PKI FAQ

1. What is PKI?
2. What are the security services PKI provides?
3. What are the main elements of a PKI?
4. What is a Certification Authority?
5. What is a digital certificate?
6. What are public and private keys, and what is their relationship?
7. What are Certificate Policies (CPs)?
8. How does a PKI ensure data confidentiality?
9. What is a digital signature?
10. How does a digital signature work?

1. What is PKI?

PKI is acronym of Public Key Infrastructure. A PKI is a comprehensive system of policies, processes, and technologies working together to enable users of the Internet to exchange information securely and confidentially. Public Key Infrastructures are based on the use of cryptography – the scrambling of information by a mathematical formula and a virtual key so that it can only be decoded by an authorized party using a related key.

A PKI uses pairs of cryptographic keys provided by a trusted third party known as a Certification Authority (CA). Central to the workings of a PKI, a CA issues digital certificates that positively identify the holder's identity. A Certification Authority maintains accessible directories of valid certificates, and a list of certificates it has revoked.

2. What are the security services PKI provides?

PKI brings to the electronic world the security and confidentiality features provided by the physical documents, hand-written signatures, sealed envelopes and established trust relationships of traditional, paper-based transactions. These features are:

- Confidentiality: Ensures than only intended recipients can read files.

- Data Integrity: Ensures that files cannot be changed without detection.

- Authentication: Ensures that participants in an electronic transaction are who they claim to be.

- Non-repudiation: Prevents participants from denying involvement in an electronic transaction.

3. What are the main elements of a PKI?

A PKI includes:

- A Certification Authority

- Digital certificates

- Mathematically related key pairs, each comprising a private key and a public key

These elements work within a formal structure defined by:

- Certificate Policies

- A Certification Practice Statement.

4. What is a Certification Authority?

A Certification Authority is a trusted third party that verifies the identity of an applicant registering for a digital certificate. Once a Certification Authority is satisfied as to the authenticity of an applicant's identity, it issues that person a digital certificate binding his or her identity to a public key. (Digital certificates are also issued to organizations and devices, but we will focus on people for the purposes of this discussion.)

5. What is a digital certificate?

An electronic credential that vouches for the holder's identity, a digital certificate has characteristics similar to those of a passport – it has identifying information, is forgery-proof, and is issued by a trusted third party. Digital certificates are published in on-line directories. Typically, a digital certificate contains:

- The user's distinguished name (a unique identifier)

- The issuing Certification Authority's distinguished name

- The user's public key

- The validity period

- The certificate's serial number

The issuing Certification Authority's digital signature, verifying the information in the digital certificate.

6. What are public and private keys, and what is their relationship?

A PKI uses asymmetric cryptography to encrypt and decrypt information. In asymmetric cryptography, encryption is done by a freely available public key, and decryption is done by a closely guarded private key. Although the public and private keys in a particular key pair are mathematically related, it is impossible to determine one key from the other. Each key in an asymmetric key pair performs a function that only the other can undo.

7. What are Certificate Policies (CPs)?

Certification Authorities issue digital certificates that are appropriate to specific purposes or applications. For example, in the Government of Canada Public Key Infrastructure, digital certificates for data confidentiality are different from those used for digital signatures. Certificate Policies describe the rules governing the different uses of these certificates.

8. How does a PKI ensure data confidentiality?

Users' public keys are published in an accessible directory. A person wishing to send an encrypted message uses the recipient's public key to scramble the information in the message. Only the recipient's private key can decrypt the message.

So, if Bob wants to send a confidential message to Alice, his PKI software finds Alice's public key in the directory where it is published, and he uses it to encrypt his message. When Alice receives the encrypted message, she uses her private key to decrypt it. Because Alice keeps her private key secret, Bob can be assured that, even if his message were to be intercepted, only Alice can read it.

9. What is a digital signature?

Not to be confused with a digitized signature (a scan of a hand-written signature), a digital signature can be used with either encrypted or unencrypted messages to confirm the sender's identity and ensure the recipient that the message content has not been changed in transmission. Digital signatures incorporate the characteristics of hand-written signatures in that they can only be generated by the signer, are verifiable, and cannot easily be imitated or repudiated.

10. How does a digital signature work?

Suppose that the famous Bob and Alice wish to correspond electronically. Bob wants to assure Alice that he originated the electronic message, and that its contents have not been tampered with. He does so by signing the message with a digital signature.

When Bob clicks on the digital signature option on his e-mail application, special software applies a mathematical formula known as a hash function to the message, converting it to a fixed-length string of characters called a message digest. The digest acts as a "digital fingerprint" of the original message. If the original message is changed in any way, it will not produce the same message digest when the hash function is applied. Bob's software then encrypts the message digest with his private key, producing a digital signature of the message. He transmits the message and digital signature to Alice.

Alice uses Bob's public key to decrypt the digital signature, revealing the message digest. Since only Bob's public key can decrypt the digital signature, she is able to verify that Bob was the sender of the message. This verification process also tells Alice's software which hash function was used to create the message digest of Bob's original message. To verify the message content, Alice's software applies the hash function to the message she received from Bob. The message digests should be identical. If they are, Alice knows the message has not been changed and she is assured of its integrity. (If Bob had wanted to ensure the confidentiality of his message, he could have encrypted it with Alice's public key before applying the hash function to the message.)

The best thing about all these encryption, decryption, verifying and authenticating processes is that special software does them all transparently, so that Bob and Alice receive the assurances they need without having actually to engage in computations themselves.