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Relating two standard notions of secrecy Zalinescu, Eugen Cortier, Véronique Rusinowitch, Michaël Verification security protocols secrecy applied-pi calculus Two styles of definitions are usually considered to express that a security protocol preserves the confidentiality of a data { t s}. Reach-ability-based secrecy means that { t s} should never be disclosed while equi-valence-based secrecy states that two executions of a protocol with distinct instances for { t s} should be indistinguishable to an attacker. Although the second formulation ensures a higher level of security and is closer to cryptographic notions of secrecy, decidability results and automatic tools have mainly focused on the first definition so far. This paper initiates a systematic investigation of situations where syntactic secrecy entails strong secrecy. We show that in the passive case, reachability-based secrecy actually implies equivalence-based secrecy for signatures, symmetric and asymmetric encryption provided that the primitives are probabilistic. For active adversaries in the case of symmetric encryption, we provide sufficient (and rather tight) conditions on the protocol for this implication to hold. Schloss Dagstuhl – Leibniz-Zentrum für Informatik Eugen Zalinescu and Véronique Cortier and Michaël Rusinowitch 2006 Is Part Of OASIcs, Volume 3, Workshop on Trustworthy Software (2006) InProceedings Text doc-type:ResearchArticle publishedVersion application/pdf doi:10.4230/OASIcs.TrustworthySW.2006.691 urn:nbn:de:0030-drops-6911 https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.TrustworthySW.2006.691 eng https://creativecommons.org/licenses/by-nc-nd/3.0/legalcode