Mini Shell

# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.


from cryptography.exceptions import InvalidSignature
from cryptography.hazmat.primitives import constant_time


_POLY1305_TAG_SIZE = 16
_POLY1305_KEY_SIZE = 32


class _Poly1305Context(object):
    def __init__(self, backend, key):
        self._backend = backend

        key_ptr = self._backend._ffi.from_buffer(key)
        # This function copies the key into OpenSSL-owned memory so we don't
        # need to retain it ourselves
        evp_pkey = self._backend._lib.EVP_PKEY_new_raw_private_key(
            self._backend._lib.NID_poly1305,
            self._backend._ffi.NULL,
            key_ptr,
            len(key),
        )
        self._backend.openssl_assert(evp_pkey != self._backend._ffi.NULL)
        self._evp_pkey = self._backend._ffi.gc(
            evp_pkey, self._backend._lib.EVP_PKEY_free
        )
        ctx = self._backend._lib.EVP_MD_CTX_new()
        self._backend.openssl_assert(ctx != self._backend._ffi.NULL)
        self._ctx = self._backend._ffi.gc(
            ctx, self._backend._lib.EVP_MD_CTX_free
        )
        res = self._backend._lib.EVP_DigestSignInit(
            self._ctx,
            self._backend._ffi.NULL,
            self._backend._ffi.NULL,
            self._backend._ffi.NULL,
            self._evp_pkey,
        )
        self._backend.openssl_assert(res == 1)

    def update(self, data):
        data_ptr = self._backend._ffi.from_buffer(data)
        res = self._backend._lib.EVP_DigestSignUpdate(
            self._ctx, data_ptr, len(data)
        )
        self._backend.openssl_assert(res != 0)

    def finalize(self):
        buf = self._backend._ffi.new("unsigned char[]", _POLY1305_TAG_SIZE)
        outlen = self._backend._ffi.new("size_t *", _POLY1305_TAG_SIZE)
        res = self._backend._lib.EVP_DigestSignFinal(self._ctx, buf, outlen)
        self._backend.openssl_assert(res != 0)
        self._backend.openssl_assert(outlen[0] == _POLY1305_TAG_SIZE)
        return self._backend._ffi.buffer(buf)[: outlen[0]]

    def verify(self, tag):
        mac = self.finalize()
        if not constant_time.bytes_eq(mac, tag):
            raise InvalidSignature("Value did not match computed tag.")