Commit c6e7e9be authored by Antonio de Angelis's avatar Antonio de Angelis Committed by Fabio Utzig
Browse files

imgtool: Improve ECDSA key generation 



This patch improves the existing ECDSA key generation feature
in the imgtool by:
 - Fix a bug in the 'minimal' representation of PKCS#8 keys where
   the resulting ASN.1 DER encoding is not compliant
 - Add the option to export ECDSA private keys in SEC1 format by
   providing a command line option -f or --format that can be
   'openssl' (for SEC1 format) or 'pkcs8'. This format ends up in
   key encodings which are generally smaller than PKCS#8.

Signed-off-by: default avatarAntonio de Angelis <Antonio.deAngelis@arm.com>
parent 284b8fe3

scripts/imgtool/keys/ecdsa.py

+44 −20
Original line number Diff line number Diff line
@@ -3,6 +3,7 @@ ECDSA key management 
"""



# SPDX-License-Identifier: Apache-2.0

import os.path



from cryptography.hazmat.backends import default_backend

from cryptography.hazmat.primitives import serialization
@@ -102,17 +103,25 @@ class ECDSA256P1(ECDSA256P1Public): 
    def _get_public(self):

        return self.key.public_key()



    def _build_minimal_ecdsa_privkey(self, der):

    def _build_minimal_ecdsa_privkey(self, der, format):

        '''

        Builds a new DER that only includes the EC private key, removing the

        public key that is added as an "optional" BITSTRING.

        '''

        offset_PUB = 68



        if format == serialization.PrivateFormat.OpenSSH:

            print(os.path.basename(__file__) +

                  ': Warning: --minimal is supported only for PKCS8 '

                  'or TraditionalOpenSSL formats')

            return bytearray(der)



        EXCEPTION_TEXT = "Error parsing ecdsa key. Please submit an issue!"

        if format == serialization.PrivateFormat.PKCS8:

            offset_PUB = 68  # where the context specific TLV starts (tag 0xA1)

            if der[offset_PUB] != 0xa1:

                raise ECDSAUsageError(EXCEPTION_TEXT)

        len_PUB = der[offset_PUB + 1]

        b = bytearray(der[:-offset_PUB])

            len_PUB = der[offset_PUB + 1] + 2  # + 2 for 0xA1 0x44 bytes

            b = bytearray(der[:offset_PUB])  # remove the TLV with the PUB key

            offset_SEQ = 29

            if b[offset_SEQ] != 0x30:

                raise ECDSAUsageError(EXCEPTION_TEXT)
@@ -123,16 +132,31 @@ class ECDSA256P1(ECDSA256P1Public): 
            b[offset_OCT_STR + 1] -= len_PUB

            if b[0] != 0x30 or b[1] != 0x81:

                raise ECDSAUsageError(EXCEPTION_TEXT)

            # as b[1] has bit7 set, the length is on b[2]

            b[2] -= len_PUB

            if b[2] < 0x80:

                del(b[1])



        elif format == serialization.PrivateFormat.TraditionalOpenSSL:

            offset_PUB = 51

            if der[offset_PUB] != 0xA1:

                raise ECDSAUsageError(EXCEPTION_TEXT)

            len_PUB = der[offset_PUB + 1] + 2

            b = bytearray(der[0:offset_PUB])

            b[1] -= len_PUB



        return b



    def get_private_bytes(self, minimal):

    def get_private_bytes(self, minimal, format):

        formats = {'pkcs8': serialization.PrivateFormat.PKCS8,

                   'openssl': serialization.PrivateFormat.TraditionalOpenSSL

                   }

        priv = self.key.private_bytes(

                encoding=serialization.Encoding.DER,

                format=serialization.PrivateFormat.PKCS8,

                format=formats[format],

                encryption_algorithm=serialization.NoEncryption())

        if minimal:

            priv = self._build_minimal_ecdsa_privkey(priv)

            priv = self._build_minimal_ecdsa_privkey(priv, formats[format])

        return priv



    def export_private(self, path, passwd=None):

scripts/imgtool/keys/general.py

+3 −2
Original line number Diff line number Diff line
@@ -6,6 +6,7 @@ import sys 


AUTOGEN_MESSAGE = "/* Autogenerated by imgtool.py, do not edit. */"





class KeyClass(object):

    def _emit(self, header, trailer, encoded_bytes, indent, file=sys.stdout, len_format=None):

        print(AUTOGEN_MESSAGE, file=file)
@@ -40,11 +41,11 @@ class KeyClass(object): 
    def emit_public_pem(self, file=sys.stdout):

        print(str(self.get_public_pem(), 'utf-8'), file=file, end='')



    def emit_private(self, minimal, file=sys.stdout):

    def emit_private(self, minimal, format, file=sys.stdout):

        self._emit(

                header="const unsigned char enc_priv_key[] = {",

                trailer="};",

                encoded_bytes=self.get_private_bytes(minimal),

                encoded_bytes=self.get_private_bytes(minimal, format),

                indent="    ",

                len_format="const unsigned int enc_priv_key_len = {};",

                file=file)

scripts/imgtool/main.py

100755 → 100644
+7 −2
Original line number Diff line number Diff line
@@ -69,6 +69,7 @@ keygens = { 
    'ed25519':    gen_ed25519,

    'x25519':     gen_x25519,

}

valid_formats = ['openssl', 'pkcs8']



def load_signature(sigfile):

    with open(sigfile, 'rb') as f:
@@ -150,13 +151,17 @@ def getpub(key, encoding, lang): 
                   'might require changes to the build config. Check the docs!'

              )

@click.option('-k', '--key', metavar='filename', required=True)

@click.option('-f', '--format',

              type=click.Choice(valid_formats),

              help='Valid formats: {}'.format(', '.join(valid_formats)),

              default='pkcs8')

@click.command(help='Dump private key from keypair')

def getpriv(key, minimal):

def getpriv(key, minimal, format):

    key = load_key(key)

    if key is None:

        print("Invalid passphrase")

    try:

        key.emit_private(minimal)

        key.emit_private(minimal, format)

    except (RSAUsageError, ECDSAUsageError, Ed25519UsageError,

            X25519UsageError) as e:

        raise click.UsageError(e)

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