Authenticating binary substitutes

[ludo@gnu.org (Ludovic Courtès)]

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Hello,

Currently the “binary cache” substituter relies on DNS to authenticate
downloaded binaries: anything coming from, say, hydra.nixos.org is
considered authentic, because hydra.nixos.org is listed in the
‘trusted-binary-cache’ list.

This is obviously subject to person-in-the-middle attacks: one could
connect over Wifi to somebody else’s network, which happens to redirect
hydra.nixos.org to evil.example.com, and end up downloading evil binaries.

I was thinking of a simple extension to solve that:

  1a. The /nix-cache-info file would contain an (optional)
     ‘OpenPGPFingerprint’ field, to announce the fingerprint of the
     OpenPGP key used to sign Nars.

  1b. In addition to, or alternatively, a /nix-signing-key file would be
      served, containing the OpenPGP key used to sign Nars.

  2.  In addition to serving, say,
      /nar/zwpx7d0sv36fi4xpwqx2dak0axx5nji8-gmp-5.1.1, the server would
      also serve /nar/zwpx7d0sv36fi4xpwqx2dak0axx5nji8-gmp-5.1.1.sig, an
      OpenPGP binary signature of the uncompressed Nar.

WDYT?  Could this be implemented in Hydra?

Ludo’.

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