What do you use when you want to provide evidence that an object is authentic? What does authentic mean?
- providing evidence that an object is authentic == wanting to provide evidence that an object was endorsed by a particular person
- you use signatures
What do you use when you want to provide evidence that you are who you say you are?
authentication protocols
Consider a paper check used to transfer money from one person to another. What are the properties of the checks and the physical signatures on the checks?
- Signature confirms authenticity
- only legitimate signer can produce signature (Arguable) - in case of alleged forgery
- 3rd party can verify authenticity (arguable) - Checks are cancelled
- so they can’t be reused - Checks are not alterable
- or alterations are easily detected
What are the requirements for digital signatures?
- a mark that only one principal can make, but others can easily recognize
- unforgeable
- if P signs a message M with signature S{P,M} it is computationally infeasible for any other principal to produce the pair (M, S{P,M}) - authentic
- if R receives the pair (M, S{P,M}) purportedly from P,R can check that the signature relaly is from P - Not alterable
- after being transmitted, (M, S{P,M}) cannot be changed by P, M, or an interceptor - Not reusable
- a duplicate message will be detected by the recipient
How do digital signatures with shared keys work?
How does RSA work (Digital Signatures with Public Keys)
RSA is commutative:
- D(E(M, K), k) = E(D(M, k), K)
Opposite from normal use of PK as cipher
◆ Let KA be Alice’s public key
◆ Let kA be her private key
◆ To sign msg, Alice sends D(msg, kA)
◆ Bob can verify the message with Alice’s public key
Works! RSA: (me)d = med = (md)e
What are the advantages and disadvantages of digital signatures with public keys? What is the Alice and Bob drawing that shows how digital signatures with public keys works?
What are the variations on public key signatures?
How do A and B convince each otehr that they are each A and B?
- cryptographic authentication protocols
What is the threat model of communication over a network as it relates to cryptographic protocols?
What is the general definition of “protocol”?
What can the interceptor do?
What is an arbitrator and how do they affect protocols (arbitrated protocols)?
What are real-world examples of arbitrated protocols and what are the issues with arbitrated protocols?
How do adjudicated protocols work?
How do self-enforcing protocols work?
Is the shared key digital signature algorithm an arbitrated or adjudicated protocol?
arbitrated
Is trusted 3rd party provided authenticity an arbitrated or adjudicated protocol?
arbitrated
is public key digital signature algorithm an arbitrated or adjudicated protocol?
adjudicated
What is trusted 3rd party provided non-repudiation mean? Is it an arbitrated or adjudicated protocol?
- Bob can keep Alice’s digitally signed message
- adjudicated
What is the goal of authentication?
What are the threats to authentication?
What situation would we use shared-key authentication?
How does weak authentication work?
What is a replay attack?
What are the three strategies for defeating replay attacks?
What are nonces?
What are the uses of nonces in a challenge-response protocol?
How are time stamps used?
What are sequence numbers used for? What are the disadvantages?
How does strong(er) shared-key authentication w/ nonces work? What makes it stronger?
What is wrong with this flawed version of shared-key authentication?
What are the difficulties of protocol design?
What are the general principles of protocol design?
What are the different types of key establishments? What is the key issue?
What is bilateral out-of-band?
What is point-to-point?
What is third-party key distribution?
Explain the Needham-Schroeder Protocol
How can a replay attack occur on the Needham-Schroeder Protocol?
What are the different attack scenarios of the replay attack on the Needham-Schroeder Protocol?
What is Kerberos? What is it used for?
Draw the diagram that shows how Kerberos works
How does Kerberos login work…in notation terms?
How does Kerberos Service Request work..in notation terms?
How does Kerberos work in quasi-english?
What are the benefits to Kerberos?
What are the drawbacks to Kerberos?
What do Public Key Infrastructures do?
What do Certification Authorities do?
Certification Authority (CA)
◆ Binds identifiers to a public key
◆ Expected to perform some amount of due diligence before vouching for this binding
◆ Popular CA’s: Verisign, Thawte
◆ Note that you must trust CA
What is the PKI cErtificate ‘X.509’
Explain the SSL/TLS example of PKI
What to do about SSL/TLS if you want multiple Certificate Authorities (CA)?
What happens if a private key is lost or compromised?
What is Secure Shell?
What are the different way SSH authenticates?
How does SSH with passwords work?
What is an example use of ssh with password?
How does SSH Protocol work?
What are the problems with SSH with password?
What does ssh.com’s SSH and SSH error look like? When does the error come up?
How does SSH with client keys work?