Function single_cross_signing_key_id 

fn single_cross_signing_key_id(
    key: &CrossSigningKey,
) -> Result<&str, DatabaseError>

Extracts the single key id from a cross-signing key’s keys map, to be stored in the user_keys.key_id column.

A well-formed cross-signing key has exactly one entry in keys (per the Matrix spec, the object “must have exactly one property”); anything else is rejected as invalid data rather than silently picking one.