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.

perl/Makefile.am

@@ -1,4 +1,4 @@
-PERL_MODULES = lib/Nix/Store.pm lib/Nix/Manifest.pm lib/Nix/GeneratePatches.pm lib/Nix/SSH.pm lib/Nix/CopyClosure.pm lib/Nix/Config.pm.in lib/Nix/Utils.pm
+PERL_MODULES = lib/Nix/Store.pm lib/Nix/Manifest.pm lib/Nix/GeneratePatches.pm lib/Nix/SSH.pm lib/Nix/CopyClosure.pm lib/Nix/Config.pm.in lib/Nix/Utils.pm lib/Nix/Crypto.pm
 
 all: $(PERL_MODULES:.in=)
 

perl/lib/Nix/Config.pm.in

@@ -13,6 +13,7 @@ $storeDir = $ENV{"NIX_STORE_DIR"} || "@storedir@";
 $bzip2 = "@bzip2@";
 $xz = "@xz@";
 $curl = "@curl@";
+$openssl = "@openssl@";
 
 $useBindings = "@perlbindings@" eq "yes";
 
@@ -32,7 +33,7 @@ sub readConfig {
 
     open CONFIG, "<$config" or die "cannot open `$config'";
     while (<CONFIG>) {
-        /^\s*([\w|-]+)\s*=\s*(.*)$/ or next;
+        /^\s*([\w\-\.]+)\s*=\s*(.*)$/ or next;
         $config{$1} = $2;
     }
     close CONFIG;

perl/lib/Nix/Crypto.pm

@@ -0,0 +1,42 @@
+package Nix::Crypto;
+
+use strict;
+use MIME::Base64;
+use Nix::Store;
+use Nix::Config;
+use IPC::Open2;
+
+our @ISA = qw(Exporter);
+our @EXPORT = qw(signString isValidSignature);
+
+sub signString {
+    my ($privateKeyFile, $s) = @_;
+    my $hash = hashString("sha256", 0, $s);
+    my ($from, $to);
+    my $pid = open2($from, $to, $Nix::Config::openssl, "rsautl", "-sign", "-inkey", $privateKeyFile);
+    print $to $hash;
+    close $to;
+    local $/ = undef;
+    my $sig = <$from>;
+    close $from;
+    waitpid($pid, 0);
+    die "$0: OpenSSL returned exit code $? while signing hash\n" if $? != 0;
+    my $sig64 = encode_base64($sig, "");
+    return $sig64;
+}
+
+sub isValidSignature {
+    my ($publicKeyFile, $sig64, $s) = @_;
+    my ($from, $to);
+    my $pid = open2($from, $to, $Nix::Config::openssl, "rsautl", "-verify", "-inkey", $publicKeyFile, "-pubin");
+    print $to decode_base64($sig64);
+    close $to;
+    my $decoded = <$from>;
+    close $from;
+    waitpid($pid, 0);
+    return 0 if $? != 0;
+    my $hash = hashString("sha256", 0, $s);
+    return $decoded eq $hash;
+}
+
+1;

perl/lib/Nix/Manifest.pm

@@ -8,6 +8,7 @@ use File::stat;
 use File::Path;
 use Fcntl ':flock';
 use Nix::Config;
+use Nix::Crypto;
 
 our @ISA = qw(Exporter);
 our @EXPORT = qw(readManifest writeManifest updateManifestDB addPatch deleteOldManifests parseNARInfo);
@@ -394,9 +395,10 @@ sub deleteOldManifests {
 
 # Parse a NAR info file.
 sub parseNARInfo {
-    my ($storePath, $content) = @_;
+    my ($storePath, $content, $requireValidSig, $location) = @_;
 
-    my ($storePath2, $url, $fileHash, $fileSize, $narHash, $narSize, $deriver, $system);
+    my ($storePath2, $url, $fileHash, $fileSize, $narHash, $narSize, $deriver, $system, $sig);
+    my $signedData = "";
     my $compression = "bzip2";
     my @refs;
 
@@ -412,11 +414,13 @@ sub parseNARInfo {
         elsif ($1 eq "References") { @refs = split / /, $2; }
         elsif ($1 eq "Deriver") { $deriver = $2; }
         elsif ($1 eq "System") { $system = $2; }
+        elsif ($1 eq "Signature") { $sig = $2; last; }
+        $signedData .= "$line\n";
     }
 
     return undef if $storePath ne $storePath2 || !defined $url || !defined $narHash;
 
-    return
+    my $res =
         { url => $url
         , compression => $compression
         , fileHash => $fileHash
@@ -427,6 +431,36 @@ sub parseNARInfo {
         , deriver => $deriver
         , system => $system
         };
+
+    if ($requireValidSig) {
+        if (!defined $sig) {
+            warn "NAR info file `$location' lacks a signature; ignoring\n";
+            return undef;
+        }
+        my ($sigVersion, $keyName, $sig64) = split ";", $sig;
+        $sigVersion //= 0;
+        if ($sigVersion != 1) {
+            warn "NAR info file `$location' has unsupported version $sigVersion; ignoring\n";
+            return undef;
+        }
+        return undef unless defined $keyName && defined $sig64;
+        my $publicKeyFile = $Nix::Config::config{"binary-cache-public-key-$keyName"};
+        if (!defined $publicKeyFile) {
+            warn "NAR info file `$location' is signed by unknown key `$keyName'; ignoring\n";
+            return undef;
+        }
+        if (! -f $publicKeyFile) {
+            die "binary cache public key file `$publicKeyFile' does not exist\n";
+            return undef;
+        }
+        if (!isValidSignature($publicKeyFile, $sig64, $signedData)) {
+            warn "NAR info file `$location' has an invalid signature; ignoring\n";
+            return undef;
+        }
+        $res->{signedBy} = $keyName;
+    }
+
+    return $res;
 }