scripts/imgtool/image.py

# Copyright 2018 Nordic Semiconductor ASA
# Copyright 2017 Linaro Limited
# Copyright 2019 Arm Limited
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""
Image signing and management.
"""

from . import version as versmod
from enum import Enum
from intelhex import IntelHex
import hashlib
import struct
import os.path
from .keys import rsa, ecdsa
from cryptography.hazmat.primitives.asymmetric import ec, padding
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.primitives.kdf.hkdf import HKDF
from cryptography.hazmat.primitives.serialization import Encoding, PublicFormat
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes, hmac
from cryptography.exceptions import InvalidSignature

IMAGE_MAGIC = 0x96f3b83d
IMAGE_HEADER_SIZE = 32
BIN_EXT = "bin"
INTEL_HEX_EXT = "hex"
DEFAULT_MAX_SECTORS = 128
MAX_ALIGN = 8
DEP_IMAGES_KEY = "images"
DEP_VERSIONS_KEY = "versions"

# Image header flags.
IMAGE_F = {
        'PIC':                   0x0000001,
        'NON_BOOTABLE':          0x0000010,
        'ENCRYPTED':             0x0000004,
}

TLV_VALUES = {
        'KEYHASH': 0x01,
        'SHA256': 0x10,
        'RSA2048': 0x20,
        'ECDSA224': 0x21,
        'ECDSA256': 0x22,
        'RSA3072': 0x23,
        'ED25519': 0x24,
        'ENCRSA2048': 0x30,
        'ENCKW128': 0x31,
        'ENCEC256': 0x32,
        'DEPENDENCY': 0x40
}

TLV_SIZE = 4
TLV_INFO_SIZE = 4
TLV_INFO_MAGIC = 0x6907
TLV_PROT_INFO_MAGIC = 0x6908

boot_magic = bytes([
    0x77, 0xc2, 0x95, 0xf3,
    0x60, 0xd2, 0xef, 0x7f,
    0x35, 0x52, 0x50, 0x0f,
    0x2c, 0xb6, 0x79, 0x80, ])

STRUCT_ENDIAN_DICT = {
        'little': '<',
        'big':    '>'
}

VerifyResult = Enum('VerifyResult',
                    """
                    OK INVALID_MAGIC INVALID_TLV_INFO_MAGIC INVALID_HASH
                    INVALID_SIGNATURE
                    """)


class TLV():
    def __init__(self, endian, magic=TLV_INFO_MAGIC):
        self.magic = magic
        self.buf = bytearray()
        self.endian = endian

    def __len__(self):
        return TLV_INFO_SIZE + len(self.buf)

    def add(self, kind, payload):
        """
        Add a TLV record.  Kind should be a string found in TLV_VALUES above.
        """
        e = STRUCT_ENDIAN_DICT[self.endian]
        buf = struct.pack(e + 'BBH', TLV_VALUES[kind], 0, len(payload))
        self.buf += buf
        self.buf += payload

    def get(self):
        if len(self.buf) == 0:
            return bytes()
        e = STRUCT_ENDIAN_DICT[self.endian]
        header = struct.pack(e + 'HH', self.magic, len(self))
        return header + bytes(self.buf)


class Image():

    def __init__(self, version=None, header_size=IMAGE_HEADER_SIZE,
                 pad_header=False, pad=False, align=1, slot_size=0,
                 max_sectors=DEFAULT_MAX_SECTORS, overwrite_only=False,
                 endian="little", load_addr=0, erased_val=0xff):
        self.version = version or versmod.decode_version("0")
        self.header_size = header_size
        self.pad_header = pad_header
        self.pad = pad
        self.align = align
        self.slot_size = slot_size
        self.max_sectors = max_sectors
        self.overwrite_only = overwrite_only
        self.endian = endian
        self.base_addr = None
        self.load_addr = 0 if load_addr is None else load_addr
        self.erased_val = 0xff if erased_val is None else int(erased_val)
        self.payload = []
        self.enckey = None

    def __repr__(self):
        return "<Image version={}, header_size={}, base_addr={}, load_addr={}, \
                align={}, slot_size={}, max_sectors={}, overwrite_only={}, \
                endian={} format={}, payloadlen=0x{:x}>".format(
                    self.version,
                    self.header_size,
                    self.base_addr if self.base_addr is not None else "N/A",
                    self.load_addr,
                    self.align,
                    self.slot_size,
                    self.max_sectors,
                    self.overwrite_only,
                    self.endian,
                    self.__class__.__name__,
                    len(self.payload))

    def load(self, path):
        """Load an image from a given file"""
        ext = os.path.splitext(path)[1][1:].lower()
        if ext == INTEL_HEX_EXT:
            ih = IntelHex(path)
            self.payload = ih.tobinarray()
            self.base_addr = ih.minaddr()
        else:
            with open(path, 'rb') as f:
                self.payload = f.read()

