image.py

# Copyright 2018 Nordic Semiconductor ASA
# Copyright 2017-2020 Linaro Limited
# Copyright 2019-2021 Arm Limited
#
# SPDX-License-Identifier: Apache-2.0
#
# 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 .boot_record import create_sw_component_data
import click
from enum import Enum
from intelhex import IntelHex
import hashlib
import struct
import os.path
from .keys import rsa, ecdsa, x25519
from cryptography.hazmat.primitives.asymmetric import ec, padding
from cryptography.hazmat.primitives.asymmetric.x25519 import X25519PrivateKey
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
DEFAULT_MAX_ALIGN = 8
DEP_IMAGES_KEY = "images"
DEP_VERSIONS_KEY = "versions"
MAX_SW_TYPE_LENGTH = 12  # Bytes

# Image header flags.
IMAGE_F = {
        'PIC':                   0x0000001,
        'ENCRYPTED_AES128':      0x0000004,
        'ENCRYPTED_AES256':      0x0000008,
        'NON_BOOTABLE':          0x0000010,
        'RAM_LOAD':              0x0000020,
        'ROM_FIXED':             0x0000100,
}

TLV_VALUES = {
        'KEYHASH': 0x01,
        'PUBKEY': 0x02,
        'SHA256': 0x10,
        'RSA2048': 0x20,
        'ECDSA224': 0x21,
        'ECDSA256': 0x22,
        'RSA3072': 0x23,
        'ED25519': 0x24,
        'ENCRSA2048': 0x30,
        'ENCKW': 0x31,
        'ENCEC256': 0x32,
        'ENCX25519': 0x33,
        'DEPENDENCY': 0x40,
        'SEC_CNT': 0x50,
        'BOOT_RECORD': 0x60,
}

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

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

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

def align_up(num, align):
    assert (align & (align - 1) == 0) and align != 0
    return (num + (align - 1)) & ~(align - 1)

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]
        if isinstance(kind, int):
            buf = struct.pack(e + 'BBH', kind, 0, len(payload))
        else:
            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, confirm=False, align=1,
                 slot_size=0, max_sectors=DEFAULT_MAX_SECTORS,
                 overwrite_only=False, endian="little", load_addr=0,
                 rom_fixed=None, erased_val=None, save_enctlv=False,
                 security_counter=None, max_align=None):

        if load_addr and rom_fixed:
            raise click.UsageError("Can not set rom_fixed and load_addr at the same time")

        self.version = version or versmod.decode_version("0")
        self.header_size = header_size
        self.pad_header = pad_header
        self.pad = pad
        self.confirm = confirm
        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.rom_fixed = rom_fixed
        self.erased_val = 0xff if erased_val is None else int(erased_val, 0)
        self.payload = []
        self.enckey = None
        self.save_enctlv = save_enctlv
        self.enctlv_len = 0
        self.max_align = max(DEFAULT_MAX_ALIGN, align) if max_align is None else int(max_align)

        if self.max_align == DEFAULT_MAX_ALIGN:
            self.boot_magic = bytes([
                0x77, 0xc2, 0x95, 0xf3,
                0x60, 0xd2, 0xef, 0x7f,
                0x35, 0x52, 0x50, 0x0f,
                0x2c, 0xb6, 0x79, 0x80, ])
        else:
            align_lsb = self.max_align & 0x00ff
            align_msb = (self.max_align & 0xff00) >> 8
            self.boot_magic = bytes([
                align_lsb, align_msb, 0x2d, 0xe1,
                0x5d, 0x29, 0x41, 0x0b,
                0x8d, 0x77, 0x67, 0x9c,
                0x11, 0x0f, 0x1f, 0x8a, ])

        if security_counter == 'auto':
            # Security counter has not been explicitly provided,
            # generate it from the version number
            self.security_counter = ((self.version.major << 24)
                                     + (self.version.minor << 16)
                                     + self.version.revision)
        else:
            self.security_counter = security_counter

    def __repr__(self):
        return "<Image version={}, header_size={}, security_counter={}, \
                base_addr={}, load_addr={}, align={}, slot_size={}, \
                max_sectors={}, overwrite_only={}, endian={} format={}, \
                payloadlen=0x{:x}>".format(
                    self.version,
                    self.header_size,
                    self.security_counter,
                    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()
        try:
            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()
        except FileNotFoundError:
            raise click.UsageError("Input file not found")

