocessing involves padding a message, parsing the
padded message into m-bit blocks, and setting initialization
values to be used in the hash computation
– hash computation
The hash computation generates a message schedule from
the padded message and uses that schedule, along with
functions, constants, and word operations to iteratively
generate a series of hash values
– The final hash value generated by the hash computation is used
to determine the message digest.

Algorithm – cont’d
Step 1. Padding
-padding bits to original message
-to make a original message equal to a value which is 64 bits less than
an exact multiple of 512.
Exp. Let the length of the message is 1000 bits , add a padding of
472 bits to make the length of the message 1472 bits.
i.e. when we add 64 to 1472 we got 1536 (512 X 3).
-padding is always added even if message length is already 64 bits
Less than a multiple of 512.
Exp. If length of message is 448 bits, add a padding 512 bits to make its
length 960 bits, padding is always between 1 to 512.
Original Message + Padding 1-512
Original Message P a dPdaidndgi n1g-5 1 2
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Step 2. Appending Length
-now calculate the original length of message and add it to the end of the message,
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after padding.
Exp.: let original message is 1000 bits and we add padding of 472 bits to make the
length of message 64 bits less than 1536 , here the length is consider as 1000 1472 bits.
Original Message P a dPdaidndgi n1g-5 1 2 +
Length
Original Message P a dPdaidndgi n1g-5 1 2 Lengt
h
-the length is expressed as a 64 bit value and these 64 bits are appending to the of original message + padding

Step 3: Divide the Input
Now divide the input message into block, each of the length 512 bits.
Data to be hashed
Block 1 Block 2 Block 3 Block n
512 bits 512 bits 512 bits 512 bits
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Step 4: Initialize chaining variable
- Now , five chaining variables A to E are initialized , each of 32 bits number.
-in SHA we want to produce a message digest of length 160 bits , for that we
have
five chaining variables(5 X 32= 160 bits.)
Step 5: Copy the chaining variables.
-now copy the chaining variable A-E into variable a-e.
-The combination of a-e treated as single register for storing the temporary
intermediate as well as final result.
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A
a
B
b
C
c
D
d
E
e

Step 6: Divide a block
-now divide the current block 512 bits into 16 sub blocks , each of 32 bits.
Step 7: Round and Iterations
-SHA consists of four rounds , each round containing 20 iteration
-This make it total of 80 iterations
-Mathematical representation is:
abcde= (e + Process P + S^5 (a) +W [t] + K[t]) ,a , S^30 (b) ,c,d
Where,
abcde= The registers
Process P = The logical operation
S ^t = Circular –left shift of the 32 bit sub block by t bits
W[t] = A 32 bit derived from the current 32 bit sub block
K[t] = one of the five additive constant
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Secure Hashes Algorithm
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• One-Way
– Given f(x), hard to find x.
• Collision-Free
– Hard to find x and y so that f(x)=f(y)
• Hard to bias output
– Hard to generate a set {xi} so that we can
differentiate between f({xi}) and f(U) where U is a
uniformly distributed input.

Uses for SHA
• Message Authentication Checksums
– Prevent an attacker from changing messages
• Faster Digital Signatures
• Faster Bit-Commitment Schemes
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Related References
• http://www.packetizer.com/security/sha1/
• http://www.itl.nist.gov/fipspubs/fip180-1.htm(IMP)
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Tutorial 2
“Study and implementation of various
Hashing functions ”-Exercise
1. Write a comparison between MD5 and SHA-1
2. Explain the various authentication requirements
for context communication across a network.
3. Differentiate between Message Encryption,
Message Authentication Code, and Hash
Function.
4. Explain various applications of MAC.
5. Explain in details working of SHA 512.
Submission: Submit the Tutorial-2 on and before
28/8/2013.
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Exercise
Download a DES and AES encryption software
For further Reading
http://www.progressive-coding.com/tutorial.php#aes_description
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Today’s Agenda
Intrusion Detection Techniques
Intrusion
Intrusion Techniques
Intrusion Detection
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Intruders
Intruders: Intruder is a person whose objetive is to gain
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access
to system or to increase the range of privilege accessible
on
a system either via network or local
Classes of intruders:
• Masquerader : An individual who is not authorized to use
the computer (outsider)
• Misfeasor : A legitimate user who accesses unauthorized
data, programs, or resources (insider)
• Clandestine user : An individual who grab supervisory
control of the system and uses this control to avoid
auditing and access controls or to suppress audit
collection (either)

