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 cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes
from cryptography.exceptions import InvalidSignature

IMAGE_MAGIC = 0x96f3b83d
IMAGE_HEADER_SIZE = 32
BIN_EXT = "bin"
INTEL_HEX_EXT = "hex"
DEFAULT_MAX_SECTORS = 128
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,
        'ENCRSA2048': 0x30,
        'ENCKW128': 0x31,
        'DEPENDENCY': 0x40
}

TLV_SIZE = 4
TLV_INFO_SIZE = 4
TLV_INFO_MAGIC = 0x6907

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):
        self.buf = bytearray()
        self.endian = endian

    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):
        e = STRUCT_ENDIAN_DICT[self.endian]
        header = struct.pack(e + 'HH', TLV_INFO_MAGIC, TLV_INFO_SIZE + len(self.buf))
        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"):
        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.payload = []

    def __repr__(self):
        return "<Image version={}, header_size={}, base_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.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 = (b'\000' * self.header_size) + self.payload

        self.check()

    def save(self, path):
        """Save an image from a given file"""
        if self.pad:
            self.pad_to(self.slot_size)

        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:
                raise Exception("Input file does not provide a base address")
            h = IntelHex()
            h.frombytes(bytes=self.payload, offset=self.base_addr)
            h.tofile(path, 'hex')
        else:
            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:
            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)
            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 create(self, key, enckey, dependencies=None):
        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

        self.add_header(enckey, protected_tlv_size)

        tlv = TLV(self.endian)

        if protected_tlv_size != 0:
            for i in range(dependencies_num):
                e = STRUCT_ENDIAN_DICT[self.endian]
                payload = struct.pack(
                                e + 'I'+'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
                                )
                tlv.add('DEPENDENCY', payload)
            # Full TLV size needs to be calculated in advance, because the
            # header will be protected as well
            tlv_header_size = 4
            payload_digest_size = 32
            keyhash_size = 32
            cipherkey_size = 32

            full_size = TLV_INFO_SIZE + len(tlv.buf) + tlv_header_size \
                        + payload_digest_size
            if key is not None:
                full_size += tlv_header_size + keyhash_size \
                             + tlv_header_size + key.sig_len()
            if enckey is not None:
                full_size += tlv_header_size + cipherkey_size
            tlv_header = struct.pack(e + 'HH', TLV_INFO_MAGIC, full_size)
            self.payload += tlv_header + bytes(tlv.buf)

        # 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)

            sig = key.sign(bytes(self.payload))
            tlv.add(key.sig_tlv(), sig)

        if enckey is not None:
            plainkey = os.urandom(16)
            cipherkey = enckey._get_public().encrypt(
                plainkey, padding.OAEP(
                    mgf=padding.MGF1(algorithm=hashes.SHA256()),
                    algorithm=hashes.SHA256(),
                    label=None))
            tlv.add('ENCRSA2048', 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 += tlv.get()[protected_tlv_size:]

    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,
                0, # LoadAddr
                self.header_size,
                protected_tlv_size,  # TLV Info header + Dependency TLVs
                len(self.payload) - self.header_size, # ImageSz
                flags, # 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):
        # NOTE: should already be checked by the argument parser
        if overwrite_only:
            return 8 * 2 + 16
        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
            return m * 3 * write_size + 8 * 2 + 16

    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)
        padding = size - (len(self.payload) + tsize)
        pbytes  = b'\xff' * padding
        pbytes += b'\xff' * (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])
        if magic != IMAGE_MAGIC:
            return VerifyResult.INVALID_MAGIC

        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

        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
                else:
                    return VerifyResult.INVALID_HASH
            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:
                    key.verify(tlv_sig, payload)
                    return VerifyResult.OK
                except InvalidSignature:
                    # continue to next TLV
                    pass
            tlv_off += TLV_SIZE + tlv_len
        return VerifyResult.INVALID_SIGNATURE