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Lecture 12 Overview

Lecture 12 Overview. Key Management. public-key encryption helps address key distribution problems have two aspects of this: distribution of public keys use of public-key encryption to distribute secret keys. Distribution of Public Keys. can be considered as using one of:

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Lecture 12 Overview

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  1. Lecture 12 Overview

  2. Key Management • public-key encryption helps address key distribution problems • have two aspects of this: • distribution of public keys • use of public-key encryption to distribute secret keys CS 450/650 Lecture 12: Key Exchange

  3. Distribution of Public Keys • can be considered as using one of: • public announcement • publicly available directory • public-key authority • public-key certificates CS 450/650 Lecture 12: Key Exchange

  4. Public Announcement • users distribute public keys to recipients or broadcast to community at large • append PGP keys to email messages or post to news groups or email list • major weakness is forgery • anyone can create a key claiming to be someone else and broadcast it • until forgery is discovered attacker can masquerade as claimed user CS 450/650 Lecture 12: Key Exchange

  5. Publicly Available Directory • can obtain greater security by registering keys with a public directory • directory must be trusted with properties: • contains {name, public-key} entries • participants register securely with directory • participants can replace key at any time • directory is periodically published • directory can be accessed electronically • still vulnerable to tampering or forgery CS 450/650 Lecture 12: Key Exchange

  6. Public-Key Authority • improve security by tightening control over distribution of keys from directory • has properties of directory • requires users to know public key for the directory • users interact with directory to obtain any desired public key securely • requires real-time access to directory when keys are needed CS 450/650 Lecture 12: Key Exchange

  7. Public-Key Authority CS 450/650 Lecture 12: Key Exchange

  8. Public-Key Certificates • certificates allow key exchange without real-time access to public-key authority • a certificate binds identity to public key • usually with other info such as period of validity, rights of use • all contents signed by a trusted Public-Key or Certificate Authority (CA) • can be verified by anyone who knows the public-key authority’s public-key CS 450/650 Lecture 12: Key Exchange

  9. Public-Key Certificates CS 450/650 Lecture 12: Key Exchange

  10. Distribution of Secret Keys • use previous methods to obtain public-key • can use for secrecy or authentication • public-key algorithms are slow • usually prefer to use private-key encryption to protect message contents • hence need a session key • have several alternatives for negotiating a suitable session CS 450/650 Lecture 12: Key Exchange

  11. Public-Key Distribution of Secret Keys • if have securely exchanged public-keys: CS 450/650 Lecture 12: Key Exchange

  12. Diffie-Hellman Key Exchange • public-key type scheme • proposed in 1976 • note: now know that Williamson (UK CESG) secretly proposed the concept in 1970 • A practical method for public exchange of a secret key • Used in a number of commercial products CS 450/650 Lecture 12: Diffie-Hellman Key Exchange

  13. Diffie-Hellman Key Exchange • public-key distribution scheme • cannot be used to exchange an arbitrary message • rather it can establish a common key • known only to the two participants • based on exponentiation in a finite field • modulo a prime or a polynomial • security relies on the difficulty of computing discrete logarithms CS 450/650 Lecture 12: Diffie-Hellman Key Exchange

  14. Diffie-Hellman Setup • all users agree on global parameters: • large prime integer or polynomial p • g = primitive root mod p • for every integer a that has gcd(a, p) = 1, there is an integer k such that gk ≡ a (mod p) • each user generates their key • chooses a secret key (number): a < p • compute their public key: A = ga mod p CS 450/650 Lecture 12: Diffie-Hellman Key Exchange

  15. Diffie-Hellman Key Exchange • shared session key for users is KAB: • KAB = gabmod p = Ab mod p (which B can compute) = Ba mod p (which A can compute) • g can be small • 2 or 5 is common • a, b, p should be large • attacker needs a or b to obtain the session key • must solve discrete log CS 450/650 Lecture 12: Diffie-Hellman Key Exchange

  16. Diffie-Hellman Example • users Alice & Bob who wish to swap keys • agree on prime p=353 and g=3 • select random secret keys: • A chooses a=97, B chooses b=233 • compute respective public keys: • A=397 mod 353 = 40 (Alice) • B=3233 mod 353 = 248 (Bob) • compute shared session key as: • KAB = Ba mod 353 = 24897 = 160 (Alice) • KAB = Ab mod 353 = 40233 = 160 (Bob) CS 450/650 Lecture 12: Diffie-Hellman Key Exchange

