MODSIGN: Provide a script for generating a key ID from an X.509 cert

Provide a script to parse an X.509 certificate and certain pieces of
information from it in order to generate a key identifier to be included within
a module signature.

The script takes the Subject Name and extracts (if present) the
organizationName (O), the commonName (CN) and the emailAddress and fabricates
the signer's name from them:

 (1) If both O and CN exist, then the name will be "O: CN", unless:

     (a) CN is prefixed by O, in which case only CN is used.

     (b) CN and O share at least the first 7 characters, in which case only CN
     	 is used.

 (2) Otherwise, CN is used if present.

 (3) Otherwise, O is used if present.

 (4) Otherwise the emailAddress is used, if present.

 (5) Otherwise a blank name is used.

The script emits a binary encoded identifier in the following form:

 - 2 BE bytes indicating the length of the signer's name.

 - 2 BE bytes indicating the length of the subject key identifier.

 - The characters of the signer's name.

 - The bytes of the subject key identifier.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

scripts/x509keyid

@@ -0,0 +1,268 @@
+#!/usr/bin/perl -w
+#
+# Generate an identifier from an X.509 certificate that can be placed in a
+# module signature to indentify the key to use.
+#
+# Format:
+#
+#	./scripts/x509keyid <x509-cert> <signer's-name> <key-id>
+#
+# We read the DER-encoded X509 certificate and parse it to extract the Subject
+# name and Subject Key Identifier.  The provide the data we need to build the
+# certificate identifier.
+#
+# The signer's name part of the identifier is fabricated from the commonName,
+# the organizationName or the emailAddress components of the X.509 subject
+# name and written to the second named file.
+#
+# The subject key ID to select which of that signer's certificates we're
+# intending to use to sign the module is written to the third named file.
+#
+use strict;
+
+my $raw_data;
+
+die "Need three filenames\n" if ($#ARGV != 2);
+
+my $src = $ARGV[0];
+
+open(FD, "<$src") || die $src;
+binmode FD;
+my @st = stat(FD);
+die $src if (!@st);
+read(FD, $raw_data, $st[7]) || die $src;
+close(FD);
+
+my $UNIV = 0 << 6;
+my $APPL = 1 << 6;
+my $CONT = 2 << 6;
+my $PRIV = 3 << 6;
+
+my $CONS = 0x20;
+
+my $BOOLEAN	= 0x01;
+my $INTEGER	= 0x02;
+my $BIT_STRING	= 0x03;
+my $OCTET_STRING = 0x04;
+my $NULL	= 0x05;
+my $OBJ_ID	= 0x06;
+my $UTF8String	= 0x0c;
+my $SEQUENCE	= 0x10;
+my $SET		= 0x11;
+my $UTCTime	= 0x17;
+my $GeneralizedTime = 0x18;
+
+my %OIDs = (
+    pack("CCC", 85, 4, 3)	=> "commonName",
+    pack("CCC", 85, 4, 6)	=> "countryName",
+    pack("CCC", 85, 4, 10)	=> "organizationName",
+    pack("CCC", 85, 4, 11)	=> "organizationUnitName",
+    pack("CCCCCCCCC", 42, 134, 72, 134, 247, 13, 1, 1, 1) => "rsaEncryption",
+    pack("CCCCCCCCC", 42, 134, 72, 134, 247, 13, 1, 1, 5) => "sha1WithRSAEncryption",
+    pack("CCCCCCCCC", 42, 134, 72, 134, 247, 13, 1, 9, 1) => "emailAddress",
+    pack("CCC", 85, 29, 35)	=> "authorityKeyIdentifier",
+    pack("CCC", 85, 29, 14)	=> "subjectKeyIdentifier",
+    pack("CCC", 85, 29, 19)	=> "basicConstraints"
+);
+
