How to use the oscrypto._ffi.bytes_from_buffer function in oscrypto

# the default, and what is used by Security.framework on OS X also.
    g = 2

    try:
        byte_size = bit_size // 8
        if _backend == 'win':
            alg_handle = open_alg_handle(BcryptConst.BCRYPT_RNG_ALGORITHM)
            buffer = buffer_from_bytes(byte_size)

        while True:
            if _backend == 'winlegacy':
                rb = os.urandom(byte_size)
            else:
                res = bcrypt.BCryptGenRandom(alg_handle, buffer, byte_size, 0)
                handle_error(res)
                rb = bytes_from_buffer(buffer)

            p = int_from_bytes(rb)

            # If a number is even, it can't be prime
            if p % 2 == 0:
                continue

            # Perform the generator checks outlined in OpenSSL's
            # dh_builtin_genparams() located in dh_gen.c
            if g == 2:
                if p % 24 != 11:
                    continue
            elif g == 5:
                rem = p % 10
                if rem != 3 and rem != 7:
                    continue

ciphertext = ciphertext[::-1]

    buffer = buffer_from_bytes(ciphertext)
    out_len = new(advapi32, 'DWORD *', len(ciphertext))
    res = advapi32.CryptDecrypt(
        private_key.ex_key_handle,
        null(),
        True,
        flags,
        buffer,
        out_len
    )
    handle_error(res)

    return bytes_from_buffer(buffer, deref(out_len))

if padding is not None:
            res = libcrypto.EVP_CIPHER_CTX_set_padding(evp_cipher_ctx, int(padding))
            handle_openssl_error(res)

        buffer = buffer_from_bytes(buffer_size)
        output_length = new(libcrypto, 'int *')

        res = libcrypto.EVP_EncryptUpdate(evp_cipher_ctx, buffer, output_length, data, len(data))
        handle_openssl_error(res)

        output = bytes_from_buffer(buffer, deref(output_length))

        res = libcrypto.EVP_EncryptFinal_ex(evp_cipher_ctx, buffer, output_length)
        handle_openssl_error(res)

        output += bytes_from_buffer(buffer, deref(output_length))

        return output

    finally:
        if evp_cipher_ctx:
            libcrypto.EVP_CIPHER_CTX_free(evp_cipher_ctx)

evp_cipher = null()

        res = libcrypto.EVP_EncryptInit_ex(evp_cipher_ctx, evp_cipher, null(), key, iv)
        handle_openssl_error(res)

        if padding is not None:
            res = libcrypto.EVP_CIPHER_CTX_set_padding(evp_cipher_ctx, int(padding))
            handle_openssl_error(res)

        buffer = buffer_from_bytes(buffer_size)
        output_length = new(libcrypto, 'int *')

        res = libcrypto.EVP_EncryptUpdate(evp_cipher_ctx, buffer, output_length, data, len(data))
        handle_openssl_error(res)

        output = bytes_from_buffer(buffer, deref(output_length))

        res = libcrypto.EVP_EncryptFinal_ex(evp_cipher_ctx, buffer, output_length)
        handle_openssl_error(res)

        output += bytes_from_buffer(buffer, deref(output_length))

        return output

    finally:
        if evp_cipher_ctx:
            libcrypto.EVP_CIPHER_CTX_free(evp_cipher_ctx)

self._handshake(renegotiate=True)
                return self.read(max_length)

            elif result != Secur32Const.SEC_E_OK:
                handle_error(result, TLSError)

            valid_buffer_types = set([
                Secur32Const.SECBUFFER_EMPTY,
                Secur32Const.SECBUFFER_STREAM_HEADER,
                Secur32Const.SECBUFFER_STREAM_TRAILER
            ])
            extra_amount = None
            for buf in (buf0, buf1, buf2, buf3):
                buffer_type = buf.BufferType
                if buffer_type == Secur32Const.SECBUFFER_DATA:
                    output += bytes_from_buffer(buf.pvBuffer, buf.cbBuffer)
                    output_len = len(output)
                elif buffer_type == Secur32Const.SECBUFFER_EXTRA:
                    extra_amount = native(int, buf.cbBuffer)
                elif buffer_type not in valid_buffer_types:
                    raise OSError(pretty_message(
                        '''
                        Unexpected decrypt output buffer of type %s
                        ''',
                        buffer_type
                    ))

