This document describes and analyzes three protocols for maintaining security in a simple network application between Alice and Bob. The objective is to authenticate that messages are from Alice and correspond to Bob's requests. Protocol 1 uses a shared symmetric key and MAC. Protocol 2 uses digital signatures instead of a shared key. Protocol 3 is also vulnerable to reflection attacks without additional tracking. The document analyzes each protocol's ability to provide data origin authentication, message freshness, and assurance messages correspond to requests, noting advantages and disadvantages of each approach.

1. Created by S.Gagan Kumar
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Analysis of a
Simple Protocol

2. Created by S.Gagan Kumar
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IN THIS TOPIC
▪ We will look at a simple application when cryptographic protocols can be used.
▪ We study the application and provide insights at the protocol design.
▪ Propose 7 different ways to establish security.
▪ Point out the advantages and disadvantages.
▪ Analyze complexities in protocol design.

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SIMPLE APPLICATION
NETWORK
ALICE BOB
Hey, its Bob, are you OK?
Hey Bob, its Alice, I’m OK.
OBJECTIVE : maintain security to assure Alice’s liveness

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PROTOCOL GOALS
Data Origin Authentication of Alice’s Reply
▪ Alice may not be alive but reply is sent by an attacker
Freshness of Alice’s Reply
▪ A previous reply can be resent by the attacker
Assurance that Alice’s reply corresponds to Bob’s request
▪ Bob receives a reply that corresponds to only his Alive requests

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PROTOCOL 1
ASSUMPTION :
▪ Bob has access to randomness to generate nonce RB.
▪ Alice and Bob share a symmetric key K
▪ Alice and Bob agree to use a strong MAC algorithm

6. Created by S.Gagan Kumar
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PROTOCOL 1
ALICE BOB
RB || It’s Bob, are you OK?
RB || Bob || Yes, I’m OK
MACk (RB || Bob || Yes, I’m OK )

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PROTOCOL 1
ANALYSIS :
▪ DATA ORIGIN AUTHENTICATION : Only Bob and Alice know the
MAC algorithm hence message is authenticated.
▪ FRESHNESS OF ALICE’s REPLY : The reply contains Nonce RB
which bob generates uniquely every time.
▪ ASSURANCE THAT ALICE’s REPLY CORRESPONDS : The reply
has Nonce RB corresponding to the request made and identifier
Bob is used.

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PROTOCOL 1
DISADVANTAGES :
▪ The shared key K must not be known to any third person.
▪ Bob must always check for the authentication of the message again
in order to maintain guarantee of origin.
▪ All the action must be performed in the same order to maintain
precision.

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PROTOCOL 2
ASSUMPTION :
▪ Bob has access to randomness to generate nonce RB.
▪ Alice must have the ability to generate a digital signature. Hence no
pre-shared key is required.
▪ Alice and Bob agree on use of a strong digital signature scheme.

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PROTOCOL 2
ALICE BOB
RB || It’s Bob, are you OK?
RB || Bob || Yes, I’m OK
SIGa (RB || Bob || Yes, I’m OK )

11. Created by S.Gagan Kumar
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PROTOCOL 2
ANALYSIS :
▪ DATA ORIGIN AUTHENTICATION : Alice is the only entity who can
digitally sign the reply. Bob has to verify for the authentication.
▪ FRESHNESS OF ALICE’s REPLY : The reply contains Nonce RB
which bob generates uniquely every time.
▪ ASSURANCE THAT ALICE’s REPLY CORRESPONDS : The reply
has Nonce RB corresponding to the request made and identifier
Bob is used.

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PROTOCOL 2
DISADVANTAGES :
▪ Digital Signature requires more computational power. In case of
resource constrained environment MAC’s are more efficient.
▪ Both Alice and Bob have to invest resources on signing and verify
for the authenticity.

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PROTOCOL 3
ASSUMPTION :
▪ Bob has access to randomness to generate nonce RB.
▪ Alice must have the ability to generate a digital signature. Hence no
pre-shared key is required.
▪ Alice and Bob agree on use of a strong digital signature scheme.

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PROTOCOL 3
ALICE BOB
RB || It’s Bob, are you OK?
RB || Yes, I’m OK
MACk (RB || Yes, I’m OK )

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REFLECTION ATTACK
ALICE BOB
RB || It’s Bob, are you OK?
RB || Yes, I’m OK
MACk (RB || Yes, I’m OK )
ATTACKER
RB || It’s Alice, are you OK?
RB || Yes, I’m OK
MACk (RB || Yes, I’m OK )

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PROTOCOL 3
ANALYSIS :
▪ DATA ORIGIN AUTHENTICATION : Only Bob and Alice know the
MAC algorithm hence message is authenticated.
▪ FRESHNESS OF ALICE’s REPLY : The reply contains Nonce RB
which bob generates uniquely every time.
▪ ASSURANCE THAT ALICE’s REPLY CORRESPONDS : The reply
has Nonce RB corresponding to the request made.

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PROTOCOL 3
DISADVANTAGES :
▪ Susceptible to Reflection Attacks
▪ Need for an attack detection mechanism, such as session tracking.

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