        # Add the image header if needed.
        if self.pad_header and self.header_size > 0:
            if self.base_addr:
                # Adjust base_addr for new header
                self.base_addr -= self.header_size
            self.payload = bytes([self.erased_val] * self.header_size) + \
                self.payload

        self.check()

    def save(self, path, hex_addr=None):
        """Save an image from a given file"""
        ext = os.path.splitext(path)[1][1:].lower()
        if ext == INTEL_HEX_EXT:
            # input was in binary format, but HEX needs to know the base addr
            if self.base_addr is None and hex_addr is None:
                raise Exception("No address exists in input file neither was "
                                "it provided by user")
            h = IntelHex()
            if hex_addr is not None:
                self.base_addr = hex_addr
            h.frombytes(bytes=self.payload, offset=self.base_addr)
            if self.pad:
                trailer_size = self._trailer_size(self.align, self.max_sectors,
                                                  self.overwrite_only,
                                                  self.enckey)
                trailer_addr = (self.base_addr + self.slot_size) - trailer_size
                padding = bytes([self.erased_val] *
                                (trailer_size - len(boot_magic))) + boot_magic
                h.puts(trailer_addr, padding)
            h.tofile(path, 'hex')
        else:
            if self.pad:
                self.pad_to(self.slot_size)
            with open(path, 'wb') as f:
                f.write(self.payload)

    def check(self):
        """Perform some sanity checking of the image."""
        # If there is a header requested, make sure that the image
        # starts with all zeros.
        if self.header_size > 0 and not self.pad_header:
            if any(v != 0 for v in self.payload[0:self.header_size]):
                raise Exception("Padding requested, but image does not start with zeros")
        if self.slot_size > 0:
            tsize = self._trailer_size(self.align, self.max_sectors,
                                       self.overwrite_only, self.enckey)
            padding = self.slot_size - (len(self.payload) + tsize)
            if padding < 0:
                msg = "Image size (0x{:x}) + trailer (0x{:x}) exceeds requested size 0x{:x}".format(
                        len(self.payload), tsize, self.slot_size)
                raise Exception(msg)

    def ecies_p256_hkdf(self, enckey, plainkey):
        newpk = ec.generate_private_key(ec.SECP256R1(), default_backend())
        shared = newpk.exchange(ec.ECDH(), enckey._get_public())
        derived_key = HKDF(
            algorithm=hashes.SHA256(), 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(),
                        backend=default_backend())
        mac.update(cipherkey)
        ciphermac = mac.finalize()
        pubk = newpk.public_key().public_bytes(
            encoding=Encoding.X962,
            format=PublicFormat.UncompressedPoint)
        return cipherkey, ciphermac, pubk

    def create(self, key, enckey, dependencies=None):
        self.enckey = enckey

        if dependencies is None:
            dependencies_num = 0
            protected_tlv_size = 0
        else:
            # Size of a Dependency TLV = Header ('BBH') + Payload('IBBHI')
            # = 16 Bytes
            dependencies_num = len(dependencies[DEP_IMAGES_KEY])
            protected_tlv_size = (dependencies_num * 16) + TLV_INFO_SIZE

        # At this point the image is already on the payload, this adds
        # the header to the payload as well
        self.add_header(enckey, protected_tlv_size)

        prot_tlv = TLV(self.endian, TLV_PROT_INFO_MAGIC)

        # Protected TLVs must be added first, because they are also included
        # in the hash calculation
        protected_tlv_off = None
        if protected_tlv_size != 0:
            for i in range(dependencies_num):
                e = STRUCT_ENDIAN_DICT[self.endian]
                payload = struct.pack(
                                e + 'B3x'+'BBHI',
                                int(dependencies[DEP_IMAGES_KEY][i]),
                                dependencies[DEP_VERSIONS_KEY][i].major,
                                dependencies[DEP_VERSIONS_KEY][i].minor,
                                dependencies[DEP_VERSIONS_KEY][i].revision,
                                dependencies[DEP_VERSIONS_KEY][i].build
                                )
                prot_tlv.add('DEPENDENCY', payload)

            protected_tlv_off = len(self.payload)
            self.payload += prot_tlv.get()

        tlv = TLV(self.endian)

        # Note that ecdsa wants to do the hashing itself, which means
        # we get to hash it twice.
        sha = hashlib.sha256()
        sha.update(self.payload)
        digest = sha.digest()

        tlv.add('SHA256', digest)

        if key is not None:
            pub = key.get_public_bytes()
            sha = hashlib.sha256()
            sha.update(pub)
            pubbytes = sha.digest()
            tlv.add('KEYHASH', pubbytes)

            # `sign` expects the full image payload (sha256 done internally),
            # while `sign_digest` expects only the digest of the payload

            if hasattr(key, 'sign'):
                sig = key.sign(bytes(self.payload))
            else:
                sig = key.sign_digest(digest)
            tlv.add(key.sig_tlv(), sig)

