To form a dual signature, convert the two messages separately into two message digests
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concatenate the two messages and form the message digest of the combination
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Use this combined message digest to form dual digital signature
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If documents are 1 and 2, send document 1, digest of document 2 and dual digital signature to party that can see message 1 but for whom message 2 is not be seen.
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Analogously for other party and document 2
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This strategy allows one to intrinsically link messages in a legally provable fashion but confidentiality is preserved
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