            if extra_amount:
                self._received_bytes = self._received_bytes[data_len - extra_amount:]
            else:
                self._received_bytes = self._received_bytes[data_len:]

rsa = libcrypto.EVP_PKEY_get1_RSA(certificate_or_public_key.evp_pkey)
            if is_null(rsa):
                handle_openssl_error(0)

            buffer_size = libcrypto.EVP_PKEY_size(certificate_or_public_key.evp_pkey)
            decrypted_buffer = buffer_from_bytes(buffer_size)
            decrypted_length = libcrypto.RSA_public_decrypt(
                len(signature),
                signature,
                decrypted_buffer,
                rsa,
                LibcryptoConst.RSA_PKCS1_PADDING
            )
            handle_openssl_error(decrypted_length)

            decrypted_bytes = bytes_from_buffer(decrypted_buffer, decrypted_length)

            if not constant_compare(data, decrypted_bytes):
                raise SignatureError('Signature is invalid')
            return

        finally:
            if rsa:
                libcrypto.RSA_free(rsa)

    evp_md_ctx = None
    rsa = None
    dsa = None
    dsa_sig = None
    ec_key = None
    ecdsa_sig = None

res = bcrypt.BCryptEncrypt(
        certificate_or_public_key.key_handle,
        data,
        len(data),
        padding_info,
        null(),
        0,
        buffer,
        buffer_len,
        out_len,
        flags
    )
    handle_error(res)

    return bytes_from_buffer(buffer, deref(out_len))

if is_null(rsa):
                handle_openssl_error(0)

            buffer_size = libcrypto.EVP_PKEY_size(private_key.evp_pkey)

            signature_buffer = buffer_from_bytes(buffer_size)
            signature_length = libcrypto.RSA_private_encrypt(
                len(data),
                data,
                signature_buffer,
                rsa,
                LibcryptoConst.RSA_PKCS1_PADDING
            )
            handle_openssl_error(signature_length)

            return bytes_from_buffer(signature_buffer, signature_length)

        finally:
            if rsa:
                libcrypto.RSA_free(rsa)

    evp_md_ctx = None
    rsa = None
    dsa = None
    dsa_sig = None
    ec_key = None
    ecdsa_sig = None

    try:
        if libcrypto_version_info < (1, 1):
            evp_md_ctx = libcrypto.EVP_MD_CTX_create()
        else:

raise_disconnection()

                elif error == LibsslConst.SSL_ERROR_WANT_WRITE:
                    self._raw_write()
                    again = True
                    continue

                elif error == LibsslConst.SSL_ERROR_ZERO_RETURN:
                    self._gracefully_closed = True
                    self._shutdown(False)
                    break

                else:
                    handle_openssl_error(0, TLSError)

            output += bytes_from_buffer(self._read_buffer, result)

        if self._gracefully_closed and len(output) == 0:
            self._raise_closed()

        self._decrypted_bytes = output[max_length:]
        return output[0:max_length]

encoded_data = add_pss_padding(hash_algorithm, hash_length, private_key.bit_size, data)

    key_length = private_key.byte_size
    buffer = buffer_from_bytes(key_length)
    output_length = new(Security, 'size_t *', key_length)
    result = Security.SecKeyDecrypt(
        private_key.sec_key_ref,
        SecurityConst.kSecPaddingNone,
        encoded_data,
        len(encoded_data),
        buffer,
        output_length
    )
    handle_sec_error(result)

    return bytes_from_buffer(buffer, deref(output_length))