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Code-signing private key notes
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==
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The private keys for these certificates were generated on Gavin's main work machine,
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following the certificate authoritys' recommendations for generating certificate
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signing requests.
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For OSX, the private key was generated by Keychain.app on Gavin's main work machine.
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The key and certificate is in a separate, passphrase-protected keychain file that is
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unlocked to sign the Bitcoin-Qt.app bundle.
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For Windows, the private key was generated by Firefox running on Gavin's main work machine.
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The key and certificate were exported into a separate, passphrase-protected PKCS#12 file, and
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then deleted from Firefox's keystore. The exported file is used to sign the Windows setup.exe.
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Threat analysis
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--
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Gavin is a single point of failure. He could be coerced to divulge the secret signing keys,
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allowing somebody to distribute a Bitcoin-Qt.app or bitcoin-qt-setup.exe with a valid
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signature but containing a malicious binary.
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Or the machine Gavin uses to sign the binaries could be compromised, either remotely or
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by breaking in to his office, allowing the attacker to get the private key files and then
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install a keylogger to get the passphrase that protects them.
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Threat Mitigation
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--
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"Air gapping" the machine used to do the signing will not work, because the signing
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process needs to access a timestamp server over the network. And it would not
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prevent the "rubber hose cryptography" threat (coercing Gavin to sign a bad binary
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or divulge the private keys).
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Windows binaries are reproducibly 'gitian-built', and the setup.exe file created
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by the NSIS installer system is a 7zip archive, so you could check to make sure
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that the bitcoin-qt.exe file inside the installer had not been tampered with.
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However, an attacker could modify the installer's code, so when the setup.exe
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was run it compromised users' systems. A volunteer to write an auditing tool
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that checks the setup.exe for tampering, and checks the files in it against
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the list of gitian signatures, is needed.
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The long-term solution is something like the 'gitian downloader' system, which
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uses signatures from multiple developers to determine whether or not a binary
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should be trusted. However, that just pushes the problem to "how will
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non-technical users securely get the gitian downloader code to start?"
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