March R. Smith - University of St Thomas - Minnesota QMCS Class Today Exam clarifications needed?Exam clarifications needed? A ‘minor’ homework thingA ‘minor’ homework thing Microsoft’s RNGMicrosoft’s RNG “Real” PK applications“Real” PK applications CertificatesCertificates Homework/QuizHomework/Quiz

March R. Smith - University of St Thomas - Minnesota Homeowork #1: Finish the exam by Friday noon#1: Finish the exam by Friday noon –Deliver it to me #2: See if PGP still works#2: See if PGP still works –You need to be able to encrypt files –Create a PK certificate, if you haven’t already –This is NOT on the web site so far...

March R. Smith - University of St Thomas - Minnesota Microsoft’s random number generator Some guys in Israel just wrote a paperSome guys in Israel just wrote a paper Windows RNG is predictable in both directionsWindows RNG is predictable in both directions If you know the RNG’s current stateIf you know the RNG’s current state –Then you can figure out earlier states (crack older keys) –You can EASILY figure out later states (crack future keys) Paper illustrates several things:Paper illustrates several things: –It’s hard for an OS to do good random numbers –Security Through Obscurity doesn’t work –It’s annoying when people redefine terminology PRNG, “forward” and “backward”PRNG, “forward” and “backward”

March R. Smith - University of St Thomas - Minnesota Using Public Key Diffie HellmanDiffie Hellman –I can share one secret with another D-H user I use the other user’s PUBLIC key with my PRIVATE keyI use the other user’s PUBLIC key with my PRIVATE key RSARSA –If I have a user’s PUBLIC key, I can send them a secret I encrypt the secret with THEIR public keyI encrypt the secret with THEIR public key They decrypt with their own private keyThey decrypt with their own private key –I can use my PRIVATE key to “sign” things I encrypt a hash (checksum) with my PRIVATE keyI encrypt a hash (checksum) with my PRIVATE key Others can check the result with my PUBLIC keyOthers can check the result with my PUBLIC key

March R. Smith - University of St Thomas - Minnesota Real Public Key Applications I.e. places where it really does something valuableI.e. places where it really does something valuable Secrecy (sharing keys)Secrecy (sharing keys) –Secret file sharing (PGP) –SSL: browsers, Secure Shell Integrity (digital signatures)Integrity (digital signatures) –Verifying downloaded software –Verifying messages –Verifying public key “owners”

March R. Smith - University of St Thomas - Minnesota Creating a Certificate People generally trust Honest AbePeople generally trust Honest Abe Abe attests that has the public key 3,5555Abe attests that has the public key 3,5555www.bank.com Abe digitally signs a certificate to say thisAbe digitally signs a certificate to say this Abe is a certificate authority (CA) since he certifies the owners of public keysAbe is a certificate authority (CA) since he certifies the owners of public keys Key: 3,5555 Honest Abe’s Private Key Signature Procedure Key: 3,5555

March R. Smith - University of St Thomas - Minnesota Validating a Certificate The initial strategy in SSL-enabled BrowsersThe initial strategy in SSL-enabled Browsers Every Web server with SSL has a certificateEvery Web server with SSL has a certificate Only one Certificate Authority’s public keyOnly one Certificate Authority’s public key –RSA Security, later Verisign, serves as “Honest Abe” Problems with scalability, delegationProblems with scalability, delegation From Authentication © Used by permission

March R. Smith - University of St Thomas - Minnesota Multiple CAs in the Browser Browsers maintain a list of “Honest Abes”Browsers maintain a list of “Honest Abes” Users can add a new CA when encounteredUsers can add a new CA when encountered –Security issue – is a new CA really honest, or not? From Authentication © Used by permission

March R. Smith - University of St Thomas - Minnesota Public Key Infrastructure A catch-all term for the services required to support the widespread use of public keys Server and client software to support public keysServer and client software to support public keys Software to create and distribute certificatesSoftware to create and distribute certificates Trustworthy organizations to issue reliable certificatesTrustworthy organizations to issue reliable certificates Mechanisms so that organizations can recognize each other’s certificatesMechanisms so that organizations can recognize each other’s certificates

March R. Smith - University of St Thomas - Minnesota Commercial PKI Commercial PKIs use a hierarchical strategy Certificates are created and signed by special certificate authority softwareCertificates are created and signed by special certificate authority software Each certificate authority belongs to an enterprise and carries a unique keyEach certificate authority belongs to an enterprise and carries a unique key The enterprise is responsible for ensuring the accuracy of certificatesThe enterprise is responsible for ensuring the accuracy of certificates –Commercial certifiers like Verisign, Inc., rely on stringent, published rules and procedures defined in their Certification Practices Statement and Certificate Policy –Private corporations may rely on internal controls and limits on certificate usage

March R. Smith - University of St Thomas - Minnesota Alternative to the CA/PKI “Pretty Good Privacy” (PGP) uses web of trust strategy Traditional ‘Web of Trust’Traditional ‘Web of Trust’ –Anyone may sign a certificate –Certificates may carry multiple signatures –Individuals must personally decide on authenticity, based on the signatures –Pairwise trust relationships, extended based upon interpersonal transitive trust Current on-line key directoryCurrent on-line key directory –Directory itself “signs” its certificates –Authenticity based on an exchange (!?!)

March R. Smith - University of St Thomas - Minnesota Issues with PKI StandardizationStandardization InteroperabilityInteroperability Poorly defined trust relationshipsPoorly defined trust relationships Confidentiality of Private/Secret signing keysConfidentiality of Private/Secret signing keys DeploymentDeployment –Infrastructure cost –Infrastructure complexity –Enrollment costs –Client deployment costs

March R. Smith - University of St Thomas - Minnesota “Group quiz” How can I send an encrypted message to 2 other people without sharing a secret with all 3?How can I send an encrypted message to 2 other people without sharing a secret with all 3? Assume we’ve shared public keysAssume we’ve shared public keys Pull out a piece of paperPull out a piece of paper Draw the answer, put the group names on itDraw the answer, put the group names on it

March R. Smith - University of St Thomas - Minnesota That’s it Questions?Questions? Creative Commons License This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United States License. To view a copy of this license, visit or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.