137 lines
6.5 KiB
C++
137 lines
6.5 KiB
C++
/*
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* Copyright 2015 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "auth_encrypted_key_blob.h"
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#include <keymaster/android_keymaster_utils.h>
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#include <keymaster/authorization_set.h>
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#include <keymaster/logger.h>
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#include "ocb_utils.h"
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namespace keymaster {
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const uint32_t CURRENT_BLOB_VERSION = 0;
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keymaster_error_t SerializeAuthEncryptedBlob(const KeymasterKeyBlob& encrypted_key_material,
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const AuthorizationSet& hw_enforced,
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const AuthorizationSet& sw_enforced,
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const Buffer& nonce, const Buffer& tag,
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KeymasterKeyBlob* key_blob) {
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size_t size = 1 /* version byte */ + nonce.SerializedSize() +
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encrypted_key_material.SerializedSize() + tag.SerializedSize() +
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hw_enforced.SerializedSize() + sw_enforced.SerializedSize();
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if (!key_blob->Reset(size))
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return KM_ERROR_MEMORY_ALLOCATION_FAILED;
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uint8_t* buf = key_blob->writable_data();
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const uint8_t* end = key_blob->key_material + key_blob->key_material_size;
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*buf++ = CURRENT_BLOB_VERSION;
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buf = nonce.Serialize(buf, end);
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buf = encrypted_key_material.Serialize(buf, end);
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buf = tag.Serialize(buf, end);
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buf = hw_enforced.Serialize(buf, end);
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buf = sw_enforced.Serialize(buf, end);
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if (buf != key_blob->key_material + key_blob->key_material_size)
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return KM_ERROR_UNKNOWN_ERROR;
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return KM_ERROR_OK;
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}
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static keymaster_error_t DeserializeUnversionedBlob(const KeymasterKeyBlob& key_blob,
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KeymasterKeyBlob* encrypted_key_material,
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AuthorizationSet* hw_enforced,
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AuthorizationSet* sw_enforced, Buffer* nonce,
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Buffer* tag) {
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const uint8_t* tmp = key_blob.key_material;
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const uint8_t** buf_ptr = &tmp;
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const uint8_t* end = tmp + key_blob.key_material_size;
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if (!nonce->reserve(OCB_NONCE_LENGTH) || !tag->reserve(OCB_TAG_LENGTH))
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return KM_ERROR_MEMORY_ALLOCATION_FAILED;
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if (!copy_from_buf(buf_ptr, end, nonce->peek_write(), OCB_NONCE_LENGTH) ||
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!encrypted_key_material->Deserialize(buf_ptr, end) ||
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!copy_from_buf(buf_ptr, end, tag->peek_write(), OCB_TAG_LENGTH) ||
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!hw_enforced->Deserialize(buf_ptr, end) || //
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!sw_enforced->Deserialize(buf_ptr, end)) {
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LOG_D("Failed to deserialize unversioned blob (may be a HW-backed key)", 0);
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return KM_ERROR_INVALID_KEY_BLOB;
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}
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if (!nonce->advance_write(OCB_NONCE_LENGTH) || !tag->advance_write(OCB_TAG_LENGTH))
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return KM_ERROR_UNKNOWN_ERROR;
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return KM_ERROR_OK;
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}
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keymaster_error_t DeserializeAuthEncryptedBlob(const KeymasterKeyBlob& key_blob,
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KeymasterKeyBlob* encrypted_key_material,
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AuthorizationSet* hw_enforced,
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AuthorizationSet* sw_enforced, Buffer* nonce,
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Buffer* tag) {
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if (!key_blob.key_material || key_blob.key_material_size == 0)
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return KM_ERROR_INVALID_KEY_BLOB;
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const uint8_t* tmp = key_blob.key_material;
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const uint8_t** buf_ptr = &tmp;
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const uint8_t* end = tmp + key_blob.key_material_size;
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if (end <= *buf_ptr)
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return KM_ERROR_INVALID_KEY_BLOB;
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uint8_t version = *(*buf_ptr)++;
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if (version != CURRENT_BLOB_VERSION || //
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!nonce->Deserialize(buf_ptr, end) || nonce->available_read() != OCB_NONCE_LENGTH ||
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!encrypted_key_material->Deserialize(buf_ptr, end) || //
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!tag->Deserialize(buf_ptr, end) || tag->available_read() != OCB_TAG_LENGTH ||
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!hw_enforced->Deserialize(buf_ptr, end) || //
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!sw_enforced->Deserialize(buf_ptr, end)) {
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// This blob failed to parse. Either it's corrupted or it's a blob generated by an earlier
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// version of keymaster using a previous blob format which did not include the version byte
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// or the nonce or tag length fields. So we try to parse it as that previous version.
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//
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// Note that it's not really a problem if we erronously parse a corrupted blob, because
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// decryption will fail the authentication check.
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//
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// A bigger potential problem is: What if a valid unversioned blob appears to parse
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// correctly as a versioned blob? It would then be rejected during decryption, causing a
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// valid key to become unusable. If this is a disk encryption key, upgrading to a keymaster
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// version with the new format would destroy the user's data.
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//
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// What is the probability that an unversioned key could be successfully parsed as a version
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// 0 key? The first 12 bytes of an unversioned key are the nonce, which, in the only
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// keymaster version released with unversioned keys, is chosen randomly. In order for an
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// unversioned key to parse as a version 0 key, the following must be true about the first
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// five of those random bytes:
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//
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// 1. The first byte must be zero. This will happen with probability 1/2^8.
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//
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// 2. The second through fifth bytes must contain an unsigned integer value equal to
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// NONCE_LENGTH. This will happen with probability 1/2^32.
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//
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// Based on those two checks alone, the probability of interpreting an unversioned blob as a
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// version 0 blob is 1/2^40. That's small enough to be negligible, but there are additional
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// checks which lower it further.
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LOG_D("Failed to deserialize versioned key blob. Assuming unversioned.", 0);
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return DeserializeUnversionedBlob(key_blob, encrypted_key_material, hw_enforced,
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sw_enforced, nonce, tag);
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}
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return KM_ERROR_OK;
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}
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} // namespace keymaster
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