Unit --3.ppt

Cryptography and
Network Security

Private-Key Cryptography
 traditional private/secret/single key
cryptography uses one key
 shared by both sender and receiver
 if this key is disclosed communications are
compromised
 also is symmetric, parties are equal
 hence does not protect sender from
receiver forging a message & claiming is
sent by sender

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

Why Public-Key
Cryptography?
 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 Uni in 1976
 known earlier in classified community

Public-Key Cryptography
 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

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)

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, but
is made hard enough to be impractical to break
 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: PU={e,n}
 keep secret private decryption key: PR={d,n}

RSA Use
 to encrypt a message M the sender:
 obtains public key of recipient PU={e,n}
 computes: C = Me mod n, where 0≤M<n
 to decrypt the ciphertext C the owner:
 uses their private key PR={d,n}
 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 chose 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))k
= M1.(1)k = M1 = M mod n

RSA Example - Key Setup
1. Select primes: p=17 & q=11
2. Compute n = pq =17 x 11=187
3. Compute ø(n)=(p–1)(q-1)=16 x 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 23x7=161= 10x160+1
6. Publish public key PU={7,187}
7. Keep secret private key PR={23,187}

RSA Example - En/Decryption
 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

Exponentiation
c = 0; f = 1
for i = k downto 0
do c = 2 x c
f = (f x f) mod n
if bi == 1 then
c = c + 1
f = (f x a) mod n
return f

Efficient Encryption
 encryption uses exponentiation to power e
 hence if e small, this will be faster
 often choose e=65537 (216-1)
 also see choices of e=3 or e=17
 but if e too small (eg e=3) can attack
 using Chinese remainder theorem & 3
messages with different modulii
 if e fixed must ensure gcd(e,ø(n))=1
 ie reject any p or q not relatively prime to e

Efficient Decryption
 decryption uses exponentiation to power d
 this is likely large, insecure if not
 can use the Chinese Remainder Theorem
(CRT) to compute mod p & q separately.
then combine to get desired answer
 approx 4 times faster than doing directly
 only owner of private key who knows
values of p & q can use this technique

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
 possible approaches to attacking RSA are:
 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)
 chosen ciphertext attacks (given properties of
RSA)

Factoring Problem
 mathematical approach takes 3 forms:
 factor n=p.q, hence compute ø(n) and then d
 determine ø(n) directly and compute d
 find d directly
 currently believe all equivalent to factoring
 have seen slow improvements over the years
• as of May-05 best is 200 decimal digits (663) bit with LS
 biggest improvement comes from improved algorithm
• cf QS to GHFS to LS
 currently assume 1024-2048 bit RSA is secure
• ensure p, q of similar size and matching other constraints

Timing Attacks
 developed by Paul Kocher in mid-1990’s
 exploit timing variations in operations
 eg. multiplying by small vs large number
 or IF's varying which instructions executed
 infer operand size based on time taken
 RSA exploits time taken in exponentiation
 countermeasures
 use constant exponentiation time
 add random delays
 blind values used in calculations

Chosen Ciphertext Attacks
• RSA is vulnerable to a Chosen Ciphertext
Attack (CCA)
• attackers chooses ciphertexts & gets
decrypted plaintext back
• choose ciphertext to exploit properties of
RSA to provide info to help cryptanalysis
• can counter with random pad of plaintext
• or use Optimal Asymmetric Encryption
Padding (OASP)

Summary
 have considered:
 principles of public-key cryptography
 RSA algorithm, implementation, security

Unit --3.ppt

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Unit --3.ppt