Cryptography Flashcards
1
Q
Schemes: MAC and Encryption
A
- Encrypt-then-MAC
- MAC-then-encrypt
- Encrypt-and-MAC
2
Q
Reasons for failure of Crypto Systems
A
- Application should use cryptography but does not
-> sometimes: ignorance, clumsy interfaces, licensing issues
-> often: deployment violates designer’s assumptions - Application uses crypto but the wrong one
-> Insecure cryptography
-> Using encryption for integrity protection
-> Using asymmetric cryptography without need
-> Using digital signatures where it does not fit the business process - Secrets are not secret
- Right crypto, but unusable (too complicated, over engineered)
- Right crypto, usable but weaker links
3
Q
Blind signatures
A
Message is blinded/disguised before being signed
4
Q
Secret sharing
A
Partition secret between several parties where the secret can only be constructed with all (or a specified number of) parts
5
Q
Zero knowledge proofs
A
- Convince other party of some properties of your secret without revealing it
-> Completeness: if the statement is true, an honest verifier (that is, one following the protocol properly) will be convinced of this fact by an honest prover.
-> Soundness: if the statement is false, no cheating prover can convince an honest verifier that it is true, except with some small probability.
-> Zero-knowledge: if the statement is true, no verifier learns anything other than the fact that the statement is true.
6
Q
TLS Advantages
A
- Easy to implement and use
- Deployed in most browsers & servers
7
Q
TLS Disadvantages
A
- Protects only if used by the application
- More vulnerable to DoS attacks (malicious packets cannot be removed at the IP level)
- Can only be used in End-to-End mode
8
Q
Security Parameters for a TLS session
A
- Connection end (Who is server/Who is client?)
- Pseudo random function algorithm
- MAC algorithm
- Bulk encryption algorithm
- Compression algorithm
- Master secret and other cryptographic keys
9
Q
TLS ChangeCipherSpecProtocol
A
- Not explicitly needed in TLS 1.3, but sent to prevent middleboxes from trying to parse the following encrypted data
- Consists only of ChangeCipherSpec message which itself consists of a single byte with value 1
- From here on everything is encrypted but not authenticated yet
10
Q
TLS Server Authentication
A
- Server sends CertificateChain
- Server proves that he is the certificate’s owner using CertificateVerify message
-> Signs hash of ClientHello, ServerHello, Certificate with the private key associated with the certificate
11
Q
TLS End of Server Hello
A
- Server may require client to authenticate by sending CertificateRequest
- Sends Finished
12
Q
TLS Finished Message Properties
A
- Contains a hash of all previously sent messages
13
Q
Wie stellt man Confidentiality sicher?
A
- Within one system: User identification, access control
- In a distributed system: Encryption
- Through a distributed system: Secret sharing
14
Q
Wie stellt man Integrity sicher?
A
- Within one system: User identification, access control
- In a distributed system: MAC, digital signature
- Through a distributed system: Verifiable secret sharing, secure multiparty computations
15
Q
Wie stellt man Availability sicher?
A
- Within one system: User identification, perfection, redundancy
- In a distributed system: Redundancy
- Through a distributed system: Reliable and secure multiparty computations
