Support cryptographically signed binary caches

NAR info files in binary caches can now have a cryptographic signature
that Nix will verify before using the corresponding NAR file.

To create a private/public key pair for signing and verifying a binary
cache, do:

  $ openssl genrsa -out ./cache-key.sec 2048
  $ openssl rsa -in ./cache-key.sec -pubout > ./cache-key.pub

You should also come up with a symbolic name for the key, such as
"cache.example.org-1".  This will be used by clients to look up the
public key.  (It's a good idea to number keys, in case you ever need
to revoke/replace one.)

To create a binary cache signed with the private key:

  $ nix-push --dest /path/to/binary-cache --key ./cache-key.sec --key-name cache.example.org-1

The public key (cache-key.pub) should be distributed to the clients.
They should have a nix.conf should contain something like:

  signed-binary-caches = *
  binary-cache-public-key-cache.example.org-1 = /path/to/cache-key.pub

If all works well, then if Nix fetches something from the signed
binary cache, you will see a message like:

  *** Downloading ‘http://cache.example.org/nar/7dppcj5sc1nda7l54rjc0g5l1hamj09j-subversion-1.7.11’ (signed by ‘cache.example.org-1’) to ‘/nix/store/7dppcj5sc1nda7l54rjc0g5l1hamj09j-subversion-1.7.11’...

On the other hand, if the signature is wrong, you get a message like

  NAR info file `http://cache.example.org/7dppcj5sc1nda7l54rjc0g5l1hamj09j.narinfo' has an invalid signature; ignoring

Signatures are implemented as a single line appended to the NAR info
file, which looks like this:

  Signature: 1;cache.example.org-1;HQ9Xzyanq9iV...muQ==

Thus the signature has 3 fields: a version (currently "1"), the ID of
key, and the base64-encoded signature of the SHA-256 hash of the
contents of the NAR info file up to but not including the Signature
line.

Issue #75.

scripts/download-from-binary-cache.pl.in

@@ -42,6 +42,8 @@ my $caBundle = $ENV{"CURL_CA_BUNDLE"} // $ENV{"OPENSSL_X509_CERT_FILE"};
 
 my $userName = getpwuid($<) or die "cannot figure out user name";
 
+my $requireSignedBinaryCaches = ($Nix::Config::config{"signed-binary-caches"} // "0") ne "0";
+
 
 sub addRequest {
     my ($storePath, $url, $head) = @_;
@@ -120,9 +122,10 @@ sub processRequests {
 
 
 sub initCache {
-    my $dbPath = "$Nix::Config::stateDir/binary-cache-v2.sqlite";
+    my $dbPath = "$Nix::Config::stateDir/binary-cache-v3.sqlite";
 
     unlink "$Nix::Config::stateDir/binary-cache-v1.sqlite";
+    unlink "$Nix::Config::stateDir/binary-cache-v2.sqlite";
 
     # Open/create the database.
     $dbh = DBI->connect("dbi:SQLite:dbname=$dbPath", "", "")
@@ -159,7 +162,7 @@ EOF
             narSize          integer,
             refs             text,
             deriver          text,
-            system           text,
+            signedBy         text,
             timestamp        integer not null,
             primary key (cache, storePath),
             foreign key (cache) references BinaryCaches(id) on delete cascade
@@ -183,7 +186,7 @@ EOF
 
     $insertNAR = $dbh->prepare(
         "insert or replace into NARs(cache, storePath, url, compression, fileHash, fileSize, narHash, " .
-        "narSize, refs, deriver, system, timestamp) values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)") or die;
+        "narSize, refs, deriver, signedBy, timestamp) values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)") or die;
 
     $queryNAR = $dbh->prepare("select * from NARs where cache = ? and storePath = ?") or die;
 
@@ -309,14 +312,16 @@ sub processNARInfo {
         return undef;
     }
 
-    my $narInfo = parseNARInfo($storePath, $request->{content});
+    my $narInfo = parseNARInfo($storePath, $request->{content}, $requireSignedBinaryCaches, $request->{url});
     return undef unless defined $narInfo;
 
+    die if $requireSignedBinaryCaches && !defined $narInfo->{signedBy};
+
     # Cache the result.
     $insertNAR->execute(
         $cache->{id}, basename($storePath), $narInfo->{url}, $narInfo->{compression},
         $narInfo->{fileHash}, $narInfo->{fileSize}, $narInfo->{narHash}, $narInfo->{narSize},
-        join(" ", @{$narInfo->{refs}}), $narInfo->{deriver}, $narInfo->{system}, time())
+        join(" ", @{$narInfo->{refs}}), $narInfo->{deriver}, $narInfo->{signedBy}, time())
         if shouldCache $request->{url};
 
     return $narInfo;
@@ -330,6 +335,10 @@ sub getCachedInfoFrom {
     my $res = $queryNAR->fetchrow_hashref();
     return undef unless defined $res;
 
