Proving ownership of a DID requires proving ownership of a private key corresponding to a public key for the DID. Of course, this could be done with a new DID-specific protocol. However, there already exist standard protocols for proving ownership of a public/private key pair.
The OpenID Connect specification defines Self-Issued Identities in Section 7. A self-issued identity is represented by a public/private key pair. Logging in with the self-issued identity proves control of the private key.
Microsoft is experimenting with using self-issued identities to prove ownership of DIDs. This is straightforward. The DID key is used as the key for the self-issued identity. The self-issued identity is validated in the standard way. In addition, a "did" claim containing the DID identifier is added to the self-issued ID Token. This enables relying parties that understand that the self-issued identity is also a DID to perform DID operations after control of the key has been verified using OpenID Connect.
I look forward to discussing this potential DID validation approach with the workshop participants.
Author: Michael B. Jones https://self-issued.info/, @selfissued