Intrusion Techniques
Aim to gain access and/or increase privileges on a
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system
Basic attack methodology
– target acquisition and information gathering
– initial access
– enlarge the privilege,
– covering tracks
Key goal often is to acquire passwords
So then exercise access rights of owner

Intrusion Detection System
Need also to detect intrusions so can
– block if detected quickly
– act as deterrent(prevention)
– collect info to improve security
Assume intruder will behave differently to a legitimate user
– but will have imperfect distinction between
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Approaches to Intrusion Detection
Statistical anomaly detection
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– threshold
– profile based
Rule-based detection
– anomaly
– penetration identification

Audit Records
It is a fundamental tool for intrusion detection
native audit records
– part of all common multi-user O/S
– already present for use
– may not have info wanted in desired form
detection-specific audit records
– created specifically to collect wanted info
– at cost of additional overhead on system
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Statistical Anomaly Detection
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Threshold detection
– count occurrences of specific event over time
– if exceed reasonable value assume intrusion
– alone is a crude & ineffective detector
Profile based
– characterize past behavior of users
– detect significant deviations from this
– profile usually multi-parameter

Rule-Based Intrusion Detection
Observe events on system & apply rules to decide if activity
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is suspicious or not
Rule-based anomaly detection
– analyze historical audit records to identify usage
patterns & auto-generate rules for them
– then observe current behavior & match against rules
to see if conforms
– like statistical anomaly detection does not require
prior knowledge of security flaws

Rule-Based Intrusion Detection
Rule-based penetration identification
– uses expert systems technology
– with rules identifying known penetration, weakness
patterns, or suspicious behavior
– compare audit records or states against rules
– rules usually machine & O/S specific
– rules are generated by experts who interview & codify
knowledge of security admins
– quality depends on how well this is done
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Distributed Intrusion Detection
Traditional focus is on single systems
but typically have networked systems
More effective defense has these working together to detect intrusions
issues
– dealing with varying audit record formats
– integrity & confidentiality of networked data
– centralized or decentralized architecture
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Distributed Intrusion Detection
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Tutorial-3
Last date of submission : 6/09/2013
Survey of Current Network Intrusion Detection
Techniques
Explain various metrics useful for profile-based
detection.
Explain various techniques for learning others
passwords.
Discuss and explain the various intrusion attacks
in real life world .
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Viruses and Malicious Programs
• Computer “Viruses” and related programs have the ability to
replicate themselves on an ever increasing number of computers.
They originally spread by people sharing floppy disks. Now they
spread primarily over the Internet (a “Worm”).
Other “Malicious Programs” may be installed by hand on a
single machine. they may also be built into widely
distributed commercial software packages. these are very
hard to detect before the payload activates
(Trojan Horses, Trap Doors, and Logic Bombs).
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Taxanomy of Malicious Programs
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Definitions
• Virus - code that copies itself into other programs.
• A “Bacteria” replicates until it fills all disk space, or CPU cycles.
• Payload - harmful things the malicious program does, after it has had
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time to spread.
• Worm - a program that replicates itself across the network (usually
riding on email messages or attached documents (e.g., macro viruses).
• Trojan Horse - instructions in an otherwise good program that cause bad
things to happen (sending your data or password to an attacker over the
net).
• Logic Bomb - malicious code that activates on an event (e.g., date).
• Trap Door (or Back Door) - undocumented entry point written into code
for debugging that can allow unwanted users.
• Easter Egg - extraneous code that does something “cool.” A way for
programmers to show that they control the product.