+    # We may previously have cached this info when signature checking
+    # was disabled.  In that case, ignore the cached info.
+    return undef if $requireSignedBinaryCaches && !defined $res->{signedBy};
+
     return
         { url => $res->{url}
         , compression => $res->{compression}
@@ -339,6 +348,7 @@ sub getCachedInfoFrom {
         , narSize => $res->{narSize}
         , refs => [ split " ", $res->{refs} ]
         , deriver => $res->{deriver}
+        , signedBy => $res->{signedBy}
         } if defined $res;
 }
 
@@ -522,7 +532,8 @@ sub downloadBinary {
             next;
         }
         my $url = "$cache->{url}/$info->{url}"; # FIXME: handle non-relative URLs
-        print STDERR "\n*** Downloading ‘$url’ to ‘$storePath’...\n";
+        die if $requireSignedBinaryCaches && !defined $info->{signedBy};
+        print STDERR "\n*** Downloading ‘$url’ ", ($requireSignedBinaryCaches ? "(signed by ‘$info->{signedBy}’) " : ""), "to ‘$storePath’...\n";
         checkURL $url;
         if (system("$Nix::Config::curl --fail --location --insecure '$url' $decompressor | $Nix::Config::binDir/nix-store --restore $destPath") != 0) {
             warn "download of `$url' failed" . ($! ? ": $!" : "") . "\n";
@@ -530,6 +541,7 @@ sub downloadBinary {
         }
 
         # Tell Nix about the expected hash so it can verify it.
+        die unless defined $info->{narHash} && $info->{narHash} ne "";
         print "$info->{narHash}\n";
 
         print STDERR "\n";

scripts/nix-push.in

@@ -10,6 +10,7 @@ use Nix::Config;
 use Nix::Store;
 use Nix::Manifest;
 use Nix::Utils;
+use Nix::Crypto;
 
 my $tmpDir = tempdir("nix-push.XXXXXX", CLEANUP => 1, TMPDIR => 1)
     or die "cannot create a temporary directory";
@@ -25,6 +26,8 @@ my $writeManifest = 0;
 my $manifestPath;
 my $archivesURL;
 my $link = 0;
+my $privateKeyFile;
+my $keyName;
 my @roots;
 
 for (my $n = 0; $n < scalar @ARGV; $n++) {
@@ -57,6 +60,14 @@ for (my $n = 0; $n < scalar @ARGV; $n++) {
         $archivesURL = $ARGV[$n];
     } elsif ($arg eq "--link") {
         $link = 1;
+    } elsif ($arg eq "--key") {
+        $n++;
+        die "$0: `$arg' requires an argument\n" unless $n < scalar @ARGV;
+        $privateKeyFile = $ARGV[$n];
+    } elsif ($arg eq "--key-name") {
+        $n++;
+        die "$0: `$arg' requires an argument\n" unless $n < scalar @ARGV;
+        $keyName = $ARGV[$n];
     } elsif (substr($arg, 0, 1) eq "-") {
         die "$0: unknown flag `$arg'\n";
     } else {
@@ -99,7 +110,7 @@ foreach my $storePath (@storePaths) {
     my $pathHash = substr(basename($storePath), 0, 32);
     my $narInfoFile = "$destDir/$pathHash.narinfo";
     if (-e $narInfoFile) {
-        my $narInfo = parseNARInfo($storePath, readFile($narInfoFile));
+        my $narInfo = parseNARInfo($storePath, readFile($narInfoFile), 0, $narInfoFile) or die "cannot read `$narInfoFile'\n";
         my $narFile = "$destDir/$narInfo->{url}";
         if (-e $narFile) {
             print STDERR "skipping existing $storePath\n";
@@ -245,6 +256,11 @@ for (my $n = 0; $n < scalar @storePaths2; $n++) {
         }
     }
 
+    if (defined $privateKeyFile && defined $keyName) {
+        my $sig = signString($privateKeyFile, $info);
+        $info .= "Signature: 1;$keyName;$sig\n";
+    }
+
     my $pathHash = substr(basename($storePath), 0, 32);
 
     $dst = "$destDir/$pathHash.narinfo";