I have just tried out an idea of maartin in another Usenet group many years ago of how one could introduce a back door into a proprietary software (or defacto similarly into a huge and poorly documented and hence never by independent experts carefully scrutinized software package) for the generation of RSA keys. See the last paragraph in the Epilogue of:
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Concerning "official" back doors, see the critiques of Internet pioneer Vint Cert:
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M. K. Shen