scripts/imgtool/keys.py

"""
Cryptographic key management for imgtool.
"""

from Crypto.Hash import SHA256
from Crypto.PublicKey import RSA
from Crypto.Signature import PKCS1_v1_5, PKCS1_PSS
from ecdsa import SigningKey, NIST256p, util
import hashlib
from pyasn1.type import namedtype, univ
from pyasn1.codec.der.encoder import encode

# By default, we use RSA-PSS (PKCS 2.1).  That can be overridden on
# the command line to support the older (less secure) PKCS1.5
sign_rsa_pss = True

AUTOGEN_MESSAGE = "/* Autogenerated by imgtool.py, do not edit. */"

class RSAPublicKey(univ.Sequence):
    componentType = namedtype.NamedTypes(
            namedtype.NamedType('modulus', univ.Integer()),
            namedtype.NamedType('publicExponent', univ.Integer()))

class RSA2048():
    def __init__(self, key):
        """Construct an RSA2048 key with the given key data"""
        self.key = key

    @staticmethod
    def generate():
        return RSA2048(RSA.generate(2048))

    def export_private(self, path):
        with open(path, 'wb') as f:
            f.write(self.key.exportKey('PEM'))

    def emit_c(self):
        node = RSAPublicKey()
        node['modulus'] = self.key.n
        node['publicExponent'] = self.key.e
        print(AUTOGEN_MESSAGE)
        print("const unsigned char rsa_pub_key[] = {", end='')
        encoded = bytearray(encode(node))
        for count, b in enumerate(encoded):
            if count % 8 == 0:
                print("\n\t", end='')
            else:
                print(" ", end='')
            print("0x{:02x},".format(b), end='')
        print("\n};")
        print("const unsigned int rsa_pub_key_len = {};".format(len(encoded)))

    def sig_type(self):
        """Return the type of this signature (as a string)"""
        if sign_rsa_pss:
            return "PKCS1_PSS_RSA2048_SHA256"
        else:
            return "PKCS15_RSA2048_SHA256"

    def sig_len(self):
        return 256

    def sig_tlv(self):
        return "RSA2048"

    def sign(self, payload):
        sha = SHA256.new(payload)
        if sign_rsa_pss:
            signer = PKCS1_PSS.new(self.key)
        else:
            signer = PKCS1_v1_5.new(self.key)
        signature = signer.sign(sha)
        assert len(signature) == self.sig_len()
        return signature

class ECDSA256P1():
    def __init__(self, key):
        """Construct an ECDSA P-256 private key"""
        self.key = key

    @staticmethod
    def generate():
        return ECDSA256P1(SigningKey.generate(curve=NIST256p))

    def export_private(self, path):
        with open(path, 'wb') as f:
            f.write(self.key.to_pem())

    def emit_c(self):
        vk = self.key.get_verifying_key()
        print(AUTOGEN_MESSAGE)
        print("const unsigned char ecdsa_pub_key[] = {", end='')
        encoded = bytes(vk.to_der())
        for count, b in enumerate(encoded):
            if count % 8 == 0:
                print("\n\t", end='')
            else:
                print(" ", end='')
            print("0x{:02x},".format(b), end='')
        print("\n};")
        print("const unsigned int ecdsa_pub_key_len = {};".format(len(encoded)))

    def sign(self, payload):
        # To make this fixed length, possibly pad with zeros.
        sig = self.key.sign(payload, hashfunc=hashlib.sha256, sigencode=util.sigencode_der)
        sig += b'\000' * (self.sig_len() - len(sig))
        return sig

    def sig_len(self):
        # The DER encoding depends on the high bit, and can be
        # anywhere from 70 to 72 bytes.  Because we have to fill in
        # the length field before computing the signature, however,
        # we'll give the largest, and the sig checking code will allow
        # for it to be up to two bytes larger than the actual
        # signature.
        return 72

    def sig_type(self):
        """Return the type of this signature (as a string)"""
        return "ECDSA256_SHA256"

    def sig_tlv(self):
        return "ECDSA256"

def load(path):
    with open(path, 'rb') as f:
        pem = f.read()
    try:
        key = RSA.importKey(pem)
        if key.n.bit_length() != 2048:
            raise Exception("Unsupported RSA bit length, only 2048 supported")
        return RSA2048(key)
    except ValueError:
        key = SigningKey.from_pem(pem)
        if key.curve.name != 'NIST256p':
            raise Exception("Unsupported ECDSA curve")
        return ECDSA256P1(key)