Security Keys Signatures Encryption Slides by Jyrki

Security Keys, Signatures, Encryption

Slides by Jyrki Nummenmaa ‘

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. n

Using a secret key General 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? n

Using a secret key Encryption 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). n

Using a private/public key pair General 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. n

Using a private/public key pair Generating a key pair 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. n

Using a private/public key pair Encryption 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. n

Encryption in practice for communicating secure processes 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 onetime session key pairs (private and public). n The secret session key is used for communication. n This is what e. g. SSL does. n

Using a private/public key pair Message integrity/1 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. n

Using a private/public key pair Message integrity/2 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). n

Using a private/public key pair Message integrity/3 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. n

Using a private/public key pair Message integrity/4 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. n

Authentication/1 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? n

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’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.

Authentication/3 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. n

Authentication/4 Certificates 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). n

Authentication/5 Certificate Authorities A Certificate Authority (CA) is generallytrusted 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 Veri. Sign, see www. verisign. com) usually charges money for its services. n

Authentication/6 Certification 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. n

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 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… • Read more from Veri. Sign.

Authentication/8 Certification chains 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. n

Authentication/9 Certification risks 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. n

Available implementations Java offers a java. security package which comes with Java 2 (jdk 1. 2. 2) and a java. cryptix package, which is only available in the US, however, there are other implementations, like the on www. cryptix. org. n SSL can be used through shttp. n However, we will start with PGP, which does not require programming. n

PGP (Pretty Good Privacy) 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. n