Chapter 3 (B) – Key Management; Other Public Key CryptosystemsKey ManagementDistribution of Public KeysPublic AnnouncementPublicly Available DirectoryPublic-Key AuthoritySlide 7Public-Key CertificatesSlide 9Public-Key Distribution of Secret KeysSimple Secret Key DistributionSlide 12Diffie-Hellman Key ExchangeSlide 14Diffie-Hellman SetupSlide 16Diffie-Hellman ExampleElliptic Curve CryptographySummaryChapter 3 (B) – Key Management; Other Public Key CryptosystemsKey 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 keysDistribution of Public Keys•can be considered as using one of:–Public announcement–Publicly available directory–Public-key authority–Public-key certificatesPublic Announcement•users distribute public keys to recipients or broadcast to community at large–eg. 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 can masquerade as claimed userPublicly 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 forgeryPublic-Key Authority•improve security by tightening control over distribution of keys from directory•has properties of directory•and requires users to know public key for the directory•then users interact with directory to obtain any desired public key securely–does require real-time access to directory when keys are neededPublic-Key AuthorityPublic-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 etc•with all contents signed by a trusted Public-Key or Certificate Authority (CA)•can be verified by anyone who knows the public-key authorities public-keyPublic-Key CertificatesPublic-Key Distribution of Secret Keys•use previous methods to obtain public-key•can use for secrecy or authentication•but public-key algorithms are slow•so usually want to use private-key encryption to protect message contents•hence need a session key•have several alternatives for negotiating a suitable sessionSimple Secret Key Distribution•proposed by Merkle in 1979–A generates a new temporary public key pair–A sends B the public key and their identity–B generates a session key K sends it to A encrypted using the supplied public key–A decrypts the session key and both use•problem is that an opponent can intercept and impersonate both halves of protocolPublic-Key Distribution of Secret Keys•if have securely exchanged public-keys:Diffie-Hellman Key Exchange•first public-key type scheme proposed •by Diffie & Hellman in 1976 along with the exposition of public key concepts–note: now know that James Ellis (UK CESG) secretly proposed the concept in 1970 •is a practical method for public exchange of a secret key•used in a number of commercial productsDiffie-Hellman Key Exchange•a 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 •value of key depends on the participants (and their private and public key information) •based on exponentiation in a finite (Galois) field (modulo a prime or a polynomial) - easy•security relies on the difficulty of computing discrete logarithms (similar to factoring) – hardDiffie-Hellman Setup•all users agree on global parameters:–large prime integer or polynomial q–α a primitive root mod q•each user (eg. A) generates their key–chooses a secret key (number): xA < q –compute their public key: yA = αxA mod q• each user makes public that key yADiffie-Hellman Key Exchange•shared session key for users A & B is KAB: KAB = αxA.xB mod q= yAxB mod q (which B can compute) = yBxA mod q (which A can compute) •KAB is used as session key in private-key encryption scheme between Alice and Bob•if Alice and Bob subsequently communicate, they will have the same key as before, unless they choose new public-keys •attacker needs an x, must solve discrete logDiffie-Hellman Example •users Alice & Bob who wish to swap keys:•agree on prime q=353 and α=3•select random secret keys:–A chooses xA=97, B chooses xB=233•compute public keys:–yA=397 mod 353 = 40 (Alice)–yB=3233 mod 353 = 248 (Bob)•compute shared session key as:KAB= yBxA mod 353 = 24897 = 160 (Alice)KAB= yAxB mod 353 = 40233 = 160 (Bob)Elliptic Curve Cryptography•majority of public-key crypto (RSA, D-H) use either integer or polynomial arithmetic with very large numbers/polynomials•imposes a significant load in storing and processing keys and messages•an alternative is to use elliptic curves•offers same security with smaller bit sizesSummary•have considered:–distribution of public keys–public-key distribution of secret keys–Diffie-Hellman key exchange–Elliptic Curve
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