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Chapter: Cryptography and Network Security

Direct Digital Signatures

1. Arbitrated Digital Signatures 2. Replay Attacks

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

 

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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