Title: Notes on PKI
Course: Introduction to SSL/PKI > Section: Information For Teachers

Notes on PKI

PKI can provide

- Authentication
- Integrity
- Confidentiality
- Non-repudiation

See the "Basic IT Security Requirements."

Two or three PKI certificates are usually issued. One for encryption, one for digital signature, and sometimes one for authentication.

PKI uses key pairs. One key is for encryption and the other for decryption. One is made public and one is kept private.

For encryption the encryption key is make public. This way, anyone can send you a encrypted message that only you can read.

For digital signature the decryption key is make public. This way, when you send a digitally signed message to someone, they can decrypt the digital signature to verify it came from you.

If a private key is stolen or made public, someone can use the key and pretend to be you.

Why PKI?

On thing PKI does is to manage large numbers of keys

For example, if I want to communicate security with person A, all we need to do is to have a shared secret key. We will be the only ones to be able to read the messages encrypted with the key.

If I want to communicate security with person B, all we need to do is to have a different shared secret key. The same with person C, person D, .... Person 9999, etc. Soon managing the keys becomes a nightmare.

There is also the problem of setting up a unique secret key with person A in such a way that someone else can not get a copy of it and see our messages.

PKI eliminates each person managing their own set of keys. They must only manage their own private keys and have access to everyone elses public keys.

PGP is also a technology that uses key pairs (one of which is public; one of which is private).

PKI vs PGP, which one is better? That is a long a technical discussion. I will just say that the federal government and other are using PKI.

This course is a overview, introduction to SSL and PKI. The descriptions of how PKI "works" are simplified "cartoons". They give a "flavor" of how things are done, but do not accurately describe the complete process. Note: There are complete descriptions available of the web.

For example, things that are not described:

- There are many (different) encryption algorithems. Which algorithem was used must be included with the data inorder for the correct decryption algorithem be used.
- Commerical CA can store the private encryption key used to decrypted encrypted data.  This allows a decryption key to be "recovered" when a user "looses" their private decryption key. Digital signature private keys are usually only held by the users and can not be "recovered".
- Encryption and decryption with PKI keys is "expensive". (It takes a lot of CPU cycles.) Symetrical keys are "cheaper". When data is encrypted, a randon symetrical key is generated and used. The symetrical key is then encrypted with a PKI asymmetrical encryption key. This greatly speeds up the the processing.
- Digitally signed messages also include the sender's verification PKI certificate and the PKI verification certificate of the CA that created it.
  1. The receiver does not need to access the web to obtrain the sender and CA PKI information.
  2. The receiver can add the CA's certificate to their trusted CA list.
  3. Older messages can be verified even after the sender's certificate expires, has changed or has been re-issued, etc.

Note: Encrypted data contains "extra" information. Encrypted data can be accessed using the "extra" data long after the PKI certificate expires and without access to the PK infrastructure that created it. For example, archived data must be available for many years and email can be sent to anyone.)