btrbk: add key derivation for encrypted raw targets using external backend

Makefile

@@ -43,6 +43,7 @@ install-share:
 	install -pDm755 ssh_filter_btrbk.sh "$(DESTDIR)$(SCRIPTDIR)/ssh_filter_btrbk.sh"
 	install -pDm755 contrib/cron/btrbk-mail "$(DESTDIR)$(SCRIPTDIR)/btrbk-mail"
 	install -pDm755 contrib/migration/raw_suffix2sidecar "$(DESTDIR)$(SCRIPTDIR)/raw_suffix2sidecar"
+	install -pDm755 contrib/crypt/kdf_pbkdf2.py "$(DESTDIR)$(SCRIPTDIR)/kdf_pbkdf2.py"
 
 install-man:
 	@echo 'installing manpages...'

btrbk

@@ -114,6 +114,10 @@ my %config_options = (
   openssl_iv_size             => { default => undef,     accept => [ "no", accept_numeric => 1 ] },
   openssl_keyfile             => { default => undef,     accept_file => { absolute => 1 } },
 
+  kdf_backend                 => { default => undef,     accept_file => { absolute => 1 } },
+  kdf_keysize                 => { default => "32",      accept_numeric => 1 },
+  kdf_keygen                  => { default => "once",    accept => [ "once", "each" ] },
+
   group                       => { default => undef,     accept_regexp => qr/^$group_match(\s*,\s*$group_match)*$/, split => qr/\s*,\s*/ },
 
   backend                     => { default => "btrfs-progs", accept => [       "btrfs-progs", "btrfs-progs-btrbk", "btrfs-progs-sudo" ] },
@@ -236,6 +240,7 @@ my %raw_info_sort = (
   encrypt              => 11,
   cipher               => 12,
   iv                   => 13,
+  # kdf_* (generated by kdf_backend)
  );
 
 my %url_cache;       # map URL to btr_tree node
@@ -261,6 +266,8 @@ my @transaction_log;
 my %config_override;
 my @tm_now;  # current localtime ( sec, min, hour, mday, mon, year, wday, yday, isdst )
 my %warn_once;
+my %kdf_vars;
+my $kdf_session_key;
 
 
 $SIG{__DIE__} = sub {
@@ -1433,9 +1440,63 @@ sub btrfs_send_to_file($$$;$$)
         $raw_info{iv} = $iv;
       }
 
+      my $encrypt_key;
+      if($encrypt->{keyfile}) {
+        if($encrypt->{kdf_backend}) {
+          WARN "Both openssl_keyfile and kdf_backend are configured, ignoring kdf_backend!";
+        }
+        $encrypt_key = '$(cat ' . $encrypt->{keyfile} . ')';
+      }
+      elsif($encrypt->{kdf_backend}) {
+        if($encrypt->{kdf_keygen_each}) {
+          $kdf_session_key = undef;
+          %kdf_vars = ();
+        }
+        if($kdf_session_key) {
+          INFO "Reusing session key for: $vol_received->{PRINT}";
+        }
+        else {
+          # run kdf backend, set session key and vars
+          DEBUG "Generating session key for: $vol_received->{PRINT}";
+          my $kdf_backend_name = $encrypt->{kdf_backend};
+          $kdf_backend_name =~ s/^.*\///;
+
+          print STDOUT "\nGenerate session key for " . ($encrypt->{kdf_keygen_each} ? "\"$vol_received->{PRINT}\"" : "all raw backups") . ":\n";
+          my $kdf_values = run_cmd(cmd => [ $encrypt->{kdf_backend}, $encrypt->{kdf_keysize} ],
+                                   non_destructive => 1,
+                                   name => $kdf_backend_name
+                                  );
+          return undef unless(defined($kdf_values));
+          foreach(split("\n", $kdf_values)) {
+            chomp;
+            next if /^\s*$/; # ignore empty lines
+            if(/^KEY=([0-9a-fA-f]+)/) {
+              $kdf_session_key = $1;
+            }
+            elsif(/^([a-z_]+)=(.*)/) {
+              my $info_key = 'kdf_' . $1;
+              my $info_val = $2;
+              DEBUG "Adding raw_info from kdf_backend: $info_key=$info_val";
+              $kdf_vars{$info_key} = $info_val;
+            }
+            else {
+              ERROR "Ambiguous line from kdf_backend: $encrypt->{kdf_backend}";
+              return undef;
+            }
+          }
+          unless($kdf_session_key && (length($kdf_session_key) == ($encrypt->{kdf_keysize} * 2))) {
+            ERROR "Ambiguous key value from kdf_backend: $encrypt->{kdf_backend}";
+            return undef;
+          }
+          INFO "Generated session key for: $vol_received->{PRINT}";
+        }
+        $encrypt_key = $kdf_session_key;
+        %raw_info = ( %kdf_vars, %raw_info );
+      }
+
       my @openssl_options = (
         '-' . $encrypt->{ciphername},
-        '-K $(cat ' . $encrypt->{keyfile} . ')',
+        '-K', $encrypt_key,
        );
       push @openssl_options, ('-iv', $iv) if($iv);
 
