- What it is
- A key pair you created on your own machine, counted as the private key file plus the passphrase that unlocks that file.
- How you get one
- A key tool generates the pair in a couple of minutes on a machine you control, with no upload to anyone at any stage.
- Where to keep it
- In the key tool on the machine you use for this, with an exported copy of the secret half stored offline.
- How it goes wrong
- The private half ends up on a machine you cannot get to, or behind a passphrase that nobody wrote down anywhere.
- What fails without it
- Anything sent to you encrypted stays unreadable, and messages that ought to carry your signature carry nothing.
- Time to acquire
- Generating the pair is quick. Arranging a copy you can still open next year takes an evening.
Which half of the pair is the object
A key pair has a public half you hand out and a private half you keep. People fixate on the public half because that is the part they paste into a profile field, and they treat the private half as an implementation detail that lives wherever the software put it. That is backwards. The public half can be republished from the private one at any time. The private half cannot be reconstructed from anything.
So when this page says you hold a key, it means two files worth of reality: the secret key material, and the passphrase without which the secret key material is a block of noise. Losing either loses both. Holding one and not the other is the same as holding neither, which is a thing people discover long after the fact.
Generated where you can see it happen
The pair should be produced on a machine you control, by software running locally, with no step that sends anything anywhere. Web pages that generate keys for you are convenient and put the most important secret you own through a browser you did not audit. A local key tool avoids the entire question.
The machine matters, which is why it is an inventory item of its own. A pair generated on a shared laptop inherits every property of that laptop, including the people who use it, and no later care makes up for that.
The passphrase on the file
A private key protected by nothing is a file anyone with the disk can use. A private key protected by a passphrase you have forgotten is a file nobody can use, including you. Both are common and the second is more painful, because it usually surfaces during a restore, months after the choice was made.
Write the key passphrase down in the same record that holds your account passphrase, on its own line, saying plainly which key it opens. If you keep more than one key, name them. Unlabelled key passphrases and unlabelled key files are the reason people give up on a restore that would otherwise have worked.
Reachable from where you actually sit
A key you cannot reach is not doing anything for you. The common shape of this failure is a pair generated on a desktop at home and needed on a laptop somewhere else, with no export and no copy, at the moment an encrypted message arrives that will not wait.
Decide early which machine is the one that holds the secret half, and keep the offline copy somewhere that survives that machine dying. The key backup covers the copy itself. This page is about the working pair you use week to week.
What the pair lets you do here
Two things, mainly. Messages encrypted to your public key can only be opened with your private half, which is how an address or a note reaches you without passing through a market inbox in the clear. And signatures made with your private half can be checked by anyone holding your public one, which is how a vendor knows a message claiming to be from you was.
Both of those sit alongside the reverse direction, where you use a vendor key you hold to encrypt outbound material, and a fingerprint you collected to check what a market has signed. Your own pair is the half of that arrangement that nobody else can supply for you.
Signs the pair has slipped out of use
Keys fail quietly. An expiry date passes and other people start seeing warnings you never see. A machine is rebuilt and the keyring is restored from a backup that was made before the last subkey rotation. A passphrase is changed in the tool and the written line beside it is not.
The symptom is almost always reported by someone else: a correspondent who says your key looks expired, or a message that will not verify. Because the report comes from outside, it arrives late, which is the argument for checking the expiry and the written passphrase on a slow evening rather than waiting to be told.
Questions that come up
Do I need my own key if I only ever read what vendors send?
Yes, because reading is exactly what it is for. Anything encrypted to you requires your private half to open, and that includes the sort of message people most want to receive intact. Without a pair of your own, a correspondent has no way to send you anything that a market inbox cannot read, and you have no way to prove that a message signed in your name came from you.
Should the key have an expiry date?
A moderate expiry is useful because it limits how long a lost key stays apparently valid, and expiry dates can be extended with the private half in hand. The catch is that extending requires that private half and its passphrase, so an expiry only helps if your storage is in order. If you are not confident of reaching the secret key next year, a long expiry causes fewer problems than a short one.
Where should the exported secret key be stored?
Offline, on something you can physically account for, in a place that is not the machine the key normally lives on. The point of the copy is to survive the failure of that machine, so anything that dies with it is not a copy. Keep the passphrase for it in your written record rather than on the same media, since media and secret travelling together defeats the purpose of encrypting the export at all.