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Subliminal Channels

ECDSA P-256 · RFC 6979

Sign the same document in different valid ways and read the additional symbol carried in a signature.

WHY IT MATTERS

A normal ECDSA verifier checks mathematical validity, not whether a signer chose one valid signature from many to encode an extra bit. Deterministic signing closes this particular choice channel, not every covert channel.

Preparing the lab…

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