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# Other public key systems

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### Other public key systems

1. 1. Other Public Key Systems
2. 2. Outline  Primitive Element Theorem  Diffie Hellman Key Distribution  ElGamal Encryption  ElGamal Digital Signatures
3. 3. Discrete Logarithm(s) (DLs)  Fix a prime p. Let a, b be nonzero integers (mod p). The problem of finding x such that ax ≡ b (mod p) is called the discrete logarithm problem. Suppose that n is the smallest integer such that an ≡1 (mod p), i.e., n=ordp(a). By assuming 0≤x<n, we denote x=La(b), and call it the discrete log of b w.r.t. a (mod p)  Ex: p=11, a=2, b=9, then x=L2(9)=6
4. 4. Discrete Logarithms  In the RSA algorithms, the difficulty of factoring a large integer yields good cryptosystems  In the ElGamal method, the difficulty of solving the discrete logarithm problem yields good cryptosystems  Given p, a, b, solve ax ≡ b (mod p)  a is suggested to be a primitive root mod p
5. 5. One-Way Function  A function f(x) is called a one-way function if f(x) is easy to compute, but, given y, it is computationally infeasible to find x with y=f(x).  La(b) is a one-way function if p is large
6. 6. Primitive Roots mod 13  The primitive root a is a number whose powers successively generate all the elements mod p.
7. 7. Example  p = 11; a = 2.  22 = 4, 23 = 8, 24 = 5, 25 = 10, 26 = 9, 27 = 7, 28 = 3, 29 = 6, 210 = 1 (mod 11)  32 = 9, 33 = 5, 34 = 4, 35 = 1 (mod 11)  The residue (2 mod 11) can create all non-zero residues mod 11 via exponentiation. It is called a generator.  The residue (3 mod 11) does not have the same property.
8. 8. 88 Public Key Distribution  The goal is for two users to securely exchange a key over an insecure channel. The key is then used in a normal cryptosystem
9. 9. Diffie-Hellman Key Exchange  first public-key type scheme proposed  by Diffie & Hellman in 1976 along with the exposition of public key concepts  note: now know that Williamson (UK CESG) secretly proposed the concept in 1970  is a practical method for public exchange of a secret key  used in a number of commercial products
10. 10. Diffie-Hellman Key Exchange  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 exponentiation in a finite (Galois) field (modulo a prime or a polynomial) - easy  security relies on the difficulty of computing discrete logarithms (similar to factoring) – hard
11. 11. Diffie-Hellman Setup  all users agree on global parameters:  large prime integer or polynomial q  a being a primitive root mod q  each user (eg. A) generates their key  chooses a secret key (number): xA < q  compute their public key: yA = a xA mod q  each user makes public that key yA
12. 12. Diffie-Hellman Key Exchange  shared session key for users A & B is KAB: KAB = a xA.xB mod q = yA xB mod q (which B can compute) = yB xA mod q (which A can compute)  KAB is used as session key in private-key encryption scheme between Alice and Bob  if Alice and Bob subsequently communicate, they will have the same key as before, unless they choose new public-keys  attacker needs an x, must solve discrete log
13. 13. Diffie-Hellman Key Exchange Bob B Bob B Alice A Alice A Public Parameters: large prime q primitive root a Choose a secret XA Compute YA = a XA mod q Send YA Choose a secret XB Compute YB = a XB mod q Send YB Shared Key KAB = YB XA = a X B XA mod q Shared Key KAB = YA X B = a X A XB mod q
14. 14. Diffie-Hellman Example  users Alice & Bob who wish to swap keys:  agree on prime q=353 and a=3  select random secret keys:  A chooses xA=97, B chooses xB=233  compute respective public keys:  yA=3 97 mod 353 = 40 (Alice)  yB=3 233 mod 353 = 248 (Bob)  compute shared session key as:  KAB= yB xA mod 353 = 248 97 = 160 (Alice)  KAB= yA xB mod 353 = 40 233 = 160 (Bob)
15. 15. Key Exchange Protocols  users could create random private/public D-H keys each time they communicate  users could create a known private/public D-H key and publish in a directory, then consulted and used to securely communicate with them  both of these are vulnerable to a meet-in- the-Middle Attack  authentication of the keys is needed
16. 16. Man-in-the-Middle Attack Man-in-the-Middle Attack Alice A Alice A Bob B Bob B Darth (Attacker) Darth (Attacker) Man-in-the-middle attacker shared a key between Alice and Bob.
