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KEYS: Provide a script to extract a module signature

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The supplied script takes a signed module file and extracts the tailmost
signature (there could theoretically be more than one) and dumps all or
part of it or the unsigned file to stdout.

Call as:

	scripts/extract-module-sig.pl -[0adnks] module-file >out

where the initial flag indicates which bit of the signed file you want dumping
to stdout:

 (*) "-0".  Dumps the unsigned data with the signature stripped.

 (*) "-a".  Dumps all of the signature data, including the magic number.

 (*) "-d".  Dumps the signature information block as a sequence of decimal
     	    numbers in text form with spaces between (crypto algorithm type,
     	    hash type, identifier type, signer's name length, key identifier
     	    length and signature length).

 (*) "-n".  Dumps the signer's name contents.

 (*) "-k".  Dumps the key identifier contents.

 (*) "-s".  Dumps the cryptographic signature contents.

In the case that the signature is a PKCS#7 (or CMS) message, -n and -k will
print a warning to stderr and dump nothing to stdout, but will otherwise
complete okay; the entire PKCS#7/CMS message will be dumped by "-s"; and "-d"
will show "0 0 2 0 0 <pkcs#7-msg-len>".

Signed-off-by: David Howells <dhowells@redhat.com>
This commit is contained in:
David Howells 2015-10-21 14:04:47 +01:00
parent 2221a6ee73
commit 4adc605edc
1 changed files with 136 additions and 0 deletions
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136
scripts/extract-module-sig.pl Executable file
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#!/usr/bin/perl -w
#
# extract-mod-sig <part> <module-file>
#
# Reads the module file and writes out some or all of the signature
# section to stdout. Part is the bit to be written and is one of:
#
# -0: The unsigned module, no signature data at all
# -a: All of the signature data, including magic number
# -d: Just the descriptor values as a sequence of numbers
# -n: Just the signer's name
# -k: Just the key ID
# -s: Just the crypto signature or PKCS#7 message
#
use strict;
die "Format: $0 -[0adnks] module-file >out\n"
if ($#ARGV != 1);
my $part = $ARGV[0];
my $modfile = $ARGV[1];
my $magic_number = "~Module signature appended~\n";
#
# Read the module contents
#
open FD, "<$modfile" || die $modfile;
binmode(FD);
my @st = stat(FD);
die "$modfile" unless (@st);
my $buf = "";
my $len = sysread(FD, $buf, $st[7]);
die "$modfile" unless (defined($len));
die "Short read on $modfile\n" unless ($len == $st[7]);
close(FD) || die $modfile;
print STDERR "Read ", $len, " bytes from module file\n";
die "The file is too short to have a sig magic number and descriptor\n"
if ($len < 12 + length($magic_number));
#
# Check for the magic number and extract the information block
#
my $p = $len - length($magic_number);
my $raw_magic = substr($buf, $p);
die "Magic number not found at $len\n"
if ($raw_magic ne $magic_number);
print STDERR "Found magic number at $len\n";
$p -= 12;
my $raw_info = substr($buf, $p, 12);
my @info = unpack("CCCCCxxxN", $raw_info);
my ($algo, $hash, $id_type, $name_len, $kid_len, $sig_len) = @info;
if ($id_type == 0) {
print STDERR "Found PGP key identifier\n";
} elsif ($id_type == 1) {
print STDERR "Found X.509 cert identifier\n";
} elsif ($id_type == 2) {
print STDERR "Found PKCS#7/CMS encapsulation\n";
} else {
print STDERR "Found unsupported identifier type $id_type\n";
}
#
# Extract the three pieces of info data
#
die "Insufficient name+kid+sig data in file\n"
unless ($p >= $name_len + $kid_len + $sig_len);
$p -= $sig_len;
my $raw_sig = substr($buf, $p, $sig_len);
$p -= $kid_len;
my $raw_kid = substr($buf, $p, $kid_len);
$p -= $name_len;
my $raw_name = substr($buf, $p, $name_len);
my $module_len = $p;
if ($sig_len > 0) {
print STDERR "Found $sig_len bytes of signature [";
my $n = $sig_len > 16 ? 16 : $sig_len;
foreach my $i (unpack("C" x $n, substr($raw_sig, 0, $n))) {
printf STDERR "%02x", $i;
}
print STDERR "]\n";
}
if ($kid_len > 0) {
print STDERR "Found $kid_len bytes of key identifier [";
my $n = $kid_len > 16 ? 16 : $kid_len;
foreach my $i (unpack("C" x $n, substr($raw_kid, 0, $n))) {
printf STDERR "%02x", $i;
}
print STDERR "]\n";
}
if ($name_len > 0) {
print STDERR "Found $name_len bytes of signer's name [$raw_name]\n";
}
#
# Produce the requested output
#
if ($part eq "-0") {
# The unsigned module, no signature data at all
binmode(STDOUT);
print substr($buf, 0, $module_len);
} elsif ($part eq "-a") {
# All of the signature data, including magic number
binmode(STDOUT);
print substr($buf, $module_len);
} elsif ($part eq "-d") {
# Just the descriptor values as a sequence of numbers
print join(" ", @info), "\n";
} elsif ($part eq "-n") {
# Just the signer's name
print STDERR "No signer's name for PKCS#7 message type sig\n"
if ($id_type == 2);
binmode(STDOUT);
print $raw_name;
} elsif ($part eq "-k") {
# Just the key identifier
print STDERR "No key ID for PKCS#7 message type sig\n"
if ($id_type == 2);
binmode(STDOUT);
print $raw_kid;
} elsif ($part eq "-s") {
# Just the crypto signature or PKCS#7 message
binmode(STDOUT);
print $raw_sig;
}