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"""
RSA Key management
"""
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.primitives.asymmetric.padding import PSS, MGF1
from cryptography.hazmat.primitives.hashes import SHA256
from .general import KeyClass
# Sizes that bootutil will recognize
RSA_KEY_SIZES = [2048, 3072]
class RSAUsageError(Exception):
pass
class RSAPublic(KeyClass):
"""The public key can only do a few operations"""
def __init__(self, key):
self.key = key
def key_size(self):
return self.key.key_size
def shortname(self):
return "rsa"
def _unsupported(self, name):
raise RSAUsageError("Operation {} requires private key".format(name))
def _get_public(self):
return self.key
def get_public_bytes(self):
# The key embedded into MCUboot is in PKCS1 format.
return self._get_public().public_bytes(
encoding=serialization.Encoding.DER,
format=serialization.PublicFormat.PKCS1)
def get_private_bytes(self, minimal):
self._unsupported('get_private_bytes')
def export_private(self, path, passwd=None):
self._unsupported('export_private')
def export_public(self, path):
"""Write the public key to the given file."""
pem = self._get_public().public_bytes(
encoding=serialization.Encoding.PEM,
format=serialization.PublicFormat.SubjectPublicKeyInfo)
with open(path, 'wb') as f:
f.write(pem)
def sig_type(self):
return "PKCS1_PSS_RSA{}_SHA256".format(self.key_size())
def sig_tlv(self):
return"RSA{}".format(self.key_size())
def sig_len(self):
return self.key_size() / 8
def verify(self, signature, payload):
k = self.key
if isinstance(self.key, rsa.RSAPrivateKey):
k = self.key.public_key()
return k.verify(signature=signature, data=payload,
padding=PSS(mgf=MGF1(SHA256()), salt_length=32),
algorithm=SHA256())
class RSA(RSAPublic):
"""
Wrapper around an RSA key, with imgtool support.
"""
def __init__(self, key):
"""The key should be a private key from cryptography"""
self.key = key
@staticmethod
def generate(key_size=2048):
if key_size not in RSA_KEY_SIZES:
raise RSAUsageError("Key size {} is not supported by MCUboot"
.format(key_size))
pk = rsa.generate_private_key(
public_exponent=65537,
key_size=key_size,
backend=default_backend())
return RSA(pk)
def _get_public(self):
return self.key.public_key()
def _build_minimal_rsa_privkey(self, der):
'''
Builds a new DER that only includes N/E/D/P/Q RSA parameters;
standard DER private bytes provided by OpenSSL also includes
CRT params (DP/DQ/QP) which can be removed.
'''
OFFSET_N = 7 # N is always located at this offset
b = bytearray(der)
off = OFFSET_N
if b[off + 1] != 0x82:
raise RSAUsageError("Error parsing N while minimizing")
len_N = (b[off + 2] << 8) + b[off + 3] + 4
off += len_N
if b[off + 1] != 0x03:
raise RSAUsageError("Error parsing E while minimizing")
len_E = b[off + 2] + 4
off += len_E
if b[off + 1] != 0x82:
raise RSAUsageError("Error parsing D while minimizing")
len_D = (b[off + 2] << 8) + b[off + 3] + 4
off += len_D
if b[off + 1] != 0x81:
raise RSAUsageError("Error parsing P while minimizing")
len_P = b[off + 2] + 3
off += len_P
if b[off + 1] != 0x81:
raise RSAUsageError("Error parsing Q while minimizing")
len_Q = b[off + 2] + 3
off += len_Q
# adjust DER size for removed elements
b[2] = (off - 4) >> 8
b[3] = (off - 4) & 0xff
return b[:off]
def get_private_bytes(self, minimal):
priv = self.key.private_bytes(
encoding=serialization.Encoding.DER,
format=serialization.PrivateFormat.TraditionalOpenSSL,
encryption_algorithm=serialization.NoEncryption())
if minimal:
priv = self._build_minimal_rsa_privkey(priv)
return priv
def export_private(self, path, passwd=None):
"""Write the private key to the given file, protecting it with the
optional password."""
if passwd is None:
enc = serialization.NoEncryption()
else:
enc = serialization.BestAvailableEncryption(passwd)
pem = self.key.private_bytes(
encoding=serialization.Encoding.PEM,
format=serialization.PrivateFormat.PKCS8,
encryption_algorithm=enc)
with open(path, 'wb') as f:
f.write(pem)
def sign(self, payload):
# The verification code only allows the salt length to be the
# same as the hash length, 32.
return self.key.sign(
data=payload,
padding=PSS(mgf=MGF1(SHA256()), salt_length=32),
algorithm=SHA256())
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