Virus Phases
• Dormant phase - the virus is idle
• Propagation phase - the virus places an identical copy of
itself into other programs
• Triggering phase – the virus is activated to perform the
function for which it was intended
• Execution phase – the function is performed
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A Compression Virus
2. Virus first compresses
3. Copy of virus is prepended to
uninfected file P2 to P2’, which is
5. The uncompressed original program is executed
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1.Program P1 is infected
compressed program..
wWithhe nv itrhuiss CprVogram invoke ,control passes to its virus.
shorter than original size.
4. The compress version
of infected program P1’
is uncompressed..

Types of Viruses
• Parasitic Virus - attaches itself to executable files as part of their code.
Runs whenever the host program runs.
• Memory-resident Virus - Lodges in main memory as part of the residual
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operating system.
• Boot Sector Virus - infects the boot sector of a disk, and spreads when the
operating system boots up (original DOS viruses).
• Stealth Virus - explicitly designed to hide from Virus Scanning programs.
• Polymorphic Virus - mutates with every new host to prevent signature
detection.

Antivirus Approaches
• 1st Generation, Scanners: searched files for any of a library
of known virus “signatures.” Checked executable files for
length changes.
• 2nd Generation, Heuristic Scanners: looks for more general
signs than specific signatures (code segments common to
many viruses). Checked files for checksum or hash
changes.
• 3rd Generation, Activity Traps: stay resident in memory
and look for certain patterns of software behavior (e.g.,
scanning files).
• 4th Generation, Full Featured: combine the best of the
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techniques above.

Advanced Antivirus Techniques
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Summary
• Intruder’s aim to gain access and/or increase privileges on a system
• There are two type of detection techniques
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statistical anomaly detection
rule-based detection
• Taxanomy of Malicious Programs
• Advanced Antivirus Techniques

Tutorial-4
last date of submission: 13/9/2013
• Explain in detail classification of Viruses.
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• Authentication
• e-mail security
• PGP,S/MIME.
• Firewalls
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Authentication
Password file User
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exrygbzyf
kgnosfix
ggjoklbsz
…
…
kiwifruit
hash function

Password based authentication
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• Setup
– User chooses password
– Hash of password stored in password file
• Authentication
– User logs into system, supplies password
– System computes hash, compares to file
• Attacks
– Online dictionary attack
• Guess passwords and try to log in
– Offline dictionary attack
• Steal password file, try to find p with hash(p) in file

Dictionary Attack – some numbers
• Typical password dictionary
– 1,000,000 entries of common passwords
• people's names, common pet names, and ordinary words.
– Suppose you generate and analyze 10 guesses per second
– Dictionary attack in at most 1,00,000 seconds = 28 hours,
or 14 hours on average
• If passwords were random
– Assume six-character password
• Upper- and lowercase letters, digits, 32 punctuation characters
• 689,869,781,056 password combinations.
• Exhaustive search requires 1,093 years on average
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Web Authentication
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Browser
• Problems
Malicious or weak-security website
• Phishing
• Common password problem
• Pharming – DNS compromise
– Malware on client machine
• Spyware
• Session hijacking, fabricated transactions
Server
password
cookie

Password Phishing Problem
• User cannot reliably identify fake sites
• Captured password can be used at target site
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Bank A
Fake Site
pwdA
pwdA

Defense: Password Hashing
• Generate a unique password per site
– HMACfido:123(banka.com)  Q7a+0ekEXb
– HMACfido:123(siteb.com)  OzX2+ICiqc
• Hashed password is not usable at any other site
– Protects against password phishing
– Protects against common password problem
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Bank A
Site B
pwdA
pwdB
=

Tutorial -5
Last date of submission : 20/9/203
• Explain in details working of client-server
based architecture.
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Today’s Agenda
• Email Overview : SMTP, POP , MIME
• Secure E-Mail Standard : PGP, S/MIME
• Firewall
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RFC 822
• Published in 1982
• Support for text format only.
• Messages are viewed as having an envelope
and contents.
• Envelop having transmission and delivery
information.
• Contents has the object to be delivered.
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RFC 822
Mail Format
• A message consists of some number of header line
( the header) followed by unrestricted text (the body).
• A blank line is used for separation.
• Lines no longer than 1000 char
• Message body - plain US-ASCII text
• Message header lines - plain US-ASCII text
• Limit on message length
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RFC Example
Date: Tue,25 feb 1985 13:45:97
From: someone@techtarget.com
To: someoneelse@techtarget.com
Subject: A demonstration of the RFC 822 message
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format.
This is the message body , which is delimited from the
message heading by a blank line.
Blank line for
Separation

