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Cryptography and Network Security Chapter 13

Cryptography and Network Security Chapter 13. Fourth Edition by William Stallings Lecture slides by Lawrie Brown & Süleyman KONDAKCI. Digital Signatures (DS). M essage authentication does not address issues of lack of trust digital signatures provide the ability to:

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Cryptography and Network Security Chapter 13

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  1. Cryptography and Network SecurityChapter 13 Fourth Edition by William Stallings Lecture slides by Lawrie Brown & Süleyman KONDAKCI

  2. Digital Signatures (DS) • Message authentication does not address issues of lack of trust • digital signatures provide the ability to: • verify author, date & time of signature • authenticate message contents • be verified by third parties to resolve disputes • DS includes message authentication function with additional capabilities

  3. Digital Signature Properties • must depend on the message signed • must use information unique to sender • to prevent both forgery and denial • must be relatively easy to produce • must be relatively easy to recognize & verify • be computationally infeasible to forge • with new message for existing digital signature • with fraudulent digital signature for given message • be practical to save digital signature in storage

  4. Direct Digital Signatures • involve only sender & receiver • assumed receiver has sender’s public-key • digital signature made by sender signing entire message or hash with private-key • can encrypt using receivers public-key • important that sign first then encrypt message & signature • security depends on sender’s private-key

  5. Digital Signature Model

  6. Digital Signature Model

  7. Arbitrated Digital Signatures • involves use of an arbiter who • validates any signed message • then dated and sent to recipient • requires suitable level of trust in arbiter • can be implemented with either private or public-key algorithms • arbiter may or may not see message

  8. Authentication Protocols • used to convince parties of each others identity and to exchange session keys • may be one-way or mutual • key issues are • confidentiality – to protect session keys • timeliness – to prevent replay attacks • published protocols are often found to have flaws and need to be modified

  9. Using Symmetric Encryption • Can use a two-level hierarchy of keys • usually with a trusted Key Distribution Center (KDC) • each party shares own master key with KDC • KDC generates session keys used for connections between parties • master keys used to distribute these to them

  10. Needham-Schroeder Protocol • Used by 2 parties who both trust a common key server • original third-party key distribution protocol • for session between A B mediated by KDC • protocol overview is: 1. A->KDC: IDA|| IDB|| N1 2. KDC ->A: EKa[Ks|| IDB|| N1 || EKb[Ks||IDA] ] 3. A ->B: EKb[Ks||IDA] 4. B ->A: EKs[N2] 5. A ->B: EKs[f(N2)] f(N2) is a function making some changes on N2 so that owner of N2 can verify the correct receipt of N2, e.g., F(23456)= 23456+1. That 1 is the verifier code.

  11. Needham-Schroeder Protocol • used to securely distribute a new session key for communications between A & B • but is vulnerable to a replay attack if an old session key has been compromised • then message 3 can be resent convincing B that is communicating with A • modifications to address this require: • timestamps • using an extra nonce

  12. Using Public-Key Encryption • have a range of approaches based on the use of public-key encryption • need to ensure have correct public keys for other parties • using a central Authentication Server (AS) • various protocols exist using timestamps or nonces

  13. Denning Authentication Server Protocol • Denning presented the following: 1. A ->AS: IDA|| IDB 2. AS ->A: EPRas[IDA||PUa||T] || EPRas[IDB||PUb||T] 3. A ->B: EPRas[IDA||PUa||T] || EPRas[IDB||PUb||T] || EPUb[EPRa[Ks||T]] • Note that to avoid the risk of exposure by the AS session key is chosen by A, hence AS need not be trusted to protect it • timestamps prevent replay attacks but require synchronized clocks

  14. One-Way Authentication • The recipient wants some assurance that the message is from the alleged sender. One-Way Authentication addresses these requirements. • Required when sender & receiver communicate in connectionless mode (eg. email) • Have header in clear text so can be delivered by email systems • May want contents of body protected & sender authenticated

