> Where are your rotation and signing keys stored? Who can acccess them?
This uses public key cryptography, so there's multiple answers here. Let's start at the beginning:
atproto uses DIDs as its identity standard: http://w3.org/TR/did-1.0/
Here is my DID, we'll use this as an example: did:plc:3danwc67lo7obz2fmdg6jxcr
There are three parts, separated by colons: The scheme (did), the method (plc), and the DID method-specific identifier (3danwc67lo7obz2fmdg6jxcr).
To use a DID, such as did:plc:3danwc67lo7obz2fmdg6jxcr, you resolve it into a DID Document https://www.w3.org/TR/did-1.0/#dfn-did-documents
> A set of data describing the DID subject, including mechanisms, such as cryptographic public keys, that the DID subject or a DID delegate can use to authenticate itself and prove its association with the DID.
That document contains various properties that describe the identity: https://www.w3.org/TR/did-1.0/#core-properties
One of those properties is the "verification method", which tells you what to do to verify the identity. So you just do whatever is in that, and that gives you the identity. In other words, the broadest part of the spec is very pluggable, it does not describe the answer to your question in all cases.
So let's get more specific: One kind of DID, and the one that's used by virtually all of atproto users, is DID:PLC. As you can see from my DID above, I'm using the PLC method for my DID.
Its specification is what I linked you to above, https://web.plc.directory/spec/v0.1/did-plc.
You can see my specific entry here, for example: https://plc.directory/did:plc:3danwc67lo7obz2fmdg6jxcr
(Note that, before we get into any other part of it, this document points at my PDS, https://morel.us-east.host.bsky.network . So already, without going further, this is why your original comment is wrong: my identity points at my PDS, my PDS does not point at my identity.)
This specifies what goes into the "verification method" of a DID document that uses this method. In this case, if you look at mine, you'll see that it points (eventually) to https://www.w3.org/TR/cid-1.0/, which is what the Multikey stuff is about. From that spec:
> Controlled identifier documents identify a subject and provide verification methods that express public cryptographic material, such as public keys, for verifying proofs created on behalf of the subject for specific purposes, such as authentication, attestation, key agreement (for encryption), and capability invocation and delegation. Controlled identifier documents also list service endpoints related to an identifier; for example, from which to request additional information for verification.
This is already getting deep in the weeds, but the point is that the publicKeyMultibase is the encoded form of the public key that controls my identity. So that's where that lives. What about its associated private key? Well, it can be anywhere! From an identity perspective, the location of the private key doesn't matter. Only that whoever has that key produces information under that identity, when signed.
So let's talk in practice: when you sign up for Bluesky, they store the private key for you. This way, for users that don't care about any of these details, they do not have to think about it at all. It's saved with the rest of your account data, you don't have to worry about backups or anything.
However, at any time, any user can register a rotation key with the PLC directory. To do this, you generate a new public and private key, and then store that private key wherever you'd like. All the usual caveats here apply. You can then use your existing private key to add this new public key to your account. Once you do that, it shows up in your DID document as a rotation key. You can use that rotation key to add more keys, remove the Bluesky owned keypair, whatever you want. Now it's not stored by Bluesky.
The majority of users have not done this, it's true. But they can. Whenever they'd like.