  Self-Sovereign Identity (SSI) is a new distributed method for identity
management, commonly used to address the problem that users are lack of control
over their identities. However, the excessive pursuit of self-sovereignty in
the most existing SSI schemes hinders sanctions against attackers. To deal with
the malicious behavior, a few SSI schemes introduce accountability mechanisms,
but they sacrifice users' privacy. What's more, the digital identities (static
strings or updatable chains) in the existing SSI schemes are as inputs to a
third-party executable program (mobile app, smart contract, etc.) to achieve
identity reading, storing and proving, users' self-sovereignty are weakened. To
solve the above problems, we present a new self-sovereign identity scheme to
strike a balance between privacy and accountability and get rid of the
dependence on the third-party program. In our scheme, one and only
individual-specific executable code is generated as a digital avatar-i for each
human to interact with others in cyberspace without a third-party program, in
which the embedding of biometrics enhances uniqueness and user control over
their identity. In addition, a joint accountability mechanism, which is based
on the shamir (t, n) threshold algorithm and a consortium blockchain, is
designed to restrict the power of each regulatory authority and protect users'
privacy. Finally, we analyze the security, SSI properties and conduct detailed
experiments in term of the cost of computation, storage and blockchain gas. The
analysis results indicate that our scheme resists the known attacks and
fulfills all the six SSI properties. Compared with the state-of-the-art
schemes, the extensive experiment results show that the cost is larger in
server storage, blockchain storage and blockchain gas, but is still low enough
for practical situations.
