Presentation on theme: "Cryptography and Network Security"— Presentation transcript:
1 Cryptography and Network Security Third Editionby William StallingsLecture slides by Lawrie Brown
2 Chapter 13 –Digital Signatures & Authentication Protocols To guard against the baneful influence exerted by strangers is therefore an elementary dictate of savage prudence. Hence before strangers are allowed to enter a district, or at least before they are permitted to mingle freely with the inhabitants, certain ceremonies are often performed by the natives of the country for the purpose of disarming the strangers of their magical powers, or of disinfecting, so to speak, the tainted atmosphere by which they are supposed to be surrounded.—The Golden Bough, Sir James George Frazer
3 Digital Signatures have looked at message authentication but does not address issues of lack of trustMary may forge a message and claim it came from JohnJohn can deny sending a meesagedigital signatures provide the ability to:verify author, date & time of signatureauthenticate message contentsbe verified by third parties to resolve disputeshence include authentication function with additional capabilities
4 Digital Signature Properties must depend on the message being signedmust use information unique to senderto prevent both forgery and denialmust be relatively easy to producemust be relatively easy to recognize & verifybe computationally infeasible to forgewith new message for existing digital signaturewith fraudulent digital signature for given messagebe practical save a copy of the digital signature in storage
5 Direct Digital Signatures involve only sender & receiverassumed receiver has sender’s public-keydigital signature made by sender signing entire message or hash with private-keycan further encrypt using receivers public-keyimportant that sign first then encrypt message & signaturesecurity depends on sender’s private-keyHave problems if lost/stolenDirect Digital Signatures involve the direct application of public-key algorithms. But are dependent on security of the sender’s private-key. Have problems if lost/stolen and signatures forged. Need time-stamps and timely key revocation.
6 Arbitrated Digital Signatures involves use of arbiter Avalidates any signed messagethen dated and sent to recipientrequires a great deal of trust in arbitercan be implemented with either private or public-key algorithmsarbiter may or may not see messageSee Stallings Table 13-1 for various alternatives.
8 Authentication Protocols used to convince parties of each others identity and to exchange session keysmay be one-way or mutualkey issues areconfidentiality – to protect session keystimeliness – to prevent replay attacks
9 Replay Attacks where a valid signed message is copied and later resent simple replayrepetition that can be loggedrepetition that cannot be detectedbackward replay without modificationcountermeasures includeuse of sequence numbers (generally impractical)timestamps (needs synchronized clocks)challenge/response (using unique nonce)
10 Using Symmetric Encryption as discussed previously can use a two-level hierarchy of keysusually with a trusted Key Distribution Center (KDC)each party shares own master key with KDCKDC generates session keys used for connections between partiesmaster keys used to distribute these to them
11 Needham-Schroeder Protocol original third-party key distribution protocolfor session between A B mediated by KDCprotocol overview is: Fig 7.91. A→KDC: IDA || IDB || N12. 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)]This is the original, basic key exchange protocol. Used by 2 parties who both trusted a common key server, it gives one party the info needed to establish a session key with the other. Note that since the key server chooses the session key, it is capable of reading/forging any messages between A&B, which is why they need to trust it absolutely!Note that all communications is between A&KDC and A&B, B&KDC don't talk directly (though indirectly a message passes from KDC via A to B, encrypted in B's key so that A is unable to read or alter it). Other variations of key distribution protocols can involve direct communications between B&KDC.
12 Improvements to the Needham-Schroeder Protocol used to securely distribute a new session key for communications between A & BSecure even if Step 3 is replayedbut is vulnerable to a replay attack if an old session key has been compromisedthen message 3 can be resent convincing B that is communicating with Amodifications to address this require:timestamps (Denning 81) (clock sync. Issue)using an extra nonce (Neuman 93) (solves sync Issue)There is a critical flaw in the protocol, as shown. This emphasises the need to be extremely careful in codifying assumptions, and tracking the timeliness of the flow of info in protocols. Designing secure protocols is not easy, and should not be done lightly. Great care and analysis is needed.
13 One-Way Authentication required when sender & receiver are not in communications at same time (eg. )have header in clear so can be delivered by systemmay want contents of body protected & sender authenticatedThe receiver wants some assurance of the identity of the alleged sender
14 Using Symmetric Encryption can refine use of KDC but can’t have final exchange of nonces:1. A→KDC: IDA || IDB || N12. KDC→A: EKa[Ks || IDB || N1 || EKb[Ks||IDA] ]3. A→B: EKb[Ks||IDA] || EKs[M]Only the intended recipient can read itCertain level of authentication of Adoes not protect against replayscould rely on timestamp in message, though delays make this problematic
15 Public-Key Approaches have seen some public-key approachesif confidentiality is major concern, can use:A→B: EKUb[Ks] || EKs[M]has encrypted session key, encrypted messageMore efficient than simply EKUb[M]if authentication is the primary concernuse a digital signature with a digital certificate:A→B: M || EKRa[H(M)], problematicEncrypt everything using receiver’s public keyA→B: M || EKRa[H(M)] || EKRas[T||IDA||KUa]with message, signature, certificate
16 Digital Signature Standard (DSS) A public-key scheme for digital signature use only, combines hash and encryptiondesigned by NIST & NSA in early 90'sDSS is the standard, DSA is the algorithmBased on number theorysecurity depends on difficulty of computing discrete logarithmscreates a 320 bit signature, but with bit securityComputationally efficientDSA is the US Govt approved signature scheme - designed to provide strong signatures without allowing easy use for encryption. The signature scheme has advantages, being both smaller (320 vs 1024bit) and faster (much of the computation is done modulo a 160 bit number) than RSA.
17 Summary have considered: authentication protocols (mutual & one-way) digital signature standard