  17. Key Exchange Protocols • users could create random Diffie-Hellman keys each time they communicate • users could create a known Diffie-Hellman key and publish in a directory, • then consult and use to securely communicate with them • both of these are vulnerable to a man-in-the-middle attack • authentication of the keys is needed CS 450/650 Lecture 12: Diffie-Hellman Key Exchange

  18. Lecture 13Digital Certificates CS 450/650 Fundamentals of Integrated Computer Security Slides are modified from Robin Burke

  19. Trusting a Public Key • We can't trust • the public key associated with a message • We might trust • an authoritative source to vouch for Alice CS 450/650 Lecture 13: Digital Certificates

  20. Digital Certificates • A digital certificate is a digital file that certifies the identity of • an individual • an institution • a server • a router seeking access to computer- based information • It is issued by a Certification Authority (CA) CS 450/650 Lecture 13: Digital Certificates

  21. wolfmail.unr.edu Digital certificates name public key X.509 certificate CS 450/650 Lecture 13: Digital Certificates

  22. Trusted third party • Certification authority (CA) • issue digital certificates and validate holders’ identity and authority • CA can • meet with Alice • look at her driver's license / birth certificate / etc • take her fingerprints • CA will then • sign her public key CS 450/650 Lecture 13: Digital Certificates

  23. Man-in-the-middle? • When Trudy tries to substitute her public key for Alice's • Bob will either notice that the key isn't certified or • notice that it is certified but not for Alice CS 450/650 Lecture 13: Digital Certificates

  24. Masquerading as CA? • Trudy could falsely issue a certificate • sign the certificate pretending to be the CA • but • strong interest in making CA’s correct public key well known • Multiple sources to access the CA's public key • some public keys are actually bundled with the browser CS 450/650 Lecture 13: Digital Certificates

  25. Public key certificate • A public key • An identifier • Certificate by the CA • Embed public key along with other identifying information • cryptographically sign it as a tamper-proof seal • verifying the integrity of the data within the certificate • validating its use CS 450/650 Lecture 13: Digital Certificates

  26. Benefits of certification • Alice and Bob can exchange certificates directly • no need for a separate way to communicate public keys • certificate is self-protecting • Many users can participate • only need to know CA's public key CS 450/650 Lecture 13: Digital Certificates

  27. Uses of Digital Certificates In a number of Internet applications that include: • Secure Socket Layer (SSL) developed by Netscape Communications Corporation • Secure Multipurpose Internet Mail Extensions (S/MIME) Standard for securing email and electronic data interchange (EDI). • Secure Electronic Transactions (SET) protocol for securing electronic payments • Internet Protocol Secure Standard (IPSec) for authenticating networking devices CS 450/650 Lecture 13: Digital Certificates

  28. Issues • Trust in the CA • issuance policies • Security of the CA's private key • very important!!! CS 450/650 Lecture 13: Digital Certificates

  29. Multiple CAs • If there is only one CA • all is simple • Multiple CAs • Alice's public key is signed by C1 • Bob's public key is signed by C2 • How can Bob be confident? • maybe C1 is really Trudy in disguise CS 450/650 Lecture 13: Digital Certificates

  30. Solutions • Full distribution • every user has the public key for every CA • Impractical • Cross certification • Suppose Alice presents Bob with C1's public key • Signed by C2 • Bob can verify the certificate C2 • C1's public key can be trusted • Therefore Alice's public key can be trusted CS 450/650 Lecture 13: Digital Certificates

  31. Hierarchical trust model • Root CA • a generally-trusted CA • e.g. Federal Reserve Bank • all parties trust root • Non-root CAs • have certificates signed by root CA, or • signed by another non-root CA • closer to the root CA • Certification path • the chain of certifications from the root to a particular public key certificate CS 450/650 Lecture 13: Digital Certificates

  32. CA relationships • Intra-organization communication • Bank ATM network • Organization can be its own CA • The third party CA • CA is an independent entity • is like a notary public • is evaluating the truth of a person's representation • may be liable if due diligence is not performed CS 450/650 Lecture 13: Digital Certificates

  33. Validity • Public key is not valid forever • limits risk associated with key compromise • 1 year is typical • Certificates have a valid period • expired certificate may still be useful • non-repudiation • new certificate issued when old one expires • Possibly the same key re-certified CS 450/650 Lecture 13: Digital Certificates

  34. Certificate assumptions • During the valid period • public key is valid for use • association with identity assumed correct • revocation notifications will be published CS 450/650 Lecture 13: Digital Certificates

