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Chapter: Cryptography and Network Security

Direct Digital Signatures

1. Arbitrated Digital Signatures 2. Replay Attacks



·        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


1. Arbitrated Digital Signatures


·        involves use of arbiter A


o   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


Authentication Protocols

·        used to convince parties of each others identity and to exchange session keys


·        may be one-way or mutual


·        key issues are

o     confidentiality – to protect session keys


o     timeliness – to prevent replay attacks


·        published protocols are often found to have flaws and need to be modified


2. Replay Attacks


where a valid signed message is copied and later resent

o     simple replay


o     repetition that can be logged


o     repetition that cannot be detected


o     backward replay without modification


countermeasures include

o     use of sequence numbers (generally impractical)


o     timestamps (needs synchronized clocks)


o     challenge/response (using unique nonce)


Using Symmetric Encryption


o   as discussed previously can use a twolevel hierarchy of keys


o   usually with a trusted Key Distribution Center (KDC)

o     each party shares own master key with KDC


o     KDC generates session keys used for connections between parties


o     master keys used to distribute these to them


·        can refine use of KDC but can‟t have final exchange of nonces, vis:

o     A->KDC: IDA || IDB || N1


o     KDC -> A: EKa [Ks || IDB || N1 || EKb [Ks||IDA] ]


o     A -> B: EKb [Ks||IDA] || EKs [M]


o   does not protect against replays could rely on timestamp in message, though email delays make this problematic


Using Public-Key Encryption


o   have a range of approaches based on the use of public-key encryption


o   need to ensure have correct public keys for other parties


o   using a central Authentication Server (AS)


o   various protocols exist using timestamps or nonces


o   if confidentiality is major concern, can use:


A->B: EPUb [Ks] || EKs [M]


has encrypted session key, encrypted message

o   if authentication needed use a digital signature with a digital certificate:


A->B: M || EPRa [H(M)] || EPRas [T||IDA||PUa]


with message, signature, certificate


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