CS 526 Information Security : Assignment 5

Before we address the question of which properties the proposed signature schemes lack, we enumerate desirable properties of any digital signature system: 1. Authenticity: A digital signature should be authentic, in that the signature will convince the recipient that the claimed author deliberately signed the document. It should be computationally infeasible for a third party to forge the author's signature. 2. Integrity: A digital signature on a document should verify as correct if and only if the document signed has not been altered after the signature was applied. 3. Unique: A digital signature on a document should be unique, in that the signature is only valid for the signed document and cannot be transferred to a different document. 4. Nonrepudiation: A digital signature on a document cannot be repudiated by the signer. That is, a digital signature of a document should have non-trivial legal value. It must be difficult to prove beyond a reasonable doubt that the signature was produced by a party other than the signer in a court of law. 5. Efficiency: A digital signature should be efficiently computable. That is, producing signatures for large documents should be minimally computationally expensive. 6. Compact Representation: A digital signature should have a compact fixed representation , independent of the length of the document signed. 7. Offline Verification: A digital signature should be verifiable without requiring interaction with the signer. That is, given a signed document, the recipient can verify the signature without communication with the signer. Goldwasser et. al. defined the fundamental notions of digital signature scheme security [4]. Foreword: The issue of Public Key Infrastructure (PKI) is non-trivial, and well beyond the scope of this assignment. Therefore, we assume that no certificate authorities were involved in any of the protocols. Thus, both the problem's signature formulation, as well as the proposed modifications, are vulnerable to a Man-In-The-Middle or Bucket Brigade attack by an adversary. 1. Offline Verification: The use of a public key system in the problem allows signatures to be verified while the signer is offline. That is, the recipient can obtain the signer's public key from any public key server to de-crypt the message digest to verify the document signature. The proposed modification requires that the signer participate in the verification protocol by decrypting the received signature and returning the result