        # 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_header()

    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 click.UsageError("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,
                                                  self.save_enctlv,
                                                  self.enctlv_len)
                trailer_addr = (self.base_addr + self.slot_size) - trailer_size
                if self.confirm and not self.overwrite_only:
                    magic_align_size = align_up(len(self.boot_magic), self.max_align)
                    image_ok_idx = -(magic_align_size + self.max_align)
                    flag = bytearray([self.erased_val] * self.max_align)
                    flag[0] = 0x01 # image_ok = 0x01
                    h.puts(trailer_addr + trailer_size + image_ok_idx, bytes(flag))
                h.puts(trailer_addr + (trailer_size - len(self.boot_magic)),
                       bytes(self.boot_magic))
            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_header(self):
        if self.header_size > 0 and not self.pad_header:
            if any(v != 0 for v in self.payload[0:self.header_size]):
                raise click.UsageError("Header padding was not requested and "
                                       "image does not start with zeros")

    def check_trailer(self):
        if self.slot_size > 0:
            tsize = self._trailer_size(self.align, self.max_sectors,
                                       self.overwrite_only, self.enckey,
                                       self.save_enctlv, self.enctlv_len)
            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 click.UsageError(msg)

    def ecies_hkdf(self, enckey, plainkey):
        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,
            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()
        if isinstance(enckey, ecdsa.ECDSA256P1Public):
            pubk = newpk.public_key().public_bytes(
                encoding=Encoding.X962,
                format=PublicFormat.UncompressedPoint)
        else:
            pubk = newpk.public_key().public_bytes(
                encoding=Encoding.Raw,
                format=PublicFormat.Raw)
        return cipherkey, ciphermac, pubk

    def create(self, key, public_key_format, enckey, dependencies=None,
               sw_type=None, custom_tlvs=None, encrypt_keylen=128, clear=False, fixed_sig=None, pub_key=None, vector_to_sign=None):
        self.enckey = enckey

        # Calculate the hash of the public key
        if key is not None:
            pub = key.get_public_bytes()
            sha = hashlib.sha256()
            sha.update(pub)
            pubbytes = sha.digest()
        elif pub_key is not None:
            if hasattr(pub_key, 'sign'):
                print(os.path.basename(__file__) + ": sign the payload")
            pub = pub_key.get_public_bytes()
            sha = hashlib.sha256()
            sha.update(pub)
            pubbytes = sha.digest()
        else:
            pubbytes = bytes(hashlib.sha256().digest_size)

        protected_tlv_size = 0

        if self.security_counter is not None:
            # Size of the security counter TLV: header ('HH') + payload ('I')
            #                                   = 4 + 4 = 8 Bytes
            protected_tlv_size += TLV_SIZE + 4

        if sw_type is not None:
            if len(sw_type) > MAX_SW_TYPE_LENGTH:
                msg = "'{}' is too long ({} characters) for sw_type. Its " \
                      "maximum allowed length is 12 characters.".format(
                       sw_type, len(sw_type))
                raise click.UsageError(msg)

            image_version = (str(self.version.major) + '.'
                             + str(self.version.minor) + '.'
                             + str(self.version.revision))

            # The image hash is computed over the image header, the image
            # itself and the protected TLV area. However, the boot record TLV
            # (which is part of the protected area) should contain this hash
            # before it is even calculated. For this reason the script fills
            # this field with zeros and the bootloader will insert the right
            # value later.
            digest = bytes(hashlib.sha256().digest_size)

            # Create CBOR encoded boot record
            boot_record = create_sw_component_data(sw_type, image_version,
                                                   "SHA256", digest,
                                                   pubbytes)

            protected_tlv_size += TLV_SIZE + len(boot_record)

        if dependencies is not None:
            # Size of a Dependency TLV = Header ('HH') + Payload('IBBHI')
            # = 4 + 12 = 16 Bytes
            dependencies_num = len(dependencies[DEP_IMAGES_KEY])
            protected_tlv_size += (dependencies_num * 16)

        if custom_tlvs is not None:
            for value in custom_tlvs.values():
                protected_tlv_size += TLV_SIZE + len(value)

        if protected_tlv_size != 0:
            # Add the size of the TLV info header
            protected_tlv_size += TLV_INFO_SIZE

        # At this point the image is already on the payload
        #
        # This adds the padding if image is not aligned to the 16 Bytes
        # in encrypted mode
        if self.enckey is not None:
            pad_len = len(self.payload) % 16
            if pad_len > 0:
                pad = bytes(16 - pad_len)
                if isinstance(self.payload, bytes):
                    self.payload += pad
                else:
                    self.payload.extend(pad)

        # This adds the header to the payload as well
        if encrypt_keylen == 256:
            self.add_header(enckey, protected_tlv_size, 256)
        else:
            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:

            e = STRUCT_ENDIAN_DICT[self.endian]

            if self.security_counter is not None:
                payload = struct.pack(e + 'I', self.security_counter)
                prot_tlv.add('SEC_CNT', payload)

            if sw_type is not None:
                prot_tlv.add('BOOT_RECORD', boot_record)

            if dependencies is not None:
                for i in range(dependencies_num):
                    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)

            if custom_tlvs is not None:
                for tag, value in custom_tlvs.items():
                    prot_tlv.add(tag, value)

