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

2. 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

3. 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

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

5. 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

6. 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

7. 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

8. 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