  Public blockchains such as Ethereum and Bitcoin provide transparency and
accountability, and have strong non-repudiation properties, but fall far short
of enterprise privacy requirements for business processes. Consequently
consortiums are exploring private blockchains to keep their membership and
transactions private. However, private blockchains do not provide adequate
protection against potential collusion by consortium members to revert the
state of the blockchain. To countenance this, the private blockchain state may
be "pinned" to a tamper resistant public blockchain. Existing solutions
offering pinning to the public blockchain would reveal the transaction rate of
the private blockchain, and do not provide a mechanism to contest the validity
of a pin. Moreover, they require that all transactions and members of the
private blockchain be revealed. These challenges are hampering the wider
adoption of private blockchain technology. We describe the primary author's
`Anonymous State Pinning approach', which overcomes these limitations and
present a security proof to demonstrate pins can be challenged without
compromising these properties. We perform a gas cost analysis of the
implementation to estimate the operating cost of this technology, which shows
that pinning a private blockchain at the rate of one pin per hour would cost
US$508 per year. A hierarchical pinning approach is proposed which would allow
many private blockchains to pin to a management blockchain which would then pin
to Ethereum MainNet. This approach saves money, but at the cost of increased
finality times.