• http://www.rfc-editor.org/rfc/rfc822.txt
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MIME
• MIME refers to an official Internet standard that specifies
how messages must be formatted so that they can be
exchanged between different email systems.
• MIME permits the inclusion of virtually any type of file or
document in an email message.
• Specifically, MIME messages can contain
– text
– images
– audio
– video
– application-specific data.
• spreadsheets
• word processing documets
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MIME Features
• Support of character sets other than ASCII
• Support of non-text content in e-mail messages
• Support for compound documents
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MIME Example
From: John Doe <example@example.com>
To: g.sakarkar@gmail.com
Subject: Hello Word
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="XXXXboundary text"
This is a multipart message in MIME format.
--XXXXboundary text
Content-Type: text/plain
this is the body text
--XXXXboundary text
Content-Type: text/plain;
Content-Disposition: attachment;
filename="test.txt"
this is the attachment text
--XXXXboundary text--
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• The "MIME-Version:" header tells the receiving UA to
treat this as a MIME message.
• The"Content-Type: “header specifies
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"multipart/mixed".
• The message has parts separated by the string
argument defined in "boundary="
• The "Content-Type:" header identifies it as "text/plain",
meaning US-ASCII characters are used exclusively and
any UA should be able to display this body part.
• The "Content-Disposition: attachment" header has a
parameter, "filename=", which specifies a suggested name
for the file.

• SMTP (Simple Mail Transfer Protocol) is the
procedure by which email data packets are transferred
from one networked machine to another.
• SMTP defines the message format and the message
transfer agent (MTA), which stores and forwards the
mail.
• SMTP is a relatively simple, text-based protocol,
where one or more recipients of a message are
specified and then the message text is transferred.
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• Transfer email between mail servers reliably and
efficiently .
• In order to send email, the client sends the message to
an outgoing mail server, which in turn contacts the
destination mail server for delivery.
• For this reason, it is necessary to specify an SMTP
server when configuring an email client.
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• SMTP uses persistent connections
• SMTP uses TCP port 25.
• SMTP requires message (header & body) to be in 7 -
bit ASCII
• SMTP server uses CRLF.CRLF to determine end of
message
• Unsecured against spam.
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• Mail client is configured with the name of a local
mail gateway (SMTP server)
• Mail client does not have to know how to deliver
mail to everywhere
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Scenario: Alice sends message to Bob
1)Alice uses UA to compose message and “to”
bob@someschool.edu
2)Alice’s UA sends message to her mail server;
message placed in message queue
3)Client side of SMTP opens TCP connection with
Bob’s mail server
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4)SMTP client sends Alice’s message over the
TCP connection
5)Bob’s mail server places the message in
Bob’s mailbox
6)Bob invokes his user agent to read message
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REPLY CODES MEANING
211 System status, or system
help reply
214 Help message
220 <domain> Service ready
221 <domain> Service closing
transmission channel
250 Requested mail action okay,
completed
354 Start mail input; end with
<CRLF>.<CRLF>
421 <Domain> Service not
available, closing
transmission channel
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REPLY CODES MEANING
450 Requested mail action not
taken: mailbox unavailable
451 Requested action aborted:
local error in processing
500 Syntax error, command
unrecognized
501 Syntax error in parameters or
arguments
503 Bad sequence of commands
550 Requested action not taken:
mailbox unavailable
551 User not local; please try
<forward-path>
554 Transaction failed
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This SMTP example shows how mail is sent by Smith at host
Alpha.ARPA, to Jones and Green at host Beta.ARPA
S: MAIL FROM:Smith@Alpha.ARPA
R: 250 OK
S: RCPT TO:Jones@Beta.ARPA
R: 250 OK
S: RCPT TO:Green@Beta.ARPA
R: 550 No such user here
S: RCPT TO:Brown@Beta.ARPA
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: <CRLF>.<CRLF>
R: 250 OK
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• HELLO: Sent by a client to identify itself, usually with a domain name
• EHLO: Enables the server to identify its support for Extended Simple Mail
Transfer Protocol (ESMTP) commands
• MAIL FROM: Identifies the sender of the message; used in the form MAIL
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FROM:
• RCPT TO: Identifies the message recipients; used in the form RCPT TO:
• TURN: Allows the client and server to switch roles and send mail in the reverse
direction without having to establish a new connection

