1. Security Keys, Signatures, Encryption

2. Slides by Jyrki Nummenmaa ‘

3. Keys n Keys are the basis for encryption. n They can be used for - identification, - encryption, - signatures, - and certificates. n We will skip the mathematics and only explain how the keys can be used.

4. Using a secret key - General n Messages are encrypted using a secret key known for both parties. n Usually faster algorithms than with public/private key pairs. n Problem: How can both parties get to know the secret key whilst keeping it secret?

5. Using a secret key - Encryption n The sender encrypts the message using an algorithm (which may be publicly well know) and the secret key (which is secret, as the name implies) n The recipient decrypts the message using a well-known algorithm and the secret key (of course, the algorithms must be matching).

6. Using a private/public key pair - General n Each communicating agent has a private key, only known to herself, and a public key, known to all. n Principle: both keys are needed for certain operations (next slides will explain more). n Slower algorithms than with the single secret key.

7. Using a private/public key pair - Generating a key pair n The key pair is generated using a random number algorithm in such a way that the keys match. n The private key can be protected with a passphrase, which you must know to be able to use the private key (to create the “real” private key from the passphrase and the permanently stored private key.

8. Using a private/public key pair - Encryption n A sender encrypts a document using the recipient’s public key (known to everyone) and a well-known algorithm. n Decryption is in practice only possible with the recipients private key (known to the recipient only) - no-one else can read the encrypted document.

9. Encryption in practice for communicating secure processes n As secret key algorithms are faster, it is practical to use the public keys to agree on a one-time session secret key. n For this, both parties can create one- time session key pairs (private and public). n The secret session key is used for communication. n This is what e.g. SSL does.

10. Using a private/public key pair - Message integrity/1 n Message integrity = the message has not been changed or corrupted n Tentative solution: calculate a code from the document and send it along. On receipt, a new code is being calculated and compared with the code that was sent. If they match, the message has not changed.

11. Using a private/public key pair - Message integrity/2 n The tentative solution works against corruption (a checksum). n However, if someone wants to change the message, then she could also change the code (checksum).

12. Using a private/public key pair - Message integrity/3 n Improved solution: compute the code using the senders private key and the message. (Encrypt the code using the private key.) = Create a signature or electronically sign the document. n The recipient can use sender’s public key to decrypt the code. Then it is possible to check message integrity.

13. Using a private/public key pair - Message integrity/4 n Q: What’s the difference between this and encryption? n A: The information the sender is giving out can be public and non-encrypted. It is only if you want to verify that the message has not been changed that you use the sender’s public key to check this.

14. Authentication/1 n Suppose you receive mail from Elvis.Presley@heaven.com and you also get Elvis’ public key. n You receive messages which open with the public key -> you conclude that they have been sent using a matching private key. n How can you be sure who the sender is?

15. Authentication/2 n Several possibilities: It is Elvis himself, it is really his e-mail address, it is really his public key.It is Elvis himself, it is really his e-mail address, it is really his public key. It is Elvis’s e-mail address, but someone is misusing it somehow, and he has generated a public/private key pair and sent you the public key (public part).It is Elvis’s e-mail address, but someone is misusing it somehow, and he has generated a public/private key pair and sent you the public key (public part). It is not even Elvis’ e-mail address.It is not even Elvis’ e-mail address.

16. Authentication/3 n The real question is: How can you be sure of a sender’s identity in the Internet world? n Quite often, you are convinced that such a person or company exists. n Then, you need to know if the e-mail and the identity match.

17. Authentication/4 Certificates n A certificate is a document where someone states that a public key really belongs to the right person/company. n A certificate must be digitally signed by someone. n That someone may be a person, but more generally, it is a Certificate Authority (CA).

18. Authentication/5 Certificate Authorities n A Certificate Authority (CA) is generally- trusted generally-known enterprise. n The CA makes it’s public key (or a message digest of it) publicly available so widely that it is not practical for anyone else to claim to be that CA. n The CA (like VeriSign, see www.verisign.com) usually charges money for its services.

19. Authentication/6 Certification n The CA digitally signs public keys. (Or gives digital identities with private keys and matching digitally signed public keys.) n Anyone can check the certificate against the CA’s public key, thus making sure that the CA certifies the public key.

20. Authentication/7 Levels of certification n There are different levels of certification, on the following lines Certifying that a public key and an e-mail address belong together.Certifying that a public key and an e-mail address belong together. Certifying that a public key and a person’s identity belong together (for this you need to visit the CA in person).Certifying that a public key and a person’s identity belong together (for this you need to visit the CA in person). Certifying that a company’s name and a public key belong together…Certifying that a company’s name and a public key belong together… Read more from VeriSign.Read more from VeriSign.

21. Authentication/8 Certification chains n A certified person or company can give a certificate to another. n For example, if a CA (A) gives a certificate to Netscape (B), and Netscape gives a certificate to some Java applet programming company (C), then you can verify B’s public key using A’s and C’s public key using B’s.

22. Authentication/9 Certification risks n It all comes back to a CA, either directly or via a certificate chain. n In fact, it all comes back to the CA’s private key. n If someone guesses or steals or is able to compute (shouldn’t be possible) the CA’s private key, everything collapses.

23. Available implementations n Java offers a java.security package which comes with Java2 (jdk1.2.2) and a java.cryptix package, which is only available in the US, however, there are other implementations, like the one on www.cryptix.org. n SSL can be used through shttp. n However, we will start with PGP, which does not require programming.

24. PGP (Pretty Good Privacy) n PGP is a public/private key pair system. n PGP is publicly available on e.g. kielo.uta.fi - start with pgp -h n PGP can be used for encryption and signatures. n You will need to create yourself a key pair, after which you can start operating with it.