HP-UX IPSec Version A.03.00 Administrator's Guide

Overview
You must use security certificates if you are using digital signatures (RSA signatures) for IKE
authentication. HP-UX IPSec uses the certificates to obtain cryptography keys for digital signatures
and to verify the digital signatures. If you are not using digital signatures for IKE authentication,
you can skip this chapter.
Security Certificates and Public Key Cryptography
Security certificates are used for public key cryptography , also referred to as asymmetric key
cryptography. Public key cryptography uses a pair of related, but different keys. One key, the
private key , is associated with a specific system or entity and is kept secret; the other key is the
public key and can be distributed freely. The public and private keys are mathematically related
so that data encrypted with the public key can only be decrypted with the private key.
Public Key Distribution
With asymmetric key cryptography, the public key can be freely distributed over a non-secure
communication channel.
However, there must be some assurance that a particular public key is the actual public key of
the entity with which you want to communicate. This is usually done by distributing public keys
in the form of public-key certificates, commonly referred to as security certificates.
Security Certificates
A security certificate associates (or binds) a public key with a particular person, device, or other
entity. The certificate is issued by an entity, in whom users have put their trust, called a certificate
authority (CA) that guarantees or confirms the identity of the holder (person, device, or other
entity) of the corresponding private key. The CA digitally signs the certificate with the CA’s
private key, so the certificate can be verified using the CA’s public key.
The format for security certificates (public-key certificates) is defined by the International
Organization for Standardization (ISO) X.509 standard, Version 3.
Certificates are issued with a specific lifetime, defined by a start date/time and an expiration
date/time. However, situations can arise, such as a compromised key value, that necessitate the
revocation of the certificate. In this case, the certificate authority can revoke the certificate. This
is accomplished by including the certificate’s serial number on a Certificate Revocation List
(CRL) updated and published on a regular basis by the CA and made available to certificate
users.
Digital Signatures
With digital signatures, the sender uses its private key to create a digital signature value, and
sends the digital signature with the data. The recipient uses the sender’s public key and the data
to verify the digital signature.
There are different methods to generate and verify the digital signature. In one method, the
sender generates a one-way hash value and encrypts it with its private key to form the digital
signature. The recipient uses the sender’s public key to decrypt the digital signature and extract
the hash value; it then generates its own hash value and compares the hash values. In another
method, the sender uses its private key and the data as input to a keyed secure hash algorithm
that outputs the digital signature. The receiver uses the data, the sender's public key and the
digital signature as input to a verification algorithm that verifies the digital signature.
One difference between a digital signature and a symmetric-key hash value is that only the holder
of the private key can generate the digital signature, while either holder of a symmetric key can
generate a symmetric-key hash value. Since only the private key holder can generate the digital
signature, a digital signature also provides non-repudiation which makes it difficult for the
sender to deny sending the message.
114 Using Certificates with HP-UX IPSec