imgtool: image signature export

Extend sign/create command so it now allow to export the image
signature to the file pointed by --sig-out option.
The image signature will be encoded as base64 formatted string.
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

scripts/imgtool/image.py

@@ -447,12 +447,14 @@ class Image():
             else:
                 sig = key.sign_digest(digest)
             tlv.add(key.sig_tlv(), sig)
+            self.signature = sig
         elif fixed_sig is not None and key is None:
             if public_key_format == 'hash':
                 tlv.add('KEYHASH', pubbytes)
             else:
                 tlv.add('PUBKEY', pub)
             tlv.add(pub_key.sig_tlv(), fixed_sig['value'])
+            self.signature = fixed_sig['value']
         else:
             raise click.UsageError("Can not sign using key and provide fixed-signature at the same time")
 

scripts/imgtool/main.py

@@ -74,6 +74,11 @@ def load_signature(sigfile):
         signature = base64.b64decode(f.read())
         return signature
 
+def save_signature(sigfile, sig):
+    with open(sigfile, 'wb') as f:
+        signature = base64.b64encode(sig)
+        f.write(signature)
+
 def load_key(keyfile):
     # TODO: better handling of invalid pass-phrase
     key = keys.load(keyfile)
@@ -313,6 +318,9 @@ class BasedIntParamType(click.ParamType):
               'the signature calculated using the public key')
 @click.option('--fix-sig-pubkey', metavar='filename',
               help='public key relevant to fixed signature')
+@click.option('--sig-out', metavar='filename',
+              help='Path to the file to which signature will be written'
+              'The image signature will be encoded as base64 formatted string')
 @click.command(help='''Create a signed or unsigned image\n
                INFILE and OUTFILE are parsed as Intel HEX if the params have
                .hex extension, otherwise binary format is used''')
@@ -321,7 +329,7 @@ def sign(key, public_key_format, align, version, pad_sig, header_size,
          endian, encrypt_keylen, encrypt, infile, outfile, dependencies,
          load_addr, hex_addr, erased_val, save_enctlv, security_counter,
          boot_record, custom_tlv, rom_fixed, max_align, clear, fix_sig,
-         fix_sig_pubkey):
+         fix_sig_pubkey, sig_out):
 
     if confirm:
         # Confirmed but non-padded images don't make much sense, because
@@ -388,6 +396,10 @@ def sign(key, public_key_format, align, version, pad_sig, header_size,
                custom_tlvs, int(encrypt_keylen), clear, baked_signature, pub_key)
     img.save(outfile, hex_addr)
 
+    if sig_out is not None:
+        new_signature = img.get_signature()
+        save_signature(sig_out, new_signature)
+
 
 class AliasesGroup(click.Group):