Digital Signatures Reference: Pfleeger , Charles P., Security in Computing, 2nd Edition, Prentice Hall, 1996. 4/7/2019 Ref: Pfleeger96, Ch.4.

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Presentation transcript:

Digital Signatures Reference: Pfleeger , Charles P., Security in Computing, 2nd Edition, Prentice Hall, 1996. 4/7/2019 Ref: Pfleeger96, Ch.4

Definition of Digital Signatures A digital signature is a protocol that produces the same effect as a real signature: it is a mark that only the sender can make, but other people can easily recognize as belonging to the sender. 4/7/2019 Ref: Pfleeger96, Ch.4

Requirements of Digital Signatures Unforgeable If person P signs message M with signature S(P,M), it is impossible for anyone else to produce the pair [M, S(P,M)]. Authentic If a person R receives the pair [M, S(P,M)] purportedly from P,R can check that the signature is really from P. 4/7/2019 Ref: Pfleeger96, Ch.4

Additional Desirable Properties of Digital Signatures Not Alterable After being transmitted, M cannot be changed by S,R, or an interceptor. Not Reusable A previous message presented will be instantly detected by R. 4/7/2019 Ref: Pfleeger96, Ch.4

Symmetric Key Digital Signatures The secrecy of the key guarantees the authenticity of the message. Does not prevent forgery, and cannot prevent against repudiation (denial of sending the message). Thus an arbiter is needed to prevent forgery. 4/7/2019 Ref: Pfleeger96, Ch.4

Digital Signatures without Encryption If secrecy is not an issue, then a cryptographic sealing function can be used as a signature. The sealing function is a mathematical function of every bit of its input. A hash could be used as a sealing function. 4/7/2019 Ref: Pfleeger96, Ch.4

Preventing Reuse or Alteration A time stamp (or sequence number) could be used to prevent reuse. The time stamp could also become part of every block of a block cipher, to prevent alteration of a piece of an encrypted message. 4/7/2019 Ref: Pfleeger96, Ch.4

Asymmetric Digital Signatures Public key systems can be used for digital signatures. A sender could encrypt a message using their private key. The encrypted message could be read by anyone using the sender’s public key. The message could not be decrypted using someone else’s public key. 4/7/2019 Ref: Pfleeger96, Ch.4