Other public key systems

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  • Lecture slides by Lawrie Brown for “Cryptography and Network Security”, 5/e, by William Stallings, Chapter 10 – “Other Public Key Cryptosystems”.
  • This chapter continues our overview of public-key cryptography systems (PKCSs), and begins with a description of one of the earliest and simplest PKCS, Diffie-Hellman key exchange. This first published public-key algorithm appeared in the seminal paper by Diffie and Hellman that defined public-key cryptography [DIFF76b] and is generally referred to as Diffie-Hellman key exchange. The concept had been previously described in a classified report in 1970 by Williamson (UK CESG) - and subsequently declassified in 1987, see [ELLI99]. The purpose of the algorithm is to enable two users to securely exchange a key that can then be used for subsequent encryption of messages. The algorithm itself is limited to the exchange of secret values. A number of commercial products employ this key exchange technique.
  • The purpose of the algorithm is to enable two users to securely exchange a key that can then be used for subsequent encryption of messages. The algorithm itself is limited to the exchange of secret values, which depends on the value of the public/private keys of the participants. The Diffie-Hellman algorithm uses exponentiation in a finite (Galois) field (modulo a prime or a polynomial), and depends for its effectiveness on the difficulty of computing discrete logarithms.
  • In the Diffie-Hellman key exchange algorithm, there are two publicly known numbers: a prime number q and an integer a that is a primitive root of q. The prime q and primitive root a can be common to all using some instance of the D-H scheme. Note that the primitive root a is a number whose powers successively generate all the elements mod q. Users Alice and Bob choose random secrets x's, and then "protect" them using exponentiation to create their public y's. For an attacker monitoring the exchange of the y's to recover either of the x's, they'd need to solve the discrete logarithm problem, which is hard.
  • The actual key exchange for either party consists of raising the others "public key' to power of their private key. The resulting number (or as much of as is necessary) is used as the key for a block cipher or other private key scheme. For an attacker to obtain the same value they need at least one of the secret numbers, which means solving a discrete log, which is computationally infeasible given large enough numbers. Note that if Alice and Bob subsequently communicate, they will have the same key as before, unless they choose new public-keys.
  • Here is an example of Diffie-Hellman from the text using prime q=353, showing how each computes its public key, and then how after they exchange public keys, each can compute the common secret key.I n this simple example, it would be possible by brute force to determine the secret key 160. In particular, an attacker E can determine the common key by discovering a solution to the equation 3a mod 353 = 40 or the equation 3b mod 353 = 248. The brute-force approach is to calculate powers of 3 modulo 353, stopping when the result equals either 40 or 248. The desired answer is reached with the exponent value of 97, which provides 397 mod 353 = 40. With larger numbers, the problem becomes impractical.
  • Now consider a simple protocol that makes use of the Diffie-Hellman calculation. Suppose that user A wishes to set up a connection with user B and use a secret key to encrypt messages on that connection. User A can generate a one-time private key XA, calculate YA, and send that to user B. User B responds by generating a private value XB, calculating YB, and sending YB to user A. Both users can now calculate the key. The necessary public values q and a would need to be known ahead of time. Alternatively, user A could pick values for q and a and include those in the first message.
  • The protocol described on the previous slide is insecure against a man-in-the-middle attack. Suppose Alice and Bob wish to exchange keys, and Darth is the adversary. The attack proceeds as follows:
    Darth prepares for the attack by generating two random private keys XD1 and XD2 and then computing the corresponding public keys YD1 and YD2
    Alice transmits YA to Bob.
    Darth intercepts YA and transmits YD1 to Bob. Darth also calculates K2 = (YA )^ XD2 mod q
    Bob receives YD1 and calculates K1=(YD1 )^ XB mod q
    Bob transmits YB to Alice.
    Darth intercepts YB and transmits YD2 to Alice. Darth calculates K1=(YB )^ XD1 mod q
    Alice receives YD2 and calculates K2=(YD2 )^ XA mod q .
    At this point, Bob and Alice think that they share a secret key, but instead Bob and Darth share secret key K1 and Alice and Darth share secret key K2. All future communication between Bob and Alice is compromised in the following way:
    Alice sends an encrypted message M: E(K2, M).
    Darth intercepts the encrypted message and decrypts it, to recover M.
    Darth sends Bob E(K1, M) or E(K1, M'), where M' is any message. In the first case, Darth simply wants to eavesdrop on the communication without altering it. In the second case, Darth wants to modify the message going to Bob. The key exchange protocol is vulnerable to such an attack because it does not authenticate the participants. This vulnerability can be overcome with the use of digital signatures and public- key certificates.
  • In 1984, T. Elgamal announced a public-key scheme based on discrete logarithms, closely related to the Diffie-Hellman technique [ELGA84, ELGA85]. The ElGamal cryptosystem is used in some form in a number of standards including the digital signature standard (DSS) and the S/MIME email standard. As with Diffie-Hellman, the global elements of ElGamal are a prime number q and a, which is a primitive root of q. User A generates a private/public key pair as shown. The security of ElGamal is based on the difficulty of computing discrete logarithms, to recover either x given y, or k given K (next slide).
  • Any user B that has access to A's public key can encrypt a message as shown. These steps correspond to Figure 9.1a in that Alice generates a public/private key pair; Bob encrypts using Alice's public key; and Alice decrypts using her private key. See text for details of why these steps result in M being recovered. Note that K functions as a one-time key, used to encrypt and decrypt the message. If a message must be broken up into blocks and sent as a sequence of encrypted blocks, a unique value of k should be used for each block. If k is used for more than one block, knowledge of one block m of the message enables the user to compute other blocks as shown in the text.
    The basic idea with El Gamal encryption is to choose a random key, protect it, then use it to scramble the message by multiplying the message with it. Two bits of info have to be sent: the first to recover this temporary key, the second the actual scrambled message. See that El Gamal encryption involves 1 modulo exponentiation and a multiplication (vs 1 exponentiation for RSA).
  • Here is an example of ElGamal from the text using the prime field GF(19); that is, q = 19. It has primitive roots {2, 3, 10, 13, 14, 15}, as shown in Table 8.3. We choose a = 10. Alice generates a key pair as shown. Suppose Bob wants to send the message with the value M = 17. Then he computes the ciphertext pair (11, 5) and sends this to Alice. Alice recovers the message by first recovering K, then computing its inverse (using the Extended Euclids Algorithm – see Ch 4), and finally recovering M.
  • 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.

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