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Sign and Encrypt vs Encrypt and Sign PowerPoint Presentation

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Sign and Encrypt vs Encrypt and Sign Presentation Transcript

Slide 1 - Sign and Encrypt vs Encrypt and Sign Part 1  Cryptography 1
Slide 2 - Confidentiality and Non-repudiation Notation Sign M with Alice’s private key: [M]Alice Encrypt M with Alice’s public key: {M}Alice Want confidentiality and non-repudiation Can public key crypto achieve both? Alice sends message to Bob Sign and encrypt {[M]Alice}Bob Encrypt and sign [{M}Bob]Alice Can the order possibly matter? Part 1  Cryptography 2
Slide 3 - Sign and Encrypt Part 1  Cryptography 3 Alice Bob {[M]Alice}Bob Q: What is the problem? A: Charlie misunderstands crypto! Charlie {[M]Alice}Charlie M = “I love you”
Slide 4 - Encrypt and Sign Part 1  Cryptography 4 Alice Bob [{M}Bob]Alice Note that Charlie cannot decrypt M Q: What is the problem? A: Bob misunderstands crypto! Charlie [{M}Bob]Charlie M = “My theory, which is mine….”
Slide 5 - Public Key Infrastructure Part 1  Cryptography 5
Slide 6 - Public Key Certificate Contains name of user and user’s public key (and possibly other info) Certificate is signed by the issuer (such as VeriSign) who vouches for it Signature on certificate is verified using signer’s public key Part 1  Cryptography 6
Slide 7 - Certificate Authority Certificate authority (CA) is a trusted 3rd party (TTP) that issues and signs cert’s Verifying signature verifies the identity of the owner of corresponding private key Verifying signature does not verify the identity of the source of certificate! Certificates are public! Big problem if CA makes a mistake (a CA once issued Microsoft certificate to someone else) Common format for certificates is X.509 Part 1  Cryptography 7
Slide 8 - PKI Public Key Infrastructure (PKI) consists of all pieces needed to securely use public key cryptography Key generation and management Certificate authorities Certificate revocation (CRLs), etc. No general standard for PKI We consider a few “trust models” Part 1  Cryptography 8
Slide 9 - PKI Trust Models Monopoly model One universally trusted organization is the CA for the known universe Favored by VeriSign (for obvious reasons) Big problems if CA is ever compromised Big problem if you don’t trust the CA! Part 1  Cryptography 9
Slide 10 - PKI Trust Models Oligarchy Multiple trusted CAs This approach used in browsers today Browser may have 80 or more certificates, just to verify signatures! User can decide which CAs to trust Part 1  Cryptography 10
Slide 11 - PKI Trust Models Anarchy model Everyone is a CA! Users must decide which “CAs” to trust This approach used in PGP (Web of trust) Why do they call it “anarchy”? Suppose cert. is signed by Frank and I don’t know Frank, but I do trust Bob and Bob says Alice is trustworthy and Alice vouches for Frank. Should I trust Frank? Many other PKI trust models Part 1  Cryptography 11
Slide 12 - Confidentiality in the Real World Part 1  Cryptography 12
Slide 13 - Symmetric Key vs Public Key Symmetric key +’s Speed No public key infrastructure (PKI) needed Public Key +’s Signatures (non-repudiation) No shared secret Part 1  Cryptography 13
Slide 14 - Notation Reminder Public key notation [M]Alice Sign M with Alice’s private key {M}Alice Encrypt M with Alice’s public key Symmetric key notation C = E(P,K) Encrypt plaintext P with key K P = D(C,K) Decrypt ciphertext C with key K Part 1  Cryptography 14
Slide 15 - Real World Confidentiality Hybrid cryptosystem Public key crypto to establish a key Symmetric key crypto to encrypt data Consider the following Part 1  Cryptography 15 Alice Bob {K}Bob E(Bob’s data, K) E(Alice’s data, K) Can Bob be sure he’s talking to Alice?