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auth_encrypted_key_blob.cpp
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/*
* Copyright 2015 The Android Open Source Project
*
* 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.
*/
#include "auth_encrypted_key_blob.h"
#include <keymaster/android_keymaster_utils.h>
#include <keymaster/authorization_set.h>
#include <keymaster/logger.h>
#include "ocb_utils.h"
namespace keymaster {
const uint32_t CURRENT_BLOB_VERSION = 0;
keymaster_error_t SerializeAuthEncryptedBlob(const KeymasterKeyBlob& encrypted_key_material,
const AuthorizationSet& hw_enforced,
const AuthorizationSet& sw_enforced,
const Buffer& nonce, const Buffer& tag,
KeymasterKeyBlob* key_blob) {
size_t size = 1 /* version byte */ + nonce.SerializedSize() +
encrypted_key_material.SerializedSize() + tag.SerializedSize() +
hw_enforced.SerializedSize() + sw_enforced.SerializedSize();
if (!key_blob->Reset(size))
return KM_ERROR_MEMORY_ALLOCATION_FAILED;
uint8_t* buf = key_blob->writable_data();
const uint8_t* end = key_blob->key_material + key_blob->key_material_size;
*buf++ = CURRENT_BLOB_VERSION;
buf = nonce.Serialize(buf, end);
buf = encrypted_key_material.Serialize(buf, end);
buf = tag.Serialize(buf, end);
buf = hw_enforced.Serialize(buf, end);
buf = sw_enforced.Serialize(buf, end);
if (buf != key_blob->key_material + key_blob->key_material_size)
return KM_ERROR_UNKNOWN_ERROR;
return KM_ERROR_OK;
}
static keymaster_error_t DeserializeUnversionedBlob(const KeymasterKeyBlob& key_blob,
KeymasterKeyBlob* encrypted_key_material,
AuthorizationSet* hw_enforced,
AuthorizationSet* sw_enforced, Buffer* nonce,
Buffer* tag) {
const uint8_t* tmp = key_blob.key_material;
const uint8_t** buf_ptr = &tmp;
const uint8_t* end = tmp + key_blob.key_material_size;
if (!nonce->reserve(OCB_NONCE_LENGTH) || !tag->reserve(OCB_TAG_LENGTH))
return KM_ERROR_MEMORY_ALLOCATION_FAILED;
if (!copy_from_buf(buf_ptr, end, nonce->peek_write(), OCB_NONCE_LENGTH) ||
!encrypted_key_material->Deserialize(buf_ptr, end) ||
!copy_from_buf(buf_ptr, end, tag->peek_write(), OCB_TAG_LENGTH) ||
!hw_enforced->Deserialize(buf_ptr, end) || //
!sw_enforced->Deserialize(buf_ptr, end)) {
LOG_D("Failed to deserialize unversioned blob (may be a HW-backed key)", 0);
return KM_ERROR_INVALID_KEY_BLOB;
}
if (!nonce->advance_write(OCB_NONCE_LENGTH) || !tag->advance_write(OCB_TAG_LENGTH))
return KM_ERROR_UNKNOWN_ERROR;
return KM_ERROR_OK;
}
keymaster_error_t DeserializeAuthEncryptedBlob(const KeymasterKeyBlob& key_blob,
KeymasterKeyBlob* encrypted_key_material,
AuthorizationSet* hw_enforced,
AuthorizationSet* sw_enforced, Buffer* nonce,
Buffer* tag) {
if (!key_blob.key_material || key_blob.key_material_size == 0)
return KM_ERROR_INVALID_KEY_BLOB;
const uint8_t* tmp = key_blob.key_material;
const uint8_t** buf_ptr = &tmp;
const uint8_t* end = tmp + key_blob.key_material_size;
if (end <= *buf_ptr)
return KM_ERROR_INVALID_KEY_BLOB;
uint8_t version = *(*buf_ptr)++;
if (version != CURRENT_BLOB_VERSION || //
!nonce->Deserialize(buf_ptr, end) || nonce->available_read() != OCB_NONCE_LENGTH ||
!encrypted_key_material->Deserialize(buf_ptr, end) || //
!tag->Deserialize(buf_ptr, end) || tag->available_read() != OCB_TAG_LENGTH ||
!hw_enforced->Deserialize(buf_ptr, end) || //
!sw_enforced->Deserialize(buf_ptr, end)) {
// This blob failed to parse. Either it's corrupted or it's a blob generated by an earlier
// version of keymaster using a previous blob format which did not include the version byte
// or the nonce or tag length fields. So we try to parse it as that previous version.
//
// Note that it's not really a problem if we erronously parse a corrupted blob, because
// decryption will fail the authentication check.
//
// A bigger potential problem is: What if a valid unversioned blob appears to parse
// correctly as a versioned blob? It would then be rejected during decryption, causing a
// valid key to become unusable. If this is a disk encryption key, upgrading to a keymaster
// version with the new format would destroy the user's data.
//
// What is the probability that an unversioned key could be successfully parsed as a version
// 0 key? The first 12 bytes of an unversioned key are the nonce, which, in the only
// keymaster version released with unversioned keys, is chosen randomly. In order for an
// unversioned key to parse as a version 0 key, the following must be true about the first
// five of those random bytes:
//
// 1. The first byte must be zero. This will happen with probability 1/2^8.
//
// 2. The second through fifth bytes must contain an unsigned integer value equal to
// NONCE_LENGTH. This will happen with probability 1/2^32.
//
// Based on those two checks alone, the probability of interpreting an unversioned blob as a
// version 0 blob is 1/2^40. That's small enough to be negligible, but there are additional
// checks which lower it further.
LOG_D("Failed to deserialize versioned key blob. Assuming unversioned.", 0);
return DeserializeUnversionedBlob(key_blob, encrypted_key_material, hw_enforced,
sw_enforced, nonce, tag);
}
return KM_ERROR_OK;
}
} // namespace keymaster