+###############################################################################
+#
+# Extract an ASN.1 element from a string and return information about it.
+#
+###############################################################################
+sub asn1_extract($$@)
+{
+    my ($cursor, $expected_tag, $optional) = @_;
+
+    return [ -1 ]
+	if ($cursor->[1] == 0 && $optional);
+
+    die $src, ": ", $cursor->[0], ": ASN.1 data underrun (elem ", $cursor->[1], ")\n"
+	if ($cursor->[1] < 2);
+
+    my ($tag, $len) = unpack("CC", substr(${$cursor->[2]}, $cursor->[0], 2));
+
+    if ($expected_tag != -1 && $tag != $expected_tag) {
+	return [ -1 ]
+	    if ($optional);
+	die $src, ": ", $cursor->[0], ": ASN.1 unexpected tag (", $tag,
+	" not ", $expected_tag, ")\n";
+    }
+
+    $cursor->[0] += 2;
+    $cursor->[1] -= 2;
+
+    die $src, ": ", $cursor->[0], ": ASN.1 long tag\n"
+	if (($tag & 0x1f) == 0x1f);
+    die $src, ": ", $cursor->[0], ": ASN.1 indefinite length\n"
+	if ($len == 0x80);
+
+    if ($len > 0x80) {
+	my $l = $len - 0x80;
+	die $src, ": ", $cursor->[0], ": ASN.1 data underrun (len len $l)\n"
+	    if ($cursor->[1] < $l);
+
+	if ($l == 0x1) {
+	    $len = unpack("C", substr(${$cursor->[2]}, $cursor->[0], 1));
+	} elsif ($l = 0x2) {
+	    $len = unpack("n", substr(${$cursor->[2]}, $cursor->[0], 2));
+	} elsif ($l = 0x3) {
+	    $len = unpack("C", substr(${$cursor->[2]}, $cursor->[0], 1)) << 16;
+	    $len = unpack("n", substr(${$cursor->[2]}, $cursor->[0] + 1, 2));
+	} elsif ($l = 0x4) {
+	    $len = unpack("N", substr(${$cursor->[2]}, $cursor->[0], 4));
+	} else {
+	    die $src, ": ", $cursor->[0], ": ASN.1 element too long (", $l, ")\n";
+	}
+
+	$cursor->[0] += $l;
+	$cursor->[1] -= $l;
+    }
+
+    die $src, ": ", $cursor->[0], ": ASN.1 data underrun (", $len, ")\n"
+	if ($cursor->[1] < $len);
+
+    my $ret = [ $tag, [ $cursor->[0], $len, $cursor->[2] ] ];
+    $cursor->[0] += $len;
+    $cursor->[1] -= $len;
+
+    return $ret;
+}
+
+###############################################################################
+#
+# Retrieve the data referred to by a cursor
+#
+###############################################################################
+sub asn1_retrieve($)
+{
+    my ($cursor) = @_;
+    my ($offset, $len, $data) = @$cursor;
+    return substr($$data, $offset, $len);
+}
+
+###############################################################################
+#
+# Roughly parse the X.509 certificate
+#
+###############################################################################
+my $cursor = [ 0, length($raw_data), \$raw_data ];
+
+my $cert = asn1_extract($cursor, $UNIV | $CONS | $SEQUENCE);
+my $tbs = asn1_extract($cert->[1], $UNIV | $CONS | $SEQUENCE);
+my $version = asn1_extract($tbs->[1], $CONT | $CONS | 0, 1);
+my $serial_number = asn1_extract($tbs->[1], $UNIV | $INTEGER);
+my $sig_type = asn1_extract($tbs->[1], $UNIV | $CONS | $SEQUENCE);
+my $issuer = asn1_extract($tbs->[1], $UNIV | $CONS | $SEQUENCE);
+my $validity = asn1_extract($tbs->[1], $UNIV | $CONS | $SEQUENCE);
+my $subject = asn1_extract($tbs->[1], $UNIV | $CONS | $SEQUENCE);
+my $key = asn1_extract($tbs->[1], $UNIV | $CONS | $SEQUENCE);
+my $issuer_uid = asn1_extract($tbs->[1], $CONT | $CONS | 1, 1);
