# Key Pair Change

Let’s learn why we need to revoke keys and the different techniques for changing keys.

## We'll cover the following

## Revocation of public-key certificates

The main reason why key change is challenging for public keys is that it is almost impossible (and in many cases undesirable) to control who has access to a public key. This makes withdrawing an existing public key very difficult. This process is normally referred to as *revoking* the public key since it involves ‘calling back’ information that has been released into the public domain and is now no longer valid. In contrast, establishing a new public key is relatively easy. Thus, our discussion of key change for public keys will focus on public-key revocation.

We observe that it does not suffice to establish a new public key because we cannot always determine who has access to the old public key, and hence we cannot guarantee that all holders of the old public key will realize that a new public key has been issued.

Revoking a public key essentially means revoking the public-key certificate. With this in mind, it is worth observing that there may be situations where a public-key certificate needs to be revoked and then a new public-key certificate created for the same public-key value. We will assume that revocation of a public-key certificate only takes place prior to its expiration date. A public-key certificate should not rely on any relying parties if its expiration date has been exceeded.

Why would we need to revoke a public-key certificate?

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