Cryptography and Cryptography and Network Security Network Security Chapter 13 Chapter 13 Fourth Edition Fourth Edition by William Stallings by William Stallings Lecture slides by Lawrie Brown Lecture slides by Lawrie Brown Chapter 13 – Chapter 13 – Digital Signatures & Digital Signatures & Authentication Protocols Authentication Protocols To guard against the baneful influence exerted by strangers To guard against the baneful influence exerted by strangers is therefore an elementary dictate of savage prudence. is therefore an elementary dictate of savage prudence. Hence before strangers are allowed to enter a district, or Hence before strangers are allowed to enter a district, or at least before they are permitted to mingle freely with at least before they are permitted to mingle freely with the inhabitants, certain ceremonies are often performed the inhabitants, certain ceremonies are often performed by the natives of the country for the purpose of disarming by the natives of the country for the purpose of disarming the strangers of their magical powers, or of disinfecting, the strangers of their magical powers, or of disinfecting, so to speak, the tainted atmosphere by which they are so to speak, the tainted atmosphere by which they are supposed to be surrounded. supposed to be surrounded. — — The Golden Bough The Golden Bough , Sir James George Frazer , Sir James George Frazer Digital Signatures Digital Signatures  have looked at have looked at message authentication message authentication  but does not address issues of lack of trust but does not address issues of lack of trust  digital signatures provide the ability to: digital signatures provide the ability to:  verify author, date & time of signature verify author, date & time of signature  authenticate message contents authenticate message contents  be verified by third parties to resolve disputes be verified by third parties to resolve disputes  hence include authentication function with hence include authentication function with additional capabilities additional capabilities Digital Signature Properties Digital Signature Properties  must depend on the message signed must depend on the message signed  must use information unique to sender must use information unique to sender  to prevent both forgery and denial to prevent both forgery and denial  must be relatively easy to produce must be relatively easy to produce  must be relatively easy to recognize & verify must be relatively easy to recognize & verify  be computationally infeasible to forge be computationally infeasible to forge  with new message for existing digital signature with new message for existing digital signature  with fraudulent digital signature for given message with fraudulent digital signature for given message  be practical save digital signature in storage be practical save digital signature in storage Direct Digital Signatures Direct Digital Signatures  involve only sender & receiver involve only sender & receiver  assumed receiver has sender’s public-key assumed receiver has sender’s public-key  digital signature made by sender signing digital signature made by sender signing entire message or hash with private-key entire message or hash with private-key  can encrypt using receivers public-key can encrypt using receivers public-key  important that sign first then encrypt important that sign first then encrypt message & signature message & signature  security depends on sender’s private-key security depends on sender’s private-key Arbitrated Digital Signatures Arbitrated Digital Signatures  involves use of arbiter A involves use of arbiter A  validates any signed message validates any signed message  then dated and sent to recipient then dated and sent to recipient  requires suitable level of trust in arbiter requires suitable level of trust in arbiter  can be implemented with either private or can be implemented with either private or public-key algorithms public-key algorithms  arbiter may or may not see message arbiter may or may not see message Authentication Protocols Authentication Protocols  used to convince parties of each others used to convince parties of each others identity and to exchange session keys identity and to exchange session keys  may be one-way or mutual may be one-way or mutual  key issues are key issues are  confidentiality – to protect session keys confidentiality – to protect session keys  timeliness – to prevent replay attacks timeliness – to prevent replay attacks  published protocols are often found to published protocols are often found to have flaws and need to be modified have flaws and need to be modified Replay Attacks Replay Attacks  where a valid signed message is copied and where a valid signed message is copied and later resent later resent  simple replay simple replay  repetition that can be logged repetition that can be logged  repetition that cannot be detected repetition that cannot be detected  backward replay without modification backward replay without modification  countermeasures include countermeasures include  use of sequence numbers (generally impractical) use of sequence numbers (generally impractical)  timestamps (needs synchronized clocks) timestamps (needs synchronized clocks)  challenge/response (using unique nonce) challenge/response (using unique nonce) Using Symmetric Encryption Using Symmetric Encryption  as discussed previously can use a two- as discussed previously can use a two- level hierarchy of keys level hierarchy of keys  usually with a trusted Key Distribution usually with a trusted Key Distribution Center (KDC) Center (KDC)  each party shares own master key with KDC each party shares own master key with KDC  KDC generates session keys used for KDC generates session keys used for connections between parties connections between parties  master keys used to distribute these to them master keys used to distribute these to them Needham-Schroeder Protocol Needham-Schroeder Protocol  original third-party key distribution protocol original third-party key distribution protocol  for session between A B mediated by KDC for session between A B mediated by KDC  protocol overview is: protocol overview is: 1. 1. A->KDC: A->KDC: ID ID A A || || ID ID B B || || N N 1 1 2 2 . KDC -> . KDC -> A: E A: E Ka Ka [Ks [Ks || || ID ID B B || || N N 1 1 || E || E Kb Kb [ [ Ks Ks || || ID ID A A ] ] ] ] 3. 3. A -> A -> B: B: E E Kb Kb [ [ Ks Ks || || ID ID A A ] ] 4. 4. B -> B -> A: A: E E Ks Ks [ [ N N 2 2 ] ] 5. 5. A -> A -> B: B: E E Ks Ks [f( [f( N N 2 2 )] )] [...]... security  smaller and faster than RSA  a digital signature scheme only  security depends on difficulty of computing discrete logarithms  variant of ElGamal & Schnorr schemes Digital Signature Algorithm (DSA) DSA Key Generation  have shared global public key values (p,q,g):   choose q, a 160 bit choose a large prime p = 2L • where L= 512 to 1024 bits and is a multiple of 64 • and q is a prime factor... 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 Digital Signature Algorithm (DSA)  creates a 320 bit signature  with 512-1024 bit security. .. p) > 1  users choose private & compute public key:   choose x . Cryptography and Cryptography and Network Security Network Security Chapter 13 Chapter 13 Fourth Edition Fourth Edition by. 320 bit signature  with 512-1024 bit security with 512-1024 bit security  smaller and faster than RSA smaller and faster than RSA  a digital signature