imgtool: Add support for HMAC/HKDF-SHA512 with ECIES-X25519

Commit adds imgtool command line option --hmac-sha allowing
to select between SHA256 and SHA512 for HMAC/HKDF.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

Author: de-nordic

scripts/imgtool/image.py

@@ -88,6 +88,7 @@
         'ENCKW': 0x31,
         'ENCEC256': 0x32,
         'ENCX25519': 0x33,
+        'ENCX25519_SHA512': 0x34,
         'DEPENDENCY': 0x40,
         'SEC_CNT': 0x50,
         'BOOT_RECORD': 0x60,
@@ -436,21 +437,21 @@ def check_trailer(self):
                           len(self.payload), tsize, self.slot_size)
                 raise click.UsageError(msg)
 
-    def ecies_hkdf(self, enckey, plainkey):
+    def ecies_hkdf(self, enckey, plainkey, hmac_sha_alg):
         if isinstance(enckey, ecdsa.ECDSA256P1Public):
             newpk = ec.generate_private_key(ec.SECP256R1(), default_backend())
             shared = newpk.exchange(ec.ECDH(), enckey._get_public())
         else:
             newpk = X25519PrivateKey.generate()
             shared = newpk.exchange(enckey._get_public())
         derived_key = HKDF(
-            algorithm=hashes.SHA256(), length=48, salt=None,
+            algorithm=hmac_sha_alg, length=48, salt=None,
             info=b'MCUBoot_ECIES_v1', backend=default_backend()).derive(shared)
         encryptor = Cipher(algorithms.AES(derived_key[:16]),
                            modes.CTR(bytes([0] * 16)),
                            backend=default_backend()).encryptor()
         cipherkey = encryptor.update(plainkey) + encryptor.finalize()
-        mac = hmac.HMAC(derived_key[16:], hashes.SHA256(),
+        mac = hmac.HMAC(derived_key[16:], hmac_sha_alg,
                         backend=default_backend())
         mac.update(cipherkey)
         ciphermac = mac.finalize()
@@ -468,7 +469,8 @@ def create(self, key, public_key_format, enckey, dependencies=None,
                sw_type=None, custom_tlvs=None, compression_tlvs=None,
                compression_type=None, encrypt_keylen=128, clear=False,
                fixed_sig=None, pub_key=None, vector_to_sign=None,
-               user_sha='auto', is_pure=False, keep_comp_size=False, dont_encrypt=False):
+               user_sha='auto', hmac_sha='auto', is_pure=False, keep_comp_size=False,
+               dont_encrypt=False):
         self.enckey = enckey
 
         # key decides on sha, then pub_key; of both are none default is used
@@ -675,6 +677,17 @@ def create(self, key, public_key_format, enckey, dependencies=None,
             else:
                 plainkey = os.urandom(16)
 
+            if not isinstance(enckey, rsa.RSAPublic):
+                if hmac_sha == 'auto' or hmac_sha == '256':
+                    hmac_sha = '256'
+                    hmac_sha_alg = hashes.SHA256()
+                elif hmac_sha == '512':
+                    if not isinstance(enckey, x25519.X25519Public):
+                        raise click.UsageError("Currently only ECIES-X25519 supports HMAC-SHA512")
+                    hmac_sha_alg = hashes.SHA512()
+                else:
+                    raise click.UsageError("Unsupported HMAC-SHA")
+
             if isinstance(enckey, rsa.RSAPublic):
                 cipherkey = enckey._get_public().encrypt(
                     plainkey, padding.OAEP(
@@ -683,15 +696,19 @@ def create(self, key, public_key_format, enckey, dependencies=None,
                         label=None))
                 self.enctlv_len = len(cipherkey)
                 tlv.add('ENCRSA2048', cipherkey)
-            elif isinstance(enckey, (ecdsa.ECDSA256P1Public,
-                                     x25519.X25519Public)):
-                cipherkey, mac, pubk = self.ecies_hkdf(enckey, plainkey)
+            elif isinstance(enckey, ecdsa.ECDSA256P1Public):
+                cipherkey, mac, pubk = self.ecies_hkdf(enckey, plainkey, hmac_sha_alg)
                 enctlv = pubk + mac + cipherkey
                 self.enctlv_len = len(enctlv)
-                if isinstance(enckey, ecdsa.ECDSA256P1Public):
-                    tlv.add('ENCEC256', enctlv)
-                else:
+                tlv.add('ENCEC256', enctlv)
+            elif isinstance(enckey, x25519.X25519Public):
+                cipherkey, mac, pubk = self.ecies_hkdf(enckey, plainkey, hmac_sha_alg)
+                enctlv = pubk + mac + cipherkey
+                self.enctlv_len = len(enctlv)
+                if (hmac_sha == '256'):
                     tlv.add('ENCX25519', enctlv)
+                else:
+                    tlv.add('ENCX25519_SHA512', enctlv)
 
