espsecure: Add ECDSA signature support

espsecure.py

@@ -20,21 +20,28 @@
 import sys
 import os
 import hashlib
+import struct
 import pyaes
+import ecdsa
 
 def get_chunks(source, chunk_len):
     """ Returns an iterator over 'chunk_len' chunks of 'source' """
     return (source[i: i+chunk_len] for i in range(0, len(source), chunk_len))
 
 def generate_key(args):
+    """ Save 32 random bytes for use as a pre-generated secure bootloader siging key """
     if os.path.exists(args.keyfile):
         raise FatalError("Output key file %s already exists." % args.keyfile)
     with open(args.keyfile, "wb") as f:
         # Python docs say os.urandom is "suitable for cryptographic use"
         f.write(os.urandom(32))
+    print "Private secure bootloader key written to %s" % args.keyfile
 
 
 def digest_secure_bootloader(args):
+    """ Calculate the digest of a bootloader image, in the same way the hardware
+    secure boot engine would do so. Can be used with a pre-loaded key to update a
+    secure bootloader. """
     if args.iv is not None:
         print "WARNING: --iv argument is for TESTING PURPOSES ONLY"
         iv = args.iv.read(128)
@@ -79,6 +86,51 @@ def digest_secure_bootloader(args):
     print "digest+image written to %s" % args.output
 
 
+def generate_signing_key(args):
+    """ Generate an ECDSA signing key for signing secure boot images (post-bootloader) """
+    if os.path.exists(args.keyfile):
+        raise esptool.FatalError("ERROR: Key file %s already exists" % args.keyfile)
+    sk = ecdsa.SigningKey.generate(curve=ecdsa.NIST256p)
+    with open(args.keyfile, "wb") as f:
+        f.write(sk.to_pem())
+    print("ECDSA NIST256p private key in PEM format written to %s" % args.keyfile)
+
+
+def _load_key(args):
+    sk = ecdsa.SigningKey.from_pem(args.keyfile.read())
+    if sk.curve != ecdsa.NIST256p:
+        raise esptool.FatalError("Signing key uses incorrect curve. ESP32 Secure Boot only supports NIST256p (openssl calls this curve 'prime256v1")
+    return sk
+
+def sign_data(args):
+    """ Sign a data file with a ECDSA private key, append binary signature to file contents """
+    sk = _load_key(args)
+
+    # calculate SHA512 digest of binary & sign it
+    digest = hashlib.sha512()
+    binary_content = args.datafile.read()
+    digest.update(binary_content)
+    signature = sk.sign_deterministic(digest.digest())
+
+    if args.output is None or os.path.abspath(args.output) == os.path.abspath(args.datafile.name):  # append signature to input file
+        args.datafile.close()
+        outfile = open(args.datafile.name, "ab")
+    else:  # write file & signature to new file
+        outfile = open(args.output, "wb")
+        outfile.write(binary_content)
+    outfile.write(struct.pack("I", 0))  # Version indicator, allow for different curves/formats later
+    outfile.write(signature)
+    outfile.close()
+    print "Signed %d bytes of data from %s with key %s" % (len(binary_content), args.datafile.name, args.keyfile.name)
+
+
+def extract_public_key(args):
+    """ Load an ECDSA private key and extract the embedded public key as raw binary data. """
+    sk = _load_key(args)
+    vk = sk.get_verifying_key()
+    args.public_keyfile.write(vk.to_string())
+    print "%s public key extracted to %s" % (args.keyfile.name, args.public_keyfile.name)
+
 def main():
     parser = argparse.ArgumentParser(description='espsecure.py v%s - ESP32 Secure Boot & Flash Encryption tool' % esptool.__version__, prog='espsecure')
 
@@ -91,10 +143,26 @@ def main():
 
     p = subparsers.add_parser('digest_secure_bootloader',
                           help='Take a bootloader binary image and a secure boot key, and output a combined digest+binary suitable for flashing along with the precalculated secure boot key.')
-    p.add_argument('--keyfile', '-k', help="256 bit key for secure boot digest.", type=file, required=True)
+    p.add_argument('--keyfile', '-k', help="256 bit key for secure boot digest.", type=argparse.FileType('rb'), required=True)
     p.add_argument('--output', '-o', help="Output file for signed digest image.")
-    p.add_argument('--iv', help="128 byte IV file. Supply a file for testing purposes only, if not supplied an IV will be randomly generated.", type=file)
-    p.add_argument('image', help="Bootloader image file to calculate digest from", type=file)
+    p.add_argument('--iv', help="128 byte IV file. Supply a file for testing purposes only, if not supplied an IV will be randomly generated.", type=argparse.FileType('rb'))
+    p.add_argument('image', help="Bootloader image file to calculate digest from", type=argparse.FileType('rb'))
+
+    p = subparsers.add_parser('generate_signing_key',
+                              help='Generate a private key for signing secure boot images. Key file is generated in PEM format, and contains a ECDSA NIST256p private key and matching public key.')
+    p.add_argument('keyfile', help="Filename for private key file (embedded public key)")
+
+    p = subparsers.add_parser('sign_data',
+                              help='Sign a data file for use with secure boot. Signing algorithm is determinsitic ECDSA w/ SHA-512.')
+    p.add_argument('--keyfile', '-k', help="Private key file for signing. Key is in PEM format, ECDSA NIST256p curve. generate_signing_key command can be used to generate a suitable signing key.", type=argparse.FileType('rb'), required=True)
+    p.add_argument('--output', '-o', help="Output file for signed digest image. Default is to append signature to existing file.")
+    p.add_argument('datafile', help="Data file to sign.", type=argparse.FileType('rb'))
+
+    p = subparsers.add_parser('extract_public_key',
+                              help='Extract the public verification key for signatures, save it as a raw binary file.')
+    p.add_argument('--keyfile', '-k', help="Private key file to extract the public verification key from.", type=argparse.FileType('rb'),
+                   required=True)
+    p.add_argument('public_keyfile', help="File to save new public key) into", type=argparse.FileType('wb'))
 
     args = parser.parse_args()
     print 'espsecure.py v%s' % esptool.__version__

esptool.py

@@ -1340,7 +1340,6 @@ def div_roundup(a, b):
     """
     return (int(a) + int(b) - 1) / int(b)
 
-
 def align_file_position(f, size):
     """ Align the position in the file to the next block of specified size """
     align = (size - 1) - (f.tell() % size)