        # At this point the image was hashed + signed, we can remove the
        # protected TLVs from the payload (will be re-added later)
        if protected_tlv_off is not None:
            self.payload = self.payload[:protected_tlv_off]

        if enckey is not None:
            plainkey = os.urandom(16)

            if isinstance(enckey, rsa.RSAPublic):
                cipherkey = enckey._get_public().encrypt(
                    plainkey, padding.OAEP(
                        mgf=padding.MGF1(algorithm=hashes.SHA256()),
                        algorithm=hashes.SHA256(),
                        label=None))
                tlv.add('ENCRSA2048', cipherkey)
            elif isinstance(enckey, ecdsa.ECDSA256P1Public):
                cipherkey, mac, pubk = self.ecies_p256_hkdf(enckey, plainkey)
                tlv.add('ENCEC256', pubk + mac + cipherkey)

            nonce = bytes([0] * 16)
            cipher = Cipher(algorithms.AES(plainkey), modes.CTR(nonce),
                            backend=default_backend())
            encryptor = cipher.encryptor()
            img = bytes(self.payload[self.header_size:])
            self.payload[self.header_size:] = \
                encryptor.update(img) + encryptor.finalize()

        self.payload += prot_tlv.get()
        self.payload += tlv.get()

    def add_header(self, enckey, protected_tlv_size):
        """Install the image header."""

        flags = 0
        if enckey is not None:
            flags |= IMAGE_F['ENCRYPTED']

        e = STRUCT_ENDIAN_DICT[self.endian]
        fmt = (e +
               # type ImageHdr struct {
               'I' +     # Magic    uint32
               'I' +     # LoadAddr uint32
               'H' +     # HdrSz    uint16
               'H' +     # PTLVSz   uint16
               'I' +     # ImgSz    uint32
               'I' +     # Flags    uint32
               'BBHI' +  # Vers     ImageVersion
               'I'       # Pad1     uint32
               )  # }
        assert struct.calcsize(fmt) == IMAGE_HEADER_SIZE
        header = struct.pack(fmt,
                IMAGE_MAGIC,
                self.load_addr,
                self.header_size,
                protected_tlv_size,  # TLV Info header + Protected TLVs
                len(self.payload) - self.header_size,  # ImageSz
                flags,
                self.version.major,
                self.version.minor or 0,
                self.version.revision or 0,
                self.version.build or 0,
                0)  # Pad1
        self.payload = bytearray(self.payload)
        self.payload[:len(header)] = header

    def _trailer_size(self, write_size, max_sectors, overwrite_only, enckey):
        # NOTE: should already be checked by the argument parser
        magic_size = 16
        if overwrite_only:
            return MAX_ALIGN * 2 + magic_size
        else:
            if write_size not in set([1, 2, 4, 8]):
                raise Exception("Invalid alignment: {}".format(write_size))
            m = DEFAULT_MAX_SECTORS if max_sectors is None else max_sectors
            trailer = m * 3 * write_size  # status area
            if enckey is not None:
                trailer += 16 * 2  # encryption keys
            trailer += MAX_ALIGN * 4  # image_ok/copy_done/swap_info/swap_size
            trailer += magic_size
            return trailer

    def pad_to(self, size):
        """Pad the image to the given size, with the given flash alignment."""
        tsize = self._trailer_size(self.align, self.max_sectors,
                                   self.overwrite_only, self.enckey)
        padding = size - (len(self.payload) + tsize)
        pbytes = bytes([self.erased_val] * padding)
        pbytes += bytes([self.erased_val] * (tsize - len(boot_magic)))
        pbytes += boot_magic
        self.payload += pbytes

    @staticmethod
    def verify(imgfile, key):
        with open(imgfile, "rb") as f:
            b = f.read()

        magic, _, header_size, _, img_size = struct.unpack('IIHHI', b[:16])
        version = struct.unpack('BBHI', b[20:28])

        if magic != IMAGE_MAGIC:
            return VerifyResult.INVALID_MAGIC, None

        tlv_info = b[header_size+img_size:header_size+img_size+TLV_INFO_SIZE]
        magic, tlv_tot = struct.unpack('HH', tlv_info)
        if magic != TLV_INFO_MAGIC:
            return VerifyResult.INVALID_TLV_INFO_MAGIC, None

        sha = hashlib.sha256()
        sha.update(b[:header_size+img_size])
        digest = sha.digest()

        tlv_off = header_size + img_size
        tlv_end = tlv_off + tlv_tot
        tlv_off += TLV_INFO_SIZE  # skip tlv info
        while tlv_off < tlv_end:
            tlv = b[tlv_off:tlv_off+TLV_SIZE]
            tlv_type, _, tlv_len = struct.unpack('BBH', tlv)
            if tlv_type == TLV_VALUES["SHA256"]:
                off = tlv_off + TLV_SIZE
                if digest == b[off:off+tlv_len]:
                    if key is None:
                        return VerifyResult.OK, version
                else:
                    return VerifyResult.INVALID_HASH, None
            elif key is not None and tlv_type == TLV_VALUES[key.sig_tlv()]:
                off = tlv_off + TLV_SIZE
                tlv_sig = b[off:off+tlv_len]
                payload = b[:header_size+img_size]
                try:
                    if hasattr(key, 'verify'):
                        key.verify(tlv_sig, payload)
                    else:
                        key.verify_digest(tlv_sig, digest)
                    return VerifyResult.OK, version
                except InvalidSignature:
                    # continue to next TLV
                    pass
            tlv_off += TLV_SIZE + tlv_len
        return VerifyResult.INVALID_SIGNATURE, None