            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 vector_to_sign == 'payload':
            # Stop amending data to the image
            # Just keep data vector which is expected to be signed
            print(os.path.basename(__file__) + ': export payload')
            return
        elif vector_to_sign == 'digest':
            self.payload = digest
            print(os.path.basename(__file__) + ': export digest')
            return

        if key is not None or fixed_sig is not None:
            if public_key_format == 'hash':
                tlv.add('KEYHASH', pubbytes)
            else:
                tlv.add('PUBKEY', pub)

            if key is not None and fixed_sig is None:
                # `sign` expects the full image payload (sha256 done internally),
                # while `sign_digest` expects only the digest of the payload

                if hasattr(key, 'sign'):
                    print(os.path.basename(__file__) + ": sign the payload")
                    sig = key.sign(bytes(self.payload))
                else:
                    print(os.path.basename(__file__) + ": sign the digest")
                    sig = key.sign_digest(digest)
                tlv.add(key.sig_tlv(), sig)
                self.signature = sig
            elif fixed_sig is not None and key is None:
                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")

        # 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:
            if encrypt_keylen == 256:
                plainkey = os.urandom(32)
            else:
                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))
                self.enctlv_len = len(cipherkey)
                tlv.add('ENCRSA2048', cipherkey)
            elif isinstance(enckey, (ecdsa.ECDSA256P1Public,
                                     x25519.X25519Public)):
                cipherkey, mac, pubk = self.ecies_hkdf(enckey, plainkey)
                enctlv = pubk + mac + cipherkey
                self.enctlv_len = len(enctlv)
                if isinstance(enckey, ecdsa.ECDSA256P1Public):
                    tlv.add('ENCEC256', enctlv)
                else:
                    tlv.add('ENCX25519', enctlv)

            if not clear:
                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()

        self.check_trailer()

    def get_signature(self):
        return self.signature

    def add_header(self, enckey, protected_tlv_size, aes_length=128):
        """Install the image header."""

        flags = 0
        if enckey is not None:
            if aes_length == 128:
                flags |= IMAGE_F['ENCRYPTED_AES128']
            else:
                flags |= IMAGE_F['ENCRYPTED_AES256']
        if self.load_addr != 0:
            # Indicates that this image should be loaded into RAM
            # instead of run directly from flash.
            flags |= IMAGE_F['RAM_LOAD']
        if self.rom_fixed:
            flags |= IMAGE_F['ROM_FIXED']

        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.rom_fixed or 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,
                      save_enctlv, enctlv_len):
        # NOTE: should already be checked by the argument parser
        magic_size = 16
        magic_align_size = align_up(magic_size, self.max_align)
        if overwrite_only:
            return self.max_align * 2 + magic_align_size
        else:
            if write_size not in set([1, 2, 4, 8, 16, 32]):
                raise click.BadParameter("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:
                if save_enctlv:
                    # TLV saved by the bootloader is aligned
                    keylen = align_up(enctlv_len, self.max_align)
                else:
                    keylen = align_up(16, self.max_align)
                trailer += keylen * 2  # encryption keys
            trailer += self.max_align * 4  # image_ok/copy_done/swap_info/swap_size
            trailer += magic_align_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,
                                   self.save_enctlv, self.enctlv_len)
        padding = size - (len(self.payload) + tsize)
        pbytes = bytearray([self.erased_val] * padding)
        pbytes += bytearray([self.erased_val] * (tsize - len(self.boot_magic)))
        pbytes += self.boot_magic
        if self.confirm and not self.overwrite_only:
            magic_size = 16
            magic_align_size = align_up(magic_size, self.max_align)
            image_ok_idx = -(magic_align_size + self.max_align)
            pbytes[image_ok_idx] = 0x01  # image_ok = 0x01
        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, None

        tlv_off = header_size + img_size
        tlv_info = b[tlv_off:tlv_off+TLV_INFO_SIZE]
        magic, tlv_tot = struct.unpack('HH', tlv_info)
        if magic == TLV_PROT_INFO_MAGIC:
            tlv_off += tlv_tot
            tlv_info = b[tlv_off:tlv_off+TLV_INFO_SIZE]
            magic, tlv_tot = struct.unpack('HH', tlv_info)

        if magic != TLV_INFO_MAGIC:
            return VerifyResult.INVALID_TLV_INFO_MAGIC, None, None

        sha = hashlib.sha256()
        prot_tlv_size = tlv_off
        sha.update(b[:prot_tlv_size])
        digest = sha.digest()

        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, digest
                else:
                    return VerifyResult.INVALID_HASH, None, 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[:prot_tlv_size]
                try:
                    if hasattr(key, 'verify'):
                        key.verify(tlv_sig, payload)
                    else:
                        key.verify_digest(tlv_sig, digest)
                    return VerifyResult.OK, version, digest
                except InvalidSignature:
                    # continue to next TLV
                    pass
            tlv_off += TLV_SIZE + tlv_len
        return VerifyResult.INVALID_SIGNATURE, None, None