BackupEncryption/proto/backup_chunks_metadata.proto

/*
 * Copyright (C) 2018 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
 */

syntax = "proto2";

package android_backup_crypto;

option java_package = "com.android.server.backup.encryption.protos";
option java_outer_classname = "ChunksMetadataProto";

// Cipher type with which the chunks are encrypted. For now we only support AES/GCM/NoPadding, but
// this is for backwards-compatibility in case we need to change the default Cipher in the future.
enum CipherType {
    UNKNOWN_CIPHER_TYPE = 0;
    // Chunk is prefixed with a 12-byte nonce. The tag length is 16 bytes.
    AES_256_GCM = 1;
}

// Checksum type with which the plaintext is verified.
enum ChecksumType {
    UNKNOWN_CHECKSUM_TYPE = 0;
    SHA_256 = 1;
}

enum ChunkOrderingType {
    CHUNK_ORDERING_TYPE_UNSPECIFIED = 0;
    // The chunk ordering