+my $subject_uid = asn1_extract($tbs->[1], $CONT | $CONS | 2, 1);
+my $extension_list = asn1_extract($tbs->[1], $CONT | $CONS | 3, 1);
+
+my $subject_key_id = ();
+my $authority_key_id = ();
+
+#
+# Parse the extension list
+#
+if ($extension_list->[0] != -1) {
+    my $extensions = asn1_extract($extension_list->[1], $UNIV | $CONS | $SEQUENCE);
+
+    while ($extensions->[1]->[1] > 0) {
+	my $ext = asn1_extract($extensions->[1], $UNIV | $CONS | $SEQUENCE);
+	my $x_oid = asn1_extract($ext->[1], $UNIV | $OBJ_ID);
+	my $x_crit = asn1_extract($ext->[1], $UNIV | $BOOLEAN, 1);
+	my $x_val = asn1_extract($ext->[1], $UNIV | $OCTET_STRING);
+
+	my $raw_oid = asn1_retrieve($x_oid->[1]);
+	next if (!exists($OIDs{$raw_oid}));
+	my $x_type = $OIDs{$raw_oid};
+
+	my $raw_value = asn1_retrieve($x_val->[1]);
+
+	if ($x_type eq "subjectKeyIdentifier") {
+	    my $vcursor = [ 0, length($raw_value), \$raw_value ];
+
+	    $subject_key_id = asn1_extract($vcursor, $UNIV | $OCTET_STRING);
+	}
+    }
+}
+
+###############################################################################
+#
+# Determine what we're going to use as the signer's name.  In order of
+# preference, take one of: commonName, organizationName or emailAddress.
+#
+###############################################################################
+my $org = "";
+my $cn = "";
+my $email = "";
+
+while ($subject->[1]->[1] > 0) {
+    my $rdn = asn1_extract($subject->[1], $UNIV | $CONS | $SET);
+    my $attr = asn1_extract($rdn->[1], $UNIV | $CONS | $SEQUENCE);
+    my $n_oid = asn1_extract($attr->[1], $UNIV | $OBJ_ID);
+    my $n_val = asn1_extract($attr->[1], -1);
+
+    my $raw_oid = asn1_retrieve($n_oid->[1]);
+    next if (!exists($OIDs{$raw_oid}));
+    my $n_type = $OIDs{$raw_oid};
+
+    my $raw_value = asn1_retrieve($n_val->[1]);
+
+    if ($n_type eq "organizationName") {
+	$org = $raw_value;
+    } elsif ($n_type eq "commonName") {
+	$cn = $raw_value;
+    } elsif ($n_type eq "emailAddress") {
+	$email = $raw_value;
+    }
+}
+
+my $id_name = $email;
+
+if ($org && $cn) {
+    # Don't use the organizationName if the commonName repeats it
+    if (length($org) <= length($cn) &&
+	substr($cn, 0, length($org)) eq $org) {
+	$id_name = $cn;
+	goto got_id_name;
+    }
+
+    # Or a signifcant chunk of it
+    if (length($org) >= 7 &&
+	length($cn) >= 7 &&
+	substr($cn, 0, 7) eq substr($org, 0, 7)) {
+	$id_name = $cn;
+	goto got_id_name;
+    }
+
+    $id_name = $org . ": " . $cn;
+} elsif ($org) {
+    $id_name = $org;
+} elsif ($cn) {
+    $id_name = $cn;
+}
+
+got_id_name:
+
+###############################################################################
+#
+# Output the signer's name and the key identifier that we're going to include
+# in module signatures.
+#
+###############################################################################
+die $src, ": ", "X.509: Couldn't find the Subject Key Identifier extension\n"
+    if (!$subject_key_id);
+
+my $id_key_id = asn1_retrieve($subject_key_id->[1]);
+
+open(OUTFD, ">$ARGV[1]") || die $ARGV[1];
+print OUTFD $id_name;
+close OUTFD || die $ARGV[1];
+
+open(OUTFD, ">$ARGV[2]") || die $ARGV[2];
+print OUTFD $id_key_id;
+close OUTFD || die $ARGV[2];