# Copyright 2018 Nordic Semiconductor ASA # Copyright 2017 Linaro Limited # Copyright 2019-2020 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 .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 MAX_ALIGN = 8 DEP_IMAGES_KEY = "images" DEP_VERSIONS_KEY = "versions" MAX_SW_TYPE_LENGTH = 12 # Bytes # Image header flags. IMAGE_F = { 'PIC': 0x0000001, 'NON_BOOTABLE': 0x0000010, 'RAM_LOAD': 0x0000020, 'ENCRYPTED': 0x0000004, } TLV_VALUES = { 'KEYHASH': 0x01, 'PUBKEY': 0x02, 'SHA256': 0x10, 'RSA2048': 0x20, 'ECDSA224': 0x21, 'ECDSA256': 0x22, 'RSA3072': 0x23, 'ED25519': 0x24, 'ENCRSA2048': 0x30, 'ENCKW128': 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 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, confirm=False, align=1, slot_size=0, max_sectors=DEFAULT_MAX_SECTORS, overwrite_only=False, endian="little", load_addr=0, erased_val=None, save_enctlv=False, security_counter=None): 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.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 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 "".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 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_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): 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() 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 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 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: 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) 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: if public_key_format == 'hash': tlv.add('KEYHASH', pubbytes) else: tlv.add('PUBKEY', pub) # `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)) 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) 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 add_header(self, enckey, protected_tlv_size): """Install the image header.""" flags = 0 if enckey is not None: flags |= IMAGE_F['ENCRYPTED'] 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'] 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, save_enctlv, enctlv_len): # 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 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 = (int((enctlv_len - 1) / MAX_ALIGN) + 1) * MAX_ALIGN else: keylen = 16 trailer += keylen * 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, 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(boot_magic))) if self.confirm and not self.overwrite_only: pbytes[-MAX_ALIGN] = 0x01 # image_ok = 0x01 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