1"""
2RSA Key management
3"""
4
5# SPDX-License-Identifier: Apache-2.0
6
7from cryptography.hazmat.backends import default_backend
8from cryptography.hazmat.primitives import serialization
9from cryptography.hazmat.primitives.asymmetric import rsa
10from cryptography.hazmat.primitives.asymmetric.padding import PSS, MGF1
11from cryptography.hazmat.primitives.hashes import SHA256
12
13from .general import KeyClass
14from .privatebytes import PrivateBytesMixin
15
16
17# Sizes that bootutil will recognize
18RSA_KEY_SIZES = [2048, 3072]
19
20
21class RSAUsageError(Exception):
22    pass
23
24
25class RSAPublic(KeyClass):
26    """The public key can only do a few operations"""
27    def __init__(self, key):
28        self.key = key
29
30    def key_size(self):
31        return self.key.key_size
32
33    def shortname(self):
34        return "rsa"
35
36    def _unsupported(self, name):
37        raise RSAUsageError("Operation {} requires private key".format(name))
38
39    def _get_public(self):
40        return self.key
41
42    def get_public_bytes(self):
43        # The key embedded into MCUboot is in PKCS1 format.
44        return self._get_public().public_bytes(
45                encoding=serialization.Encoding.DER,
46                format=serialization.PublicFormat.PKCS1)
47
48    def get_public_pem(self):
49        return self._get_public().public_bytes(
50                encoding=serialization.Encoding.PEM,
51                format=serialization.PublicFormat.SubjectPublicKeyInfo)
52
53    def get_private_bytes(self, minimal, format):
54        self._unsupported('get_private_bytes')
55
56    def export_private(self, path, passwd=None):
57        self._unsupported('export_private')
58
59    def export_public(self, path):
60        """Write the public key to the given file."""
61        pem = self._get_public().public_bytes(
62                encoding=serialization.Encoding.PEM,
63                format=serialization.PublicFormat.SubjectPublicKeyInfo)
64        with open(path, 'wb') as f:
65            f.write(pem)
66
67    def sig_type(self):
68        return "PKCS1_PSS_RSA{}_SHA256".format(self.key_size())
69
70    def sig_tlv(self):
71        return"RSA{}".format(self.key_size())
72
73    def sig_len(self):
74        return self.key_size() / 8
75
76    def verify(self, signature, payload):
77        k = self.key
78        if isinstance(self.key, rsa.RSAPrivateKey):
79            k = self.key.public_key()
80        return k.verify(signature=signature, data=payload,
81                        padding=PSS(mgf=MGF1(SHA256()), salt_length=32),
82                        algorithm=SHA256())
83
84
85class RSA(RSAPublic, PrivateBytesMixin):
86    """
87    Wrapper around an RSA key, with imgtool support.
88    """
89
90    def __init__(self, key):
91        """The key should be a private key from cryptography"""
92        self.key = key
93
94    @staticmethod
95    def generate(key_size=2048):
96        if key_size not in RSA_KEY_SIZES:
97            raise RSAUsageError("Key size {} is not supported by MCUboot"
98                                .format(key_size))
99        pk = rsa.generate_private_key(
100                public_exponent=65537,
101                key_size=key_size,
102                backend=default_backend())
103        return RSA(pk)
104
105    def _get_public(self):
106        return self.key.public_key()
107
108    def _build_minimal_rsa_privkey(self, der):
109        '''
110        Builds a new DER that only includes N/E/D/P/Q RSA parameters;
111        standard DER private bytes provided by OpenSSL also includes
112        CRT params (DP/DQ/QP) which can be removed.
113        '''
114        OFFSET_N = 7  # N is always located at this offset
115        b = bytearray(der)
116        off = OFFSET_N
117        if b[off + 1] != 0x82:
118            raise RSAUsageError("Error parsing N while minimizing")
119        len_N = (b[off + 2] << 8) + b[off + 3] + 4
120        off += len_N
121        if b[off + 1] != 0x03:
122            raise RSAUsageError("Error parsing E while minimizing")
123        len_E = b[off + 2] + 4
124        off += len_E
125        if b[off + 1] != 0x82:
126            raise RSAUsageError("Error parsing D while minimizing")
127        len_D = (b[off + 2] << 8) + b[off + 3] + 4
128        off += len_D
129        if b[off + 1] != 0x81:
130            raise RSAUsageError("Error parsing P while minimizing")
131        len_P = b[off + 2] + 3
132        off += len_P
133        if b[off + 1] != 0x81:
134            raise RSAUsageError("Error parsing Q while minimizing")
135        len_Q = b[off + 2] + 3
136        off += len_Q
137        # adjust DER size for removed elements
138        b[2] = (off - 4) >> 8
139        b[3] = (off - 4) & 0xff
140        return b[:off]
141
142    _VALID_FORMATS = {
143        'openssl': serialization.PrivateFormat.TraditionalOpenSSL
144    }
145    _DEFAULT_FORMAT = 'openssl'
146
147    def get_private_bytes(self, minimal, format):
148        _, priv = self._get_private_bytes(minimal, format, RSAUsageError)
149        if minimal:
150            priv = self._build_minimal_rsa_privkey(priv)
151        return priv
152
153    def export_private(self, path, passwd=None):
154        """Write the private key to the given file, protecting it with the
155        optional password."""
156        if passwd is None:
157            enc = serialization.NoEncryption()
158        else:
159            enc = serialization.BestAvailableEncryption(passwd)
160        pem = self.key.private_bytes(
161                encoding=serialization.Encoding.PEM,
162                format=serialization.PrivateFormat.PKCS8,
163                encryption_algorithm=enc)
164        with open(path, 'wb') as f:
165            f.write(pem)
166
167    def sign(self, payload):
168        # The verification code only allows the salt length to be the
169        # same as the hash length, 32.
170        return self.key.sign(
171                data=payload,
172                padding=PSS(mgf=MGF1(SHA256()), salt_length=32),
173                algorithm=SHA256())
174