@@ -2852,6 +2913,9 @@ sub config_encrypt_hash($$)
     iv_size => config_key($config, "openssl_iv_size"),
     ciphername => config_key($config, "openssl_ciphername"),
     keyfile => config_key($config, "openssl_keyfile"),
+    kdf_keygen_each => (config_key($config, "kdf_keygen") eq "each"),
+    kdf_backend => config_key($config, "kdf_backend"),
+    kdf_keysize => config_key($config, "kdf_keysize"),
   };
 }
 

contrib/crypt/kdf_pbkdf2.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+#
+# kdf_pbkdf2.py - (kdf_backend for btrbk)
+#
+# Copyright (c) 2017 Axel Burri
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# ---------------------------------------------------------------------
+# The official btrbk website is located at:
+# http://digint.ch/btrbk/
+#
+# Author:
+# Axel Burri <axel@tty0.ch>
+# ---------------------------------------------------------------------
+
+import sys
+import os
+import getpass
+import hashlib
+
+def passprompt():
+    pprompt = lambda: (getpass.getpass("Passphrase: "), getpass.getpass("Retype passphrase: "))
+    p1, p2 = pprompt()
+    while p1 != p2:
+        print("No match, please try again", file=sys.stderr)
+        p1, p2 = pprompt()
+    return p1
+
+if len(sys.argv) <= 1:
+    print("Usage: {} <dklen>".format(sys.argv[0]), file=sys.stderr)
+    sys.exit(1)
+
+hash_name = "sha256"
+iterations = 300000
+dklen = int(sys.argv[1])
+salt = os.urandom(16)
+password = passprompt().encode("utf-8")
+
+dk = hashlib.pbkdf2_hmac(hash_name=hash_name, password=password, salt=salt, iterations=iterations, dklen=dklen)
+
+salt_hex = "".join(["{:02x}".format(x) for x in salt])
+dk_hex = "".join(["{:02x}".format(x) for x in dk])
+
+print("KEY=" + dk_hex);
+print("algoritm=pbkdf2_hmac");
+print("hash_name=" + hash_name);
+print("salt=" + salt_hex);
+print("iterations=" + str(iterations));

doc/btrbk.conf.5

@@ -456,6 +456,12 @@ openssl_ciphername  <name>  (defaults to "aes-256-cbc")
 openssl_iv_size  <size-in-bytes>|no  (depends on selected cipher)
 .PP
 openssl_keyfile  <file>|no
+.PP
+kdf_backend  <file>|no
+.PP
+kdf_keysize  <size>  (defaults to "32")
+.PP
+kdf_keygen  once|each  (defaults to "once")
 .RE
 .PD
 .PP