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Logical Analysis and Verification of Cryptographic Protocols - Loria

Logical Analysis and Verification of Cryptographic Protocols - Loria

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64 CHAPTER 3. PROTOCOLS WITH VULNERABLE HASH FUNCTIONS<br />

x1, . . . , xn → f(x1, . . . , xn)<br />

with f a public function symbol in F.<br />

Unfortunately, such intruder deduction rules are not sufficient to represent<br />

an intruder taking advantage <strong>of</strong> the collision vulnerability property for hash<br />

functions. To this end, we introduce next another definition <strong>of</strong> intruder deduction<br />

rules. This representation <strong>of</strong> intruder deduction rules have been given<br />

initially in [72].<br />

Definition 35 (Intruder deduction rules) An intruder deduction rule is a rule <strong>of</strong> the<br />

form<br />

with<br />

• l1, . . . , ln, l ∈ T (F, X ),<br />

l1, . . . , ln → l<br />

• all.Cons((lσ)↓) ⊆ � n<br />

i=1 all.Cons((liσ)↓) ∪ Cspec, for any ground substitution<br />

σ.<br />

This second condition in the definition above is very similar to the origination<br />

condition for well-definedness in [157]. It is easy to see that if the equation<br />

theory H verifies the property: V ar(u) = V ar(v) for each u · = v ∈ H, then<br />

the second condition in the definition above is verified if <strong>and</strong> only if V ar(l) ⊆<br />

V ar({l1, . . . , ln}).<br />

Example 16 Let F = {., ɛ} with . denotes the concatenation, <strong>and</strong> ɛ denotes the empty<br />

word, <strong>and</strong> let<br />

⎧<br />

⎨ (x.y).z = x.(y.z)<br />

H = x.ɛ = x<br />

⎩<br />

ɛ.x = x<br />

be the associated equational theory. The following rule<br />

x.y → x<br />

is an intruder deduction rule as per definition 35.<br />

Definition 36 (Intruder deduction system) An intruder deduction system I, also<br />

called an intruder system, is a triple I = 〈F, LI, H〉 where F is a signature, LI is<br />

a set <strong>of</strong> intruder deduction rules (as per Definition 35), <strong>and</strong> H is an equational theory<br />

over T (F, X ).<br />

Given two set <strong>of</strong> terms E, F ⊆ T (F, X ), E →I F <strong>and</strong> I-derivations are defined<br />

as in Chapter 2.

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