`cosign` is my new program for cooperatively signing documents with RSA (PKCS#1 v1.5 + SHA256) in a way that requires all the parties to produce partial signatures on the document without any of them having access to the private key. https://github.com/osresearch/cosign
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Inspired by Boyd's 1986 paper "Digital Multisignatures", which introduced additive secret sharing for splitting RSA keys. Unfortunately I haven't found an online version of that paper, although a followup from 1988 is available. https://link.springer.com/content/pdf/10.1007/3-540-45961-8_40.pdf
