Procedures and techniques to confirm upper limits on the number of nuclear warheads will become a key verification objective should future arms-control agreements place limits on the total number of nuclear weapons in the arsenals. This would require baseline declarations, and the challenge would then be to confirm their correctness and completeness. So far, most states remain reluctant to make baseline declarations, however, and ways ought to be found to encourage steps at an early date. Here, we explore a cryptographic escrow scheme that would both provide a secure information-sharing mechanism and a basis to confirm numerical limits without using tagging techniques. It leverages well established cryptographic primitives, in particular commitment schemes. Such schemes allow a party to commit to a particular piece of information, or value, while keeping it hidden from others. The value can be released at a later stage while ensuring other parties it was not altered. Cryptographic commitments to declaration can use a hash (or "message digest") that is much shorter than the message itself. Our escrow could have two distinct but equally important roles. First, it would commit weapon states to the current status of their nuclear arsenal (including, for example, warhead numbers, types, and storage locations) without having to make this information public at this time. Having these records available later on could significantly increase the confidence in a future verification regime because a weapon state could demonstrate the history or "provenance" of particular treaty accountable items. Second, such declarations could also lay the basis for future verification approaches that do not rely on tagging treaty accountable items at all. Instead, host countries would reveal cleartext for selected entries as needed, and inspections would then confirm the consistency between the declared and the actual inventory of treaty accountable items at a particular site.
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