New python module to encode a PSA key for storage
Construct an object given the attributes and material for a PSA crypto key
and get the Mbed TLS storage representation.
The code to generate the storage representation was written based on the
specification in docs/architecture/mbed-crypto-storage-specification.md,
without looking at the code.
The data in the unit tests is from the AES-128 format_storage_data_check
test case in test_suite_psa_crypto_persistent_key.data, tweaked manually.
This commit creates a basic framework for using symbolic values for
attributes, but does not yet implement obtaining the corresponding numerical
values from an external source.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/scripts/mbedtls_dev/psa_storage.py b/scripts/mbedtls_dev/psa_storage.py
new file mode 100644
index 0000000..3e15c67
--- /dev/null
+++ b/scripts/mbedtls_dev/psa_storage.py
@@ -0,0 +1,184 @@
+"""Knowledge about the PSA key store as implemented in Mbed TLS.
+"""
+
+# Copyright The Mbed TLS Contributors
+# SPDX-License-Identifier: Apache-2.0
+#
+# 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.
+
+import re
+import struct
+from typing import Optional, Union
+import unittest
+
+
+class Expr:
+ """Representation of a C expression with a known or knowable numerical value."""
+ def __init__(self, content: Union[int, str]):
+ if isinstance(content, int):
+ digits = 8 if content > 0xffff else 4
+ self.string = '{0:#0{1}x}'.format(content, digits + 2)
+ self.value_if_known = content #type: Optional[int]
+ else:
+ self.string = content
+ self.value_if_known = None
+
+ value_cache = {
+ # Hard-coded for initial testing
+ 'PSA_KEY_LIFETIME_PERSISTENT': 0x00000001,
+ 'PSA_KEY_TYPE_RAW_DATA': 0x1001,
+ }
+
+ def update_cache(self) -> None:
+ pass #not implemented yet
+
+ @staticmethod
+ def normalize(string: str) -> str:
+ """Put the given C expression in a canonical form.
+
+ This function is only intended to give correct results for the
+ relatively simple kind of C expression typically used with this
+ module.
+ """
+ return re.sub(r'\s+', r'', string)
+
+ def value(self) -> int:
+ """Return the numerical value of the expression."""
+ if self.value_if_known is None:
+ if re.match(r'([0-9]+|0x[0-9a-f]+)\Z', self.string, re.I):
+ return int(self.string, 0)
+ normalized = self.normalize(self.string)
+ if normalized not in self.value_cache:
+ self.update_cache()
+ self.value_if_known = self.value_cache[normalized]
+ return self.value_if_known
+
+Exprable = Union[str, int, Expr]
+"""Something that can be converted to a C expression with a known numerical value."""
+
+def as_expr(thing: Exprable) -> Expr:
+ """Return an `Expr` object for `thing`.
+
+ If `thing` is already an `Expr` object, return it. Otherwise build a new
+ `Expr` object from `thing`. `thing` can be an integer or a string that
+ contains a C expression.
+ """
+ if isinstance(thing, Expr):
+ return thing
+ else:
+ return Expr(thing)
+
+
+class Key:
+ """Representation of a PSA crypto key object and its storage encoding.
+ """
+
+ LATEST_VERSION = 0
+ """The latest version of the storage format."""
+
+ def __init__(self, *,
+ version: Optional[int] = None,
+ id: Optional[int] = None, #pylint: disable=redefined-builtin
+ lifetime: Exprable = 'PSA_KEY_LIFETIME_PERSISTENT',
+ type: Exprable, #pylint: disable=redefined-builtin
+ bits: int,
+ usage: Exprable, alg: Exprable, alg2: Exprable,
+ material: bytes #pylint: disable=used-before-assignment
+ ) -> None:
+ self.version = self.LATEST_VERSION if version is None else version
+ self.id = id #pylint: disable=invalid-name #type: Optional[int]
+ self.lifetime = as_expr(lifetime) #type: Expr
+ self.type = as_expr(type) #type: Expr
+ self.bits = bits #type: int
+ self.usage = as_expr(usage) #type: Expr
+ self.alg = as_expr(alg) #type: Expr
+ self.alg2 = as_expr(alg2) #type: Expr
+ self.material = material #type: bytes
+
+ MAGIC = b'PSA\000KEY\000'
+
+ @staticmethod
+ def pack(
+ fmt: str,
+ *args: Union[int, Expr]
+ ) -> bytes: #pylint: disable=used-before-assignment
+ """Pack the given arguments into a byte string according to the given format.
+
+ This function is similar to `struct.pack`, but with the following differences:
+ * All integer values are encoded with standard sizes and in
+ little-endian representation. `fmt` must not include an endianness
+ prefix.
+ * Arguments can be `Expr` objects instead of integers.
+ * Only integer-valued elements are supported.
+ """
+ return struct.pack('<' + fmt, # little-endian, standard sizes
+ *[arg.value() if isinstance(arg, Expr) else arg
+ for arg in args])
+
+ def bytes(self) -> bytes:
+ """Return the representation of the key in storage as a byte array.
+
+ This is the content of the PSA storage file. When PSA storage is
+ implemented over stdio files, this does not include any wrapping made
+ by the PSA-storage-over-stdio-file implementation.
+ """
+ header = self.MAGIC + self.pack('L', self.version)
+ if self.version == 0:
+ attributes = self.pack('LHHLLL',
+ self.lifetime, self.type, self.bits,
+ self.usage, self.alg, self.alg2)
+ material = self.pack('L', len(self.material)) + self.material
+ else:
+ raise NotImplementedError
+ return header + attributes + material
+
+ def hex(self) -> str:
+ """Return the representation of the key as a hexadecimal string.
+
+ This is the hexadecimal representation of `self.bytes`.
+ """
+ return self.bytes().hex()
+
+
+class TestKey(unittest.TestCase):
+ # pylint: disable=line-too-long
+ """A few smoke tests for the functionality of the `Key` class."""
+
+ def test_numerical(self):
+ key = Key(version=0,
+ id=1, lifetime=0x00000001,
+ type=0x2400, bits=128,
+ usage=0x00000300, alg=0x05500200, alg2=0x04c01000,
+ material=b'@ABCDEFGHIJKLMNO')
+ expected_hex = '505341004b45590000000000010000000024800000030000000250050010c00410000000404142434445464748494a4b4c4d4e4f'
+ self.assertEqual(key.bytes(), bytes.fromhex(expected_hex))
+ self.assertEqual(key.hex(), expected_hex)
+
+ def test_names(self):
+ length = 0xfff8 // 8 # PSA_MAX_KEY_BITS in bytes
+ key = Key(version=0,
+ id=1, lifetime='PSA_KEY_LIFETIME_PERSISTENT',
+ type='PSA_KEY_TYPE_RAW_DATA', bits=length*8,
+ usage=0, alg=0, alg2=0,
+ material=b'\x00' * length)
+ expected_hex = '505341004b45590000000000010000000110f8ff000000000000000000000000ff1f0000' + '00' * length
+ self.assertEqual(key.bytes(), bytes.fromhex(expected_hex))
+ self.assertEqual(key.hex(), expected_hex)
+
+ def test_defaults(self):
+ key = Key(type=0x1001, bits=8,
+ usage=0, alg=0, alg2=0,
+ material=b'\x2a')
+ expected_hex = '505341004b455900000000000100000001100800000000000000000000000000010000002a'
+ self.assertEqual(key.bytes(), bytes.fromhex(expected_hex))
+ self.assertEqual(key.hex(), expected_hex)