• ATRN: The ATRN (Authenticated TURN) command optionally takes one
or more domains as a parameter. The ATRN command must be rejected if
the session has not been authenticated
• DATA: Sent by a client to initiate the transfer of message content
• RSET: Nullifies the entire message transaction and resets the buffer
• VRFY: Verifies that a mailbox is available for message delivery
• HELP: Returns a list of commands that are supported by the SMTP
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service
• QUIT: Terminates the session

• Simple Mail Transport Protocol (SMTP) is the
network protocol used to send email across the
Internet.
• SMTP provides reliability as it uses TCP connection.
• Current research focuses on the security issues of
SMTP.
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Tutorial –6
last date of submission : 27/9/2013
• Briefly explain the POP, IMAP protocols.
• What are the advantages and disadvantages of
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SMTP.
• List and explain the various applications of SMTP.

Pretty Good Privacy (PGP)
The first version of PGP was programmed
in 1991 by Phil R. Zimmerman, who later
founded PGP Security Consulting.
PGP is one of the most popular encryption
and authentication algorithm world-wide.
PGP is more widely used in electronic mail
security than any other areas.
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Phil R. Zimmerman

Pretty Good Privacy (PGP)
"If all the personal computers in the world - 260 million
- were put to work on a single PGP-encrypted
message, it would still take an estimated 12 million
times the age of the universe, on average, to break
a single message.”
- Deputy Director William Crowell
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National Security Agency
3/20/1997

Mr. Gopal Sakarkar
Notation
Ks = session key used in symmetric
encryption scheme
PRa = Private key of user A.
PUa = public key of user A.
EP = public key encryption
DP = public key decryption
EC =symmetric encryption
DC = symmetric decryption

Notation cont…
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H = hash function
|| = concatenation
Z = compression using ZIP algorithm
R64 = conversion to radix 64 ASCII format

PGP Working
PGP offers 5 services:
• Authentication
• Confidentiality
• Compression
• E-mail compatibility
• Segmentation
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PGP Authentication
This is a digital signature scheme with hashing.
1. Alice has (private/public) key pair (Ad/Ae) and she wants to send a
digitally signed message m to Bob.
2. Alice hashes the message using SHA-1 to obtain SHA(m).
2. Now the original message m is compressed to obtain
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M=ZIP(m)
3. Alice generates a session key k and encrypts the compressed
message and the signature using the session key
C=sk.encryptk(M,c)
4. The session key is encrypted using Bob’s public key as before.

3. Alice encrypts the hash using her private key Ad to obtain ciphertext c given
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by
c=pk.encryptAd(SHA(m))
4. Alice sends Bob the pair (m,c)
5. Bob receives (m,c) and decrypts c using Alice's public key Ae to obtain
signature s
s=pk.decryptAe(c)

6. He computes the hash of m using SHA-1 and if this hash value is
equal to s then the message is authenticated.
Bob is sure that the message is correct and that is does come from
Alice. Furthermore Alice cannot later deny sending the message since
only Alice has access to her private key Ad which works in conjunction
with the public key Ae.
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Message authentication
• based on digital signatures
• supported algorithms: RSA/SHA and DSS/SHA
m h s
hash enc
m h h s
hash compare dec
accept / reject
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Ksnd
-1
Ksnd
receiver sender