  35. Non-repudiation • Increasing legislation to allow digital signatures to serve as legally binding • Non-repudiation of digital signatures • Provides proof of the integrity and origin of data • both unforgeable, which can be verified by any third party at any time • An authentication that with high assurance • can be asserted to be genuine and • cannot subsequently be refuted CS 450/650 Lecture 13: Digital Certificates

  36. Revocation • What if Trudy hacks into Bob's computer and steals his private key? • Alice will still be sending encrypted messages, but now Trudy can read • Certificate must be revoked • can no longer be trusted • new certificate issued • how does Alice find this out? CS 450/650 Lecture 13: Digital Certificates

  37. Revoking a certificate • Reasons for revocation • Detected or suspected compromise • Change of data • e.g. subject name • Change of relationship between subject and CA • e.g. employee quitting a job from an organization which uses the current CA CS 450/650 Lecture 13: Digital Certificates

  38. Who can revoke? • who revokes? • the subject • the CA • an authorized third party • e.g. the organization with an employee quitting • Authentication of the source of revocation request is needed CS 450/650 Lecture 13: Digital Certificates

  39. Certificate Revocation List • CRL is a time-stamped list of revoked certificates • digitally signed by the CA • available to all users • Each revoked cert is identified by a certificate serial number • CRL contains digital signatures, thus can be sent via unprotected channels • Users of public key certificates should check a suitably-recent CRL CS 450/650 Lecture 13: Digital Certificates

  40. Certificate Revocation List • The user of a public key • must check the CRL • every time the key is used • not enough to check when the certificate is originally accepted • CA • must keep a revoked certificate in the CRL until it expires • list could get large CS 450/650 Lecture 13: Digital Certificates

  41. Example • Trudy steals Bob's private key • Bob discovers break-in • requests certificate revocation • Trudy sends a forged message to Alice • Alice verifies message • checks CRL • no problems with Bob's public key • CA publishes CRL with Bob's revocation • too late CS 450/650 Lecture 13: Digital Certificates

  42. CRL Distribution • Pull method • CA periodically updates CRL depository • users check when using a public key • Push method • broadcast new CRL when it changes • Both subject to denial of service attacks CS 450/650 Lecture 13: Digital Certificates

  43. Online Certificate Status Protocol • Request / response protocol • Verifier receives up-to-the-minute status info • Alice checks Bob's public key directly with CA • most effective • most costly • Costs • handling traffic for every public key use • handling cryptographic operations at high spped • maintaining high security in Internet environment • Also subject to denial of service attack CS 450/650 Lecture 13: Digital Certificates

  44. Short-Lived Certificates • Certificate valid for 1 day at a time • re-requested each day • possibly the same public key • Revocation not necessary • Suitable for limited resource systems • e.g. mobile wireless systems • Assumes efficient certificate generation CS 450/650 Lecture 13: Digital Certificates

  45. + + K K B B K CA Obtaining a certificate Digital signature (encrypt) Bob’s public key CA private key - certificate for Bob’s public key, signed by CA Bob’s identifying information CS 450/650 Lecture 13: Digital Certificates

  46. CA's key management • CA keys have many uses • signing (real-time validation) • historical validation • Short-use private keys • better security • But • a signed certificate can't have a valid period beyond the signer's certificate • CA will need multiple keys for different purposes CS 450/650 Lecture 13: Digital Certificates

  47. Certificate distribution • Alice sends Bob a two line signed email • signature ≈ message size • certificate > message size • Alice's public key + CA's signature • certificate for each CA in certification path • Certification info could easily be 10x the message size • What if Bob already has Alice's public key? CS 450/650 Lecture 13: Digital Certificates

  48. Certificate + Signature • Inefficient • Not practical in network environment • Different users might need different certification paths • can't predict which certificates to include CS 450/650 Lecture 13: Digital Certificates

  49. Directory services • General case for public key discovery • Online access to a directory • request a public key certificate for a given user • In this case • Alice sends only the signed message • Bob is responsible for getting Alice's certificate CS 450/650 Lecture 13: Digital Certificates

  50. + + K K B B K CA Obtaining an Individual’s Public Key • When Alice wants Bob’s public key: • Alice gets Bob’s certificate (from Bob or elsewhere) • apply CA’s public key to Bob’s certificate, get Bob’s public key digital signature (decrypt) Bob’s public key CA public key CS 450/650 Lecture 13: Digital Certificates

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