What are the two defining properties of a self-sovereign identity in its original, strict sense?

• The user owns their electronic identity and so holds their own personal data, never a central service or a state issuer.
• There is no central store of identity data, only confirmations (attestations) in public, decentralised registers.

The first property is about ownership and control: the identity data sits with the person, typically in a wallet on their device. No provider keeps a master copy that it could read, sell, lose in a breach or switch off.

The second property is about architecture: something still has to let a stranger check that a claim is genuine. In SSI that "something" is a public, decentralised register that holds only what is needed for verification (for example the public keys of issuers), never the personal data itself.

Note how strict the vision is: it excludes even a state as the owner of your identity. The pragmatic version used today (see the next cards) relaxes this: a state may well issue the credential, as long as the holder keeps it and the issuer is not involved when it is used.

Go deeper:

• Self-sovereign identity (Wikipedia) — how the strict definition meets real national identity systems, including the EU's.