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Logical Analysis and Verification of Cryptographic Protocols - Loria

Logical Analysis and Verification of Cryptographic Protocols - Loria

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92 CHAPTER 4. PROTOCOLS WITH VULNERABLE SIGNATURE SCHEMES<br />

The intruder system we consider to analyse our class <strong>of</strong> cryptographic protocols<br />

is given as follows:<br />

with:<br />

⎧<br />

⎪⎨<br />

⎪⎩<br />

IDSKS = 〈FDSKS, TDSKS, HDSKS〉<br />

FDSKS = FDSKSpub ∪ FDSKSpri<br />

FDSKSpub = {sig, ver, Sk ′ , P k ′ , 1}<br />

FDSKSpri = {Sk, P k}<br />

TDSKS = {sig(x, y), ver(x, y, z), Sk ′ (x, y), P k ′ (x, y), 1}<br />

The associated set <strong>of</strong> intruder deduction rules, denoted by LDSKS is given as<br />

follows:<br />

⎧<br />

⎪⎨<br />

LDSKS =<br />

⎪⎩<br />

x, y → sig(x, y)<br />

x, y, z → ver(x, y, z)<br />

x, y → Sk ′ (x, y)<br />

x, y → P k ′ (x, y)<br />

∅ → 1<br />

In what follows, we introduce the rewrite system, RDSKS, generating the equational<br />

theory HDSKS, <strong>and</strong> we prove that RDSKS is convergent. The rewrite system<br />

RDSKS is obtained by applying Knuth-Bendix completion procedure [131]<br />

on HDSKS. This completion procedure is described in Chapter 2, at Section 2.1.6.<br />

Lemma 25 HDSKS is generated by the convergent rewriting system:<br />

⎧<br />

⎪⎨<br />

RDSKS =<br />

⎪⎩<br />

ver(x, sig(x, Sk(y)), P k(y)) → 1<br />

ver(x, sig(x, Sk ′ (y1, y2)), P k ′ (y1, y2)) → 1<br />

ver(x, sig(x, Sk(y)), P k ′ (P k(y), sig(x, Sk(y)))) → 1<br />

sig(x, Sk ′ (P k(y), sig(x, Sk(y)))) → sig(x, Sk(y))<br />

PROOF.<br />

The application <strong>of</strong> the Knuth-Bendix completion procedure [131] on HDSKS<br />

terminates successfully <strong>and</strong> outputs the rewrite system RDSKS, which is a convergent<br />

rewrite system generating HDSKS (Chapter 2, Section 2.1.6). �<br />

Lemma 26 Any RDSKS-narrowing derivation starting from any term t terminates.<br />

PROOF.<br />

In Lemma 25, we proved that RDSKS is a convergent rewrite system. Following<br />

the definition <strong>of</strong> basic narrowing (Definition 13, Chapter 2), it is easy to<br />

see that any right member <strong>of</strong> any RDSKS rule is not RDSKS-basic narrowable,<br />

<strong>and</strong> hence, by Theorem 1 (Chapter 2) , we conclude that any RDSKS-narrowing<br />

derivation starting from any term terminates.�

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