             if not clear:
                 nonce = bytes([0] * 16)

scripts/imgtool/main.py

@@ -84,6 +84,7 @@ def gen_x25519(keyfile, passwd):
 }
 valid_formats = ['openssl', 'pkcs8']
 valid_sha = [ 'auto', '256', '384', '512' ]
+valid_hmac_sha = [ 'auto', '256', '512' ]
 
 
 def load_signature(sigfile):
@@ -437,6 +438,8 @@ def convert(self, value, param, ctx):
 @click.option('--sha', 'user_sha', type=click.Choice(valid_sha), default='auto',
               help='selected sha algorithm to use; defaults to "auto" which is 256 if '
               'no cryptographic signature is used, or default for signature type')
+@click.option('--hmac-sha', 'hmac_sha', type=click.Choice(valid_hmac_sha), default='auto',
+              help='sha algorithm used in HKDF/HMAC in ECIES key exchange TLV')
 @click.option('--vector-to-sign', type=click.Choice(['payload', 'digest']),
               help='send to OUTFILE the payload or payload''s digest instead '
               'of complied image. These data can be used for external image '
@@ -449,7 +452,7 @@ def sign(key, public_key_format, align, version, pad_sig, header_size,
          endian, encrypt_keylen, encrypt, compression, 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, sig_out, user_sha, is_pure,
+         clear, fix_sig, fix_sig_pubkey, sig_out, user_sha, hmac_sha, is_pure,
          vector_to_sign, non_bootable):
 
     if confirm:
@@ -526,7 +529,7 @@ def sign(key, public_key_format, align, version, pad_sig, header_size,
         img.create(key, public_key_format, enckey, dependencies, boot_record,
                custom_tlvs, compression_tlvs, None, int(encrypt_keylen), clear,
                baked_signature, pub_key, vector_to_sign, user_sha=user_sha,
-               is_pure=is_pure, keep_comp_size=False, dont_encrypt=True)
+               hmac_sha=hmac_sha, is_pure=is_pure, keep_comp_size=False, dont_encrypt=True)
         compressed_img = image.Image(version=decode_version(version),
                   header_size=header_size, pad_header=pad_header,
                   pad=pad, confirm=confirm, align=int(align),
@@ -570,14 +573,14 @@ def sign(key, public_key_format, align, version, pad_sig, header_size,
             compressed_img.create(key, public_key_format, enckey,
                dependencies, boot_record, custom_tlvs, compression_tlvs,
                compression, int(encrypt_keylen), clear, baked_signature,
-               pub_key, vector_to_sign, user_sha=user_sha,
+               pub_key, vector_to_sign, user_sha=user_sha, hmac_sha=hmac_sha,
                is_pure=is_pure, keep_comp_size=keep_comp_size)
             img = compressed_img
     else:
         img.create(key, public_key_format, enckey, dependencies, boot_record,
                custom_tlvs, compression_tlvs, None, int(encrypt_keylen), clear,
                baked_signature, pub_key, vector_to_sign, user_sha=user_sha,
-               is_pure=is_pure)
+               hmac_sha=hmac_sha, is_pure=is_pure)
     img.save(outfile, hex_addr)
     if sig_out is not None:
         new_signature = img.get_signature()