172 lines
5.1 KiB
Python
172 lines
5.1 KiB
Python
"""
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Image signing and management.
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"""
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from . import version as versmod
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import hashlib
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import struct
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IMAGE_MAGIC = 0x96f3b83c
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IMAGE_HEADER_SIZE = 32
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# Image header flags.
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IMAGE_F = {
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'PIC': 0x0000001,
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'SHA256': 0x0000002,
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'PKCS15_RSA2048_SHA256': 0x0000004,
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'ECDSA224_SHA256': 0x0000008,
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'NON_BOOTABLE': 0x0000010,
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'ECDSA256_SHA256': 0x0000020,
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'PKCS1_PSS_RSA2048_SHA256': 0x0000040, }
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TLV_VALUES = {
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'SHA256': 1,
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'RSA2048': 2,
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'ECDSA224': 3,
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'ECDSA256': 4, }
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TLV_HEADER_SIZE = 4
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# Sizes of the image trailer, depending on flash write size.
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trailer_sizes = {
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write_size: 128 * 3 * write_size + 8 * 2 + 16
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for write_size in [1, 2, 4, 8]
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}
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boot_magic = bytes([
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0x77, 0xc2, 0x95, 0xf3,
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0x60, 0xd2, 0xef, 0x7f,
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0x35, 0x52, 0x50, 0x0f,
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0x2c, 0xb6, 0x79, 0x80, ])
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class TLV():
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def __init__(self):
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self.buf = bytearray()
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def add(self, kind, payload):
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"""Add a TLV record. Kind should be a string found in TLV_VALUES above."""
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buf = struct.pack('<BBH', TLV_VALUES[kind], 0, len(payload))
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self.buf += buf
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self.buf += payload
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def get(self):
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return bytes(self.buf)
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class Image():
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@classmethod
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def load(cls, path, included_header=False, **kwargs):
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"""Load an image from a given file"""
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with open(path, 'rb') as f:
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payload = f.read()
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obj = cls(**kwargs)
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obj.payload = payload
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# Add the image header if needed.
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if not included_header and obj.header_size > 0:
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obj.payload = (b'\000' * obj.header_size) + obj.payload
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obj.check()
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return obj
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def __init__(self, version=None, header_size=IMAGE_HEADER_SIZE, pad=0):
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self.version = version or versmod.decode_version("0")
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self.header_size = header_size or IMAGE_HEADER_SIZE
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self.pad = pad
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def __repr__(self):
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return "<Image version={}, header_size={}, pad={}, payloadlen=0x{:x}>".format(
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self.version,
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self.header_size,
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self.pad,
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len(self.payload))
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def save(self, path):
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with open(path, 'wb') as f:
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f.write(self.payload)
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def check(self):
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"""Perform some sanity checking of the image."""
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# If there is a header requested, make sure that the image
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# starts with all zeros.
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if self.header_size > 0:
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if any(v != 0 for v in self.payload[0:self.header_size]):
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raise Exception("Padding requested, but image does not start with zeros")
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def sign(self, key):
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self.add_header(key)
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tlv = TLV()
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# Note that ecdsa wants to do the hashing itself, which means
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# we get to hash it twice.
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sha = hashlib.sha256()
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sha.update(self.payload)
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digest = sha.digest()
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tlv.add('SHA256', digest)
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if key is not None:
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sig = key.sign(self.payload)
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tlv.add(key.sig_tlv(), sig)
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self.payload += tlv.get()
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def add_header(self, key):
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"""Install the image header.
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The key is needed to know the type of signature, and
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approximate the size of the signature."""
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flags = 0
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tlvsz = 0
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if key is not None:
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flags |= IMAGE_F[key.sig_type()]
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tlvsz += TLV_HEADER_SIZE + key.sig_len()
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flags |= IMAGE_F['SHA256']
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tlvsz += 4 + hashlib.sha256().digest_size
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fmt = ('<' +
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# type ImageHdr struct {
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'I' + # Magic uint32
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'H' + # TlvSz uint16
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'B' + # KeyId uint8
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'B' + # Pad1 uint8
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'H' + # HdrSz uint16
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'H' + # Pad2 uint16
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'I' + # ImgSz uint32
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'I' + # Flags uint32
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'BBHI' + # Vers ImageVersion
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'I' # Pad3 uint32
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) # }
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assert struct.calcsize(fmt) == IMAGE_HEADER_SIZE
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header = struct.pack(fmt,
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IMAGE_MAGIC,
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tlvsz, # TlvSz
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0, # KeyId (TODO: allow other ids)
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0, # Pad1
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self.header_size,
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0, # Pad2
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len(self.payload) - self.header_size, # ImageSz
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flags, # Flags
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self.version.major,
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self.version.minor or 0,
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self.version.revision or 0,
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self.version.build or 0,
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0) # Pad3
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self.payload = bytearray(self.payload)
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self.payload[:len(header)] = header
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def pad_to(self, size, align):
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"""Pad the image to the given size, with the given flash alignment."""
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tsize = trailer_sizes[align]
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padding = size - (len(self.payload) + tsize)
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if padding < 0:
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msg = "Image size (0x{:x}) + trailer (0x{:x}) exceeds requested size 0x{:x}".format(
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len(self.payload), tsize, size)
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raise Exception(msg)
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pbytes = b'\xff' * padding
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pbytes += b'\xff' * (tsize - len(boot_magic))
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pbytes += boot_magic
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self.payload += pbytes
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