Create RSA signatures

The script now also creates RSA signatures. The key (image_sign.pem) to
use when signing images, can be created by running:
 $ openssl genrsa -out image_sign.pem 2048

The output now identical to what newt produces when enabling signature
verification.

The instruction for how to enable bootloader signature verification in
Mynewt can currently be found here:
https://github.com/apache/incubator-mynewt-core/blob/develop/libs/bootutil/signed_images.md

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>

1 files changed, 54 insertions, 5 deletions

diff --git a/zep2newt.py b/zep2newt.py
index 087f2a4..3ae3601 100755
--- a/zep2newt.py
+++ b/zep2newt.py
@@ -1,5 +1,4 @@
 #!/usr/bin/python2
-import hashlib
 import mmap
 import os
 import struct
@@ -7,6 +6,9 @@ import sys
 from argparse import ArgumentParser
 from newtimg import *
 from ctypes import *
+from Crypto.Signature import PKCS1_v1_5
+from Crypto.Hash import SHA256
+from Crypto.PublicKey import RSA
 
 DEBUG = False
 
@@ -140,8 +142,7 @@ def write_partial_img(binary_file, image_file, hdr, vtable_offs):
 
 
 ################################################################################
-def calculate_hash(image_file):
-    sha256 = hashlib.sha256()
+def calculate_hash(image_file, sha256):
     try:
         with open(image_file, "rb") as f:
             sha256.update(f.read())
@@ -160,7 +161,6 @@ def calculate_hash(image_file):
 def append_hash(image_file, digest):
     try:
         with open(image_file, "ab") as f:
-
             # Start by settings the TLV type
             # https://github.com/apache/incubator-mynewt-newt/blob/master/newt/image/image.go#L109-L116
             tlv_type = struct.pack('b', IMAGE_TLV_SHA256)
@@ -182,6 +182,43 @@ def append_hash(image_file, digest):
         sys.exit(1)
 
 ################################################################################
+def append_rsa_signature(image_file, key_file, sha256):
+    signature = None
+    try:
+        with open(key_file, "rb") as f:
+            rsa_key = RSA.importKey(f.read())
+            f.close()
+        rsa = PKCS1_v1_5.new(rsa_key)
+        signature = rsa.sign(sha256)
+
+    except (OSError, IOError):
+        print("[ERROR]: Cannot open %s" % (key_file))
+        sys.exit(1)
+
+    try:
+        with open(image_file, "ab") as f:
+            # Start by settings the TLV type
+            # https://github.com/apache/incubator-mynewt-newt/blob/master/newt/image/image.go#L109-L116
+            tlv_type = struct.pack('b', IMAGE_TLV_RSA2048)
+
+            # Next 1 byte padding
+            tlv_pad = '\x00'
+
+            # Finally the size of the TLV, for SHA256 that is 32 bytes
+            tlv_len = struct.pack('h', RSA_SIZE)
+
+            f.write(tlv_type)
+            f.write(tlv_pad)
+            f.write(tlv_len)
+            f.write(signature)
+            f.close()
+
+    except (OSError, IOError):
+        print("[ERROR]: Cannot open/append to %s" % (image_file))
+        sys.exit(1)
+
+
+################################################################################
 def create_jlink_script(image_file, offset, erase):
     """
     Creates a jlink script to flash the created binary.
@@ -272,10 +309,22 @@ def main(argv):
     # Write the image itself
     write_partial_img(args.binary_file, args.image_file, hdr, vtable_offs)
 
+    # We must use SHA256 from Crypto, since the RSA signature also uses some
+    # ASN.1 / oid, that will be created when using the SHA256 from Crypto
+    # (compared to hashlib that just do a pure hash).
+    sha256 = SHA256.new()
+
     # Now we have a header and the binary itself and we should get the hash of
     # those concatenated.
-    digest = calculate_hash(args.image_file)
+    digest = calculate_hash(args.image_file, sha256)
     append_hash(args.image_file, digest)
+
+    if args.sig_type == "RSA":
+        append_rsa_signature(args.image_file, args.key_file, sha256)
+    elif args.sig_type == "EC":
+        print("[ERROR]: ECDSA not implemented")
+        sys.exit(1)
+
     print("[*] Successfully created: %s" % args.image_file)
 
     # Misc function related to flashing