17. 17. Man-in-the-Middle Attack 1. Darth prepares by creating two private / public keys 2. Alice transmits her public key to Bob 3. Darth intercepts this and transmits his first public key to Bob. Darth also calculates a shared key with Alice 4. Bob receives the public key and calculates the shared key (with Darth instead of Alice) 5. Bob transmits his public key to Alice 6. Darth intercepts this and transmits his second public key to Alice. Darth calculates a shared key with Bob 7. Alice receives the key and calculates the shared key (with Darth instead of Bob)  Darth can then intercept, decrypt, re-encrypt, forward all messages between Alice & Bob
18. 18. ElGamal Cryptography  public-key cryptosystem related to D-H  so uses exponentiation in a finite (Galois)  with security based difficulty of computing discrete logarithms, as in D-H  each user (eg. A) generates their key  chooses a secret key (number): 1 < xA < q-1  compute their public key: yA = a xA mod q
19. 19. ElGamal Message Exchange  Bob encrypt a message to send to A computing  represent message M in range 0 <= M <= q-1 • longer messages must be sent as blocks  chose random integer k with 1 <= k <= q-1  compute one-time key K = yA k mod q  encrypt M as a pair of integers (C1,C2) where • C1 = a k mod q ; C2 = KM mod q  A then recovers message by  recovering key K as K = C1 xA mod q  computing M as M = C2 K-1 mod q  a unique k must be used each time  otherwise result is insecure
20. 20. ElGamal Crypto System Bob B Bob B Alice A Alice A Public Parameters: large prime q primitive root a Choose a secret XA Compute YA = a XA mod q Send YA Choose a secret XB Compute K = YA XB mod q Compute YB = a XB mod q Send YB and C = K.M mod q Shared Key KAB = YB XA = a X B XA mod q Shared Key KAB = YA X B X A XB
21. 21. ElGamal Example  use field GF(19) q=19 and a=10  Alice computes her key:  A chooses xA=5 & computes yA=10 5 mod 19 = 3  Bob send message m=17 as (11,5) by  chosing random k=6  computing K = yA k mod q = 3 6 mod 19 = 7  computing C1 = a k mod q = 10 6 mod 19 = 11; C2 = KM mod q = 7.17 mod 19 = 5  Alice recovers original message by computing:  recover K = C1 xA mod q = 11 5 mod 19 = 7  compute inverse K-1 = 7-1 = 11  recover M = C2 K-1 mod q = 5.11 mod 19 = 17
22. 22. 2222 ElGamal Digital Signature  Zp* = <g>, m ∈ Zp message  A signs message m.  Alice: A = ga , public key = (p, g,A), secret key = x.  Alice: k random with gcd(k,p-1)=1  r = gk (mod p)  s = (m – xr)(1/k) mod p-1 [m = sk + xr (mod p-1)]  Signature = (r,s)  Verify gm =rs hr
23. 23. 2323 ElGamal Public Key Cryptosystem and Digital Signature Scheme  Example 1. P=23, g=5. 2. x=3, then y=10 (for 53 mod 23=10 ). 3. Sign for the message M=8. 4. Select k=5 between 1 and 22 (P-1). 5. Compute r = gk mod P = 55 mod 23 = 20. 6. Compute s = k-1 (M-xr) mod (P-1) = 5-1 (8-3×20) mod 22 = 9×14 mod 22 = 16. 7. Verification: gM = 58 mod 23 =16 (rs )(yr ) mod P = 2016 × 1020 mod 23= 13×3 mod 23 = 16.