Public-Key Technology

Public and Private Keys

Characteristics of Public and Private Keys

The secret behind public-key technology lies in the relationship between two keys, a public key and a private key. The person or component that wants to “sign” owns these two keys. These keys have the following characteristics:

• The keys are pairs; they belong together.

• You cannot obtain the private key from the public key.

• The public key is to be made public.

  The owner of the keys distributes the public key as necessary. A recipient of a signed document needs to have knowledge of this key to verify the digital signature. To send an encrypted document (digital envelope), the sender needs to know the recipient's public-key.

• The private key is to be kept secret.

  The owner of the keys uses the private key to generate his or her digital signature and to decrypt messages protected with a digital envelope. Therefore, the owner of the keys needs to make sure that no unauthorized person has access to his or her private key.

Generating and Assigning Keys

To be able to sign digitally, the signer needs a pair of keys. Typically, your Web browser generates these keys. You then send the public key of this key pair to a central instance called a Certification Authority (CA) to be signed. This step guarantees your ownership of the key pair. You can compare this to a central office that distributes identification cards. These keys then “belong” to the owner and can be used for identification purposes.

Using a Digital Signature

Signing Documents

To sign a document, the signer uses his or her private key to create his or her digital signature.

For more information, see Digitally Signing Digital Documents.

The document, along with the signature, is passed on to the recipient.

Verifying Digital Signatures

The recipient of the document then uses the signer's public key to verify the signature and the integrity of the document (that it has not been changed since being signed).

For more information, see Verifying Digital Signatures.

Using Digital Envelopes

Creating Digital Envelopes

To create a digital envelope, you use a secret message key to “wrap” the document in a secure “envelope”. The recipient of the message also needs knowledge of this key to be able to decrypt the message. Therefore, you encrypt this message key using the recipient's public key and send it along with the document.

For more information, see Creating a Digital Envelope.

Opening Digital Envelopes

The recipient of the document then uses his or her own private key to decrypt the secret key that was used to encrypt the document. He or she can then decrypt the document using this secret key.

For more information, see Opening a Digital Envelope.

Public-Key Certificates

The questions still arise: “How do you know which public key belongs to whom?” and “How do you obtain the signer's public key?” The answers lie in the public-key certificate.

We have mentioned that the signer needs to have a pair of keys. We also mentioned that a central instance, called a CA, assigns these keys to the owner. The CA assigns these keys by issuing a digital certificate. This digital certificate contains the information needed to ensure that the public key belongs to the person indicated.

For more information, see Public-Key Certificates.

The signer distributes his or her public key by distributing his or her public-key certificate (for example, directly with an e-mail or by using X.500 Directory Services).

The recipient uses the information from the public-key certificate (namely the public key and which hash algorithm to use) to verify the signature of the signed document. The recipient also knows that this public key belongs to this person, because a CA has also signed the public-key certificate. (The recipient should also know of and trust this CA.) The recipient can also verify the validity of the CA's signature, because its signature and its public key are also included in the public-key certificate.