PGP Confidentiality
1. Alice wishes to send Bob a confidential message m.
2. Alice generates a random session key k for a symmetric
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cryptosystem.
3. Alice encrypts k using Bob’s public key Be to get
k’ = pk.encryptBe(k)
4. Alice encrypts the message m with the session key k to get
ciphertext c
c=sk.encryptk(m)
5. Alice sends Bob the values (k’,c)
6. Bob receives the values (k’,c) and decrypts k’ using his private key
Bd to obtain k
k=pk.decryptBd(k’)

7. Bob uses the session key k to decrypt the ciphertext c and recover the
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message m
m=sk.decryptk(c)
Public and symmetric key cryptosystems are combined in this way to
provide security for key exchange and then efficiency for encryption. The
session key k is used only to encrypt message m and is not stored for
any length of time.

PGP Authentication and
Confidentiality (at the same time)
The schemes for authentication and confidentiality can be combined
so that Alice can sign a confidential message which is encrypted
before transmission. The steps required are as follows:
1. Alice generates a signature c for her message m as in the
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Authentication scheme
c=pk.encryptAd(SHA(m))
2. Alice generates a random session key k and encrypts the message
m and the signature c using a symmetric cryptosystem to obtain
ciphertext C
C=sk.encryptk(m,c)
4. She encrypts the session key k using Bob’s public key
k’ = pk.encryptBe(k)
5. Alice sends Bob the values (k’,C)

6. Bob recieves k’ and C and decrypts k’ using his private key Bd to
obtain the session key k
k=pk.decryptBd(k’)
7. Bob decrypts the ciphertext C using the session key k to obtain m
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and c
(m,c) = sk.decryptk(C)
8. Bob now has the message m. In order to authenticate it he uses
Alice’s public key Ae to decrypt the signature c and hashes the
message m using SHA-1.
If SHA(m) = pk.decryptAe(c)
Then the message is authenticated.

Mr. Gopal Sakarkar
Working flow of PGP

Tutorial-7
Last date of submission: 1/10/2013
Explain the detail working of PGP
encryption and authentication algorithm
and its real life applications.
Mr. Gopal Sakarkar

S/MIME is the de-facto industry standard for secure mail over the
Internet. Secure MIME (S/MIME) was developed by an industry
consortium, and is now appearing in a number of major
products.
MIME is an extencion to the RFC 822 addressing many limitations
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of the use of SMPT.
MIME specification includes
– new message headers
– a number of content formats supproting multimedia
electronic mail
– transfer encodings
S/MIME

S/MIME Functionality (messages)
The general functionality of S/MIME is very similar to PGP buth offering the
ability to sign and/or encrypt messages.
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S/MIME Functions
The S/MIME functions are implemented as new MIME content types.
Enveloped data
– This consists of encrypted content of any type and encrypted
content encryption keys for one or more receipients.
– An enveloped data entity is prepared as follows: 1) Generate the
pseudo random session key. 2) Encrypt the session key with each
recipients public RSA key. 3) For each recipient prepare a
RecipientInfo block containing senders public key certifcate, an
identifier of the encryption algorithm and the encrypted session key.
4) Encrypt the message content with the session key.

S/MIME Functionality
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Signed data
A digital signature is formed by taking the message digest of the content to be
signed and encrypting that with the private key of the signer.
1) Compute the message digest with SHA or MD5.
2) Encrypt the message digest with senders private key
3) prepare SignerInfo block containing singer’s public key certificate, an
identifier of the message digest algorithm, and identifier of the encryption
algorithm and the encrypted message digest.
A signed data message can only be read by a recipient having S/MIME
capabilities
Clear signed data
Same as previous but now the message contents are readable without
S/MIME, which is needed if the recipient wishes to verify the identity if the
sender.
Signed and enveloped data
Signed-only and encrypted-only messages can be nested in both orderings.

S/MIME Functionality
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Registration request
An application or a user typically applies to a CA for a public-key certificate.
This content format is used to transfer such request.
Certificates-only message
This is a message containing only certificates or a certificate revocation list.
It is sent as a response to registration request

Public Key Cryptography Explained

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