  15. One-Way Authentication Using Symmetric Encryption • can refine use of KDC but can’t have final exchange of nonces: 1. A->KDC: IDA|| IDB|| N1 2. KDC -> A: EKa[Ks|| IDB|| N1 || EKb[Ks||IDA] ] 3. A -> B: EKb[Ks||IDA] || EKs[M] • does not protect against replays • could rely on timestamp in message, though email delays make this problematic

  16. One-Way Authentication Using Public-Key Approaches • have seen some public-key approaches • if confidentiality is major concern, can use: A->B: EPUb[Ks] || EKs[M] • has encrypted session key, encrypted message • if authentication needed use a digital signature with a digital certificate: A->B: M || EPRa[H(M)] || EPRa[T||IDA||PUa] • with message, signature, certificate

  17. Digital Signature Standard (DSS) • US Govt approved signature scheme • designed by NIST & NSA in early 90's • published as FIPS-186 in 1991 • revised in 1993, 1996 & then 2000 • uses the SHA hash algorithm • DSS is the standard, DSA is the algorithm • FIPS 186-2 (2000) includes alternative RSA & elliptic curve signature variants

  18. ElGamal Digital Signatures signature variant of ElGamal, related to D-H so uses exponentiation in a finite (Galois) with security based difficulty of computing discrete logarithms, as in D-H use private key for encryption (signing) uses public key for decryption (verification) each user (eg. A) generates their key chooses a secret key (number): 1 < xA < q-1 compute their public key: yA = axA mod q

  19. ElGamal Digital Signature Alice signs a message M to Bob by computing the hash m = H(M), 0 <= m <= (q-1) chose random integer K with 1 <= K <= (q-1) and gcd(K,q-1)=1 compute temporary key: S1 = ak mod q compute K-1the inverse of K mod (q-1) compute the value: S2 = K-1(m-xAS1) mod (q-1) signature is:(S1,S2) any user B can verify the signature by computing V1 = am mod q V2 = yAS1S1S2 mod q signature is valid if V1 = V2

  20. ElGamalSignature Example use field GF(19) q=19 and a=10 Alice computes her key: A chooses xA=16 & computes yA=1016 mod 19 = 4 Alice signs message with hash m=14 as (3,4): choosing random K=5 which has gcd(18,5)=1 computing S1 = 105 mod 19 = 3 finding K-1 mod (q-1) = 5-1 mod 18 = 11 computing S2 = 11(14-16.3) mod 18 = 4 any user B can verify the signature by computing V1 = 1014 mod 19 = 16 V2 = 43.34 = 5184 = 16 mod 19 since 16= 16signature is valid

  21. Schnorr Digital Signatures also uses exponentiation in a finite (Galois) security based on discrete logarithms, as in D-H minimizes message dependent computation multiplying a 2n-bit integer with an n-bit integer main work can be done in idle timeusing a prime modulus p p–1 has a prime factor qof appropriate size typicallyp1024-bit and q 160-bit numbers

  22. Schnorr Key Setup choose suitable primes p, q choose asuch that aq = 1 mod p (a,p,q) are global parameters for all each user (eg. A) generates a key chooses a secret key (number): 0 < sA < q compute their public key: vA = a-sA mod q

  23. Schnorr Signature user signs message by choosing random r with 0<r<q and computing x = ar mod p concatenate message with x and hash result to computing: e = H(M || x) computing: y = (r + se) mod q signature is pair (e, y) any other user can verify the signature as follows: computing: x' = ayve mod p verifying that: e = H(M || x’)

  24. Digital Signature Algorithm (DSA) • creates a 320 bit signature • with 512-1024 bit security • smaller and faster than RSA • a digital signature scheme only • security depends on difficulty of computing discrete logarithms • variant of ElGamal & Schnorr schemes

  25. Digital Signature Algorithm (DSA)

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

  27. DSA Signature Creation to sign a message M the sender: generates a random signature key k, k<q NB!k must be random, be destroyed after use, and never be reused 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

  28. 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 = (gu1yu2(mod p)) (mod q) • if v=r then signature is verified • see book web sitehe text for details!

  29. DSS Overview

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