Logical Analysis and Verification of Cryptographic Protocols - Loria

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86 CHAPTER 4. PROTOCOLS WITH VULNERABLE SIGNATURE SCHEMES – Directed chosen message attack: this attack is similar to chosen message attack, except that the list of messages for which the intruder obtains A ′ s signatures may be created after seeing A ′ s public key but before any signatures are seen. – Adaptive chosen message attack: an intruder is allowed to use the signer A as an oracle, that is, not only the intruder may request from the agent A signatures of messages which depend on As public key but he may also request signatures of messages which depend additionally on previously obtained signatures. The above attacks are listed in order of increasing severity, with the adaptative chosen message attack being the most severe natural attack an intruder can mount. Description of the breaks. The different notions of break a signature scheme we give below were initially introduced in [117, 153]. We say that an intruder forges a signature if he is able to produce a new signature which will be accepted as one of some other agent. One might say that the intruder has broken a signature scheme if his attack allows him to do one of the following with a non-negligible probability: • Total break: an intruder is able to compute the secret key information of an agent. • Universal forgery: an intruder is able to find an efficient signing algorithm functionally equivalent to an agent’s signing algorithm. • Selective forgery: an intruder is able to forge a signature for a particular message or class of messages chosen a priori. Creating the signature does not directly involve the legitimate signer. • Existential forgery: an intruder is able to forge a signature for at least one message. The intruder has little or no control over the message whose signature is obtained, and the legitimate signer may be involved in the deception. The kinds of “breaks” are listed above in order of decreasing severity, the least the intruder might hope for is to succeed with an existential forgery. We say that a signature scheme is respectively totally breakable, universally forgeable, selectively forgeable, or existentially forgeable if it is breakable in one of the above senses. From now on, we are interested only by signature schemes with appendix, and for simplicity, we write “signature schemes” instead of “signature schemes with appendix”.
4.2. DUPLICATE-SIGNATURE KEY SELECTION (DSKS) PROPERTY 87 4.2 Duplicate-signature key selection (DSKS) property 4.2.1 Description of DSKS property Digital signature schemes are primarily employed to authenticate documents, i.e. if a public key is known to be associated with an agent A, then a valid signature of some data according to A ′ s public key proves that the agent A endorses the content of the signed message, and approves the association between the message that has been signed and his public key. This assumes that no other agent has access to the secret key. But digital signature schemes can also be employed to authenticate the origin of a message, i.e. ensure that only one person could have signed it, or more formally, given a signed message s and A ′ s public key pk, if s is a valid signature of m according to pk then s has been produced using A ′ s secret key. The analysis of digital signature algorithms has however focused on the former authentication property. Formally speaking, the yardstick security notion for assessing the robustness of a digital signature scheme is the existential unforgeability against adaptative chosen-message attacks [117, 93]. This notion states that, given a pair of signing and verifying keys, it is infeasible for someone ignorant of the signing key to forge a signature for at least one message, and this even when messages devised by the attacker are signed beforehand. The security goal provided by this property is the impossibility (within given computing bounds) to impersonate a legitimate user (i.e. one that does not reveal its signature key) when signing a message. We note that this robustness does not address the issue of the identification of the origin of a message. However, this latter concept is also pertaining to digital signatures when they are employed in a non-repudiation protocol. While one would not differentiate the two properties (message authentication and origin authentication) at first glance, they are different since the first authentication property requires the existence of the participation of the signer in the creation of the message, while the latter mandates the unicity of a possible creator of a message. The two notions of message authentication and origin authentication collapse in the single-user setting when there exists only one pair of signing and verifying keys (or only one legitimate agent who can sign data, authenticate data). They may however be different in the multi-user setting. There has been some works defining and proving security of some of the cryptographic primitives in the multi-user setting. M. Bellare and P. Rogaway [37], S. Black-Wilson et al. [43], R. Canetti and H. Krawczyk [59] and V. Shoup [189] gave security definitions for symmetric key entity authentication schemes and key transport schemes in the multi-user setting, M. Bellare et al. [36] presented security definitions for public key encryption in the multi-user setting, and more
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Logical Analysis and Verification o
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Abstract This thesis is developed i
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178 CHAPTER 7. VOTER VERIFIABILITY
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202 BIBLIOGRAPHY [12] M. Abadi and
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204 BIBLIOGRAPHY [36] M. Bellare, A
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206 BIBLIOGRAPHY [60] U. Hustadt Ch
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208 BIBLIOGRAPHY [83] H. Comon-Lund
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216 BIBLIOGRAPHY [187] H. Seidl and
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