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PKI for the Mystified

PKI for the Mystified. A Non-technical Introduction to Public Key Infrastructure and Cryptography Mark Norman DCOCE Oxford University Computing Services. DCOCE. Der-kot-chee (…bless you!) Digital Certificate Operation in a Complex Environment. The DCOCE project.

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PKI for the Mystified

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  1. PKI for the Mystified A Non-technical Introduction to Public Key Infrastructure and Cryptography Mark NormanDCOCEOxford University Computing Services

  2. DCOCE Der-kot-chee (…bless you!) Digital Certificate Operation in a Complex Environment

  3. The DCOCE project • DCOCE is about authentication with digital certificates • Digital certificates use Public Key Infrastructure (PKI) • PKI is very secure • but can be difficult to administer • and a lot of people don't understand it too well

  4. Aim of this talk • Improve the understanding of the fundamentals of encryption and public key encryption • So that people can then go on and be confident with: • PKI • PGP • SSL (e.g. using a credit card on-line) • Encrypting documents (and email) • Signing documents (and email) • Deciding whether a signature is good or not • I won’t use any maths!

  5. Parts of this talk • Define a few terms • Symmetric keys • (A demonstration) • Breaking (or cracking) encrypted information • Asymmetric keys • Signing things with keys • Trusting keys

  6. A few terms: Authentication • The act of verifying that an electronic identity (username, login name etc.) is being employed by the person to whom it was issued • Strictly it should mean "establishing the validity of something, such as an identity". • (The procedure as indicated by the definition above can be very difficult indeed, but PKI attempts to do this.)

  7. A few terms: Authentication

  8. A few terms: Authorisation • Associating rights or capabilities with a subject • Authorisation usually comes after authentication • i.e. once the service knows who it is (Authenticated), it then proceeds to decide what that person/subject can do (Authorisation)

  9. What is Public Key Infrastructure? • A key is like a code sheet • A public key is an odd concept • why would you reveal your secret code in public? • We need to understand symmetric keys and asymmetric keys

  10. Substitution Transposition Symmetric encryption Easy to understand the principles etc. …

  11. Encryption Plaintext Ciphertext Key and encryption algorithm Decryption Symmetric encryption The (almost) technical explanation

  12. Demonstration

  13. Demonstration Algorithm = ‘Write (or read) the letters behind the boxes’. Key = ‘The boxes on this particular sheet’ (or some plotting data for their positions). You and your correspondent need to have the key… …and it needs to be secret from all other spies.

  14. Symmetric encryption On a computer… • Example using the Data Encryption Standard (DES) $> des -e “Mary had a little lamb” output.des Enter key: oucskey Enter key again: oucskey $> • The result: $> cat output.des !¢ðuýåćßÞf 謶׀ עжТφẸỆ≈∞▪ﲑ $>

  15. Symmetric encryption • Example using the Data Encryption Standard (DES) continued… • To decrypt: $> des -d output.des text.des Enter key: oucskey Enter key again: oucskey $>cat text.des Mary had a little lamb $>

  16. Symmetric encryption • Example using the Data Encryption Standard (DES) continued… • Trying to decrypt with the wrong key: $> des -d output.des text.des Enter key: oucsquay Enter key again: oucsquay Corrupted file or wrong key $>cat text.des uýåćß#¬`謶׀ φẸỆעжТ עжТ $>

  17. Symmetric encryption Attacking a cipher • What about a ‘brute force’ attack?i.e. ‘guessing’ at the key “oucskey” • DES algorithm has a 56-bit key. Therefore, there are 256 = 72,057,594,037,900,000different keys • 834 days at a billion keys per second • But for a typed key, effectively 968 (83 days) • How safe are encryption algorithms anyway? • Example using (DES) continued…

  18. Symmetric encryption Attacking a cipher • How safe are encryption algorithms anyway? • Established algorithms should remain sound • Safety is dependent on key length

  19. Symmetric encryption Demonstration

  20. Symmetric encryption Some issues • So you have to have the same key as your correspondent – is that a problem? • How do you send the key safely? • Do I try to exchange keys before I communicate? • How many keys will I need to communicate with everyone? • You need a key for everyone!

  21. Symmetric encryption • What makes it symmetric?

  22. Asymmetric encryption • Then there was asymmetric encryption • Whitfield Diffie and Martin Hellman (1975) • (Ellis and Cox, 1973) • A key pair is constructed using some complicated maths (the keys are not the same) • Each party has two keys (public and private) • Anything encrypted with key1 can only be decrypted with key2 • Asymmetric!

  23. Encryption Key 1 andencryption algorithm Decryption Plaintext Ciphertext Asymmetric encryption

  24. Encryption Key 1 andencryption algorithm Plaintext Ciphertext Key 2 andencryption algorithm Decryption Asymmetric encryption If Key 1 = private, Key2 must be corresponding public If Key 1 = public, Key2 must be corresponding private

  25. Public and private keys • Keys exist in pairs • Keep one private (very secret) and 'publish' one • Public keys can exist on certificates • Encryption can be done by either key • If it is your key pair, you can use the private key • Anyone else can use the public key to encrypt something

  26. Asymmetric encryption Demonstration Public key Private key Complex maths! 23 February 2004 26

  27. Asymmetric encryption Demonstration

  28. Asymmetric encryption Demonstration ‘s public key 30 81 89 02 81 81 00 bb 58 a2 ff ce 8e 3d 4b 59 6a 8c 8c 68 52 62 ef 9c 64 2e 94 43 a5 c3 2d 30 d2 5a c6 3a 2e dc c0 a4 24 1d 74 ce 08 46 53 fd bf 00 50 83 5a 4f 48 63 b8 99 6d df 04 d0 c5 1a 2b e5 12 b8 0c e6 e1 54 ab cf 79 74 3b 8e d7 b1 b9 f1 1e 11 03 94 df 1b 69 13 8f 14 32 d3 d7 bd 57 eb 5a 75 f2 ee 66 30 ca 7f 55 52 c4 d5 73 0e 03 89 e6 69 7f b0 cd 69 c3 67 2f 1d 9a 5a 16 f6 03 3c 68 5c 91 c4 d3 02 03 01 00 01

  29. Private keys • Extremely secret! • If you send something encrypted by a private key, it can be read by everyone, but they know it came from you. • Authentication

  30. Public keys • Not at all secret! • Widely available, but must be trusted • May be supplied as part of a certificate • If you send something using a public key, it can only be read by the entity to which it is addressed. • Secure communications • e.g. SSL

  31. How can I trust a public key? • Someone can use a public key to prove their identity to me • but only if I trust that public key • there's public keys out there that say they belong to George Bush etc. • So if someone I trusted endorsed (signed) that public key • hold that thought for a moment - we’ll come back to this...

  32. Signing things with keys • Asymmetric keys can be used to sign things • encrypt a bit of text with your private key (can be attached 'securely' to the 'document') • people can de-crypt it with the public key and know that it was signed by you • What?…

  33. Signing things with keys • You need to know something about hashes… • Message digests or one way hash functions distil the information contained in a file (very small or very large) into a single large number (usually between 128 and 256 bits in length) • So, you can actually add the hash value to the file somehow and then sign (or encrypt) that hash value with your private key.

  34. Very important document. Blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, Alg. <975764138552> Signing things with keys Very important document. Blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah, blah,

  35. How can I trust a public key? • Put that public key on a certificate • (There are other ways, but this is PKI) • Get someone you trust to sign the certificate • If the certificate is tampered with, the signature is broken • Organisations who sign public keys/certificates are called Certification Authorities (CA)

  36. Public Key Infrastructure • You create a key pair • Put one key of the pair on a certificate • Send the certificate (request) to the CA • Present yourself or identify yourself to the Registration Authority (RA) • The RA tells the CA that you are OK • The CA sends you the signed certificate

  37. Summary of PKI

  38. Public Key Infrastructure • Now you have a signed certificate, people and services can trust that you are who you say you are • Present your certificate to a service • Tell them something encrypted by your private key • They like your certificate and know it is you

  39. Public Key Infrastructure • You keep your private key very secret • Obey the rules for this! • Your public key is on the certificate • Services must trust the CA • Your certificate will have an expiry date • after which you may have to re-visit the RA • Your certificate can be revoked at any time

  40. Client authentication Mary had a little lamb Mary had a little lamb Hello Mary had a little lamb End user Authentication using certificates and public/private keys Webserver Mary had a little lamb OK. The server is happy that the end user is Mr Bloggs himself!

  41. Public Key Infrastructure • Asymmetric encryption = public/private keys • Symmetric encryption is faster • but how do you deliver the keys • Asymmetric encryption is used in SSL • Secure Sockets Layer, very common • Also used in client authentication(less common, at the moment)

  42. Setting up the session and server authentication Challenge Phrase (Random message) Random connection identifier (server) Encryptionprotocols Encryption protocols OK OK! Or client may not have CA public key (receives message that certificate is not known) | Cancel | Always Trust | Trust this time | End user Public CA key Server pub. key and cert. Authentication using certificates and public/private keys Webserver Challenge Phrase (Random message) Random connection identifier (server)

  43. Setting up the session and server authentication Symmetric key pairs (Encrypted) End user Master session key Public CA key Server pub. key and cert. Authentication using certificates and public/private keys Webserver Challenge Phrase (Random message) Random connection identifier (server)

  44. Setting up the session and server authentication Read Write Random connection identifier (server) Challenge Phrase (Random message) Symmetric key pairs Write Read End user Master session key Public CA key Symmetric key pairs Server pub. key and cert. Authentication using certificates and public/private keys Webserver Challenge Phrase (Random message) Challenge Phrase (Random message) Then client authentication begins! (as we looked at before) Random connection identifier (server) Random connection identifier (server)

  45. PKI for the Mystified Please tell me you’re not still mystified! A Non-technical Introduction to Public Key Infrastructure and Cryptography Mark NormanDCOCEOxford University Computing Services

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