move encryption key segue higher up

 * the YubiKey has been obtained from trusted channels and verified

   (see [Unpacking and authenticating a YubiKey](#unpacking-and-authenticating-a-yubikey)

### Special considerations for storing encryption keys

Here we are storing a full OpenPGP secret keyring on the YubiKey. This

implies that encryption keys end up stored on the device. Therefore,

special care need to be taken as the loss of a YubiKey could be

catastrophic: in such a case, while the encryption key can be revoked,

that doesn't allow the operator to recover past material encrypted

with the key. This can lead to severe data loss.

Encryption keys, therefore, must *not* be generated on the YubiKey as

they *MUST* be backed up. They therefore *MUST* be generated on

another device.

The general strategy here is to have three copies of the encryption

key:

 1. `main key`: a first YubiKey used for daily operation

 2. `backup key`: a *second* YubiKey available as a backup

 3. `backup disk`: a copy of the encryption key material stored on a

    normal disk, encrypted with itself

The rationale here is that if the `main key` is lost, the `backup key`

and `backup disk` can be *combined* to create a *new* `main key`.

If the `backup disk` did not exist, it would be impossible to recreate

a new `main key` and, when the `backup key` is eventually lost or

destroyed, the encrypted contents will not be readable anymore.

This is the strategy taken in this guide.

### Install software and preparation

You will need to install [GnuPG](https://gnupg.org/), its `scdaemon` component and a

TODO: using the YubiKey on a new computer

### Special considerations for storing encryption keys

For *encryption* keys, however, special care need to be taken as the

loss of a YubiKey could be catastrophic: in such a case, while the key

can be revoked, that doesn't allow the operator to recover past

material encrypted with the key.

Encryption keys, therefore, must *not* be generated on the YubiKey as

they *MUST* be backed up. They therefore *MUST* be generated on

another device.

The general strategy here is to have three copies of the encryption

key:

 1. `main key`: a first YubiKey used for daily operation

 2. `backup key`: a *second* YubiKey available as a backup

 3. `backup disk`: a copy of the encryption key material stored on a

    normal disk, encrypted with itself

The rationale here is that if the `main key` is lost, the `backup key`

and `backup disk` can be *combined* to create a *new* `main key`.

If the `backup disk` did not exist, it would be impossible to recreate

a new `main key` and, when the `backup key` is eventually lost or

destroyed, the encrypted contents will not be readable anymore.

TODO: talk about nylon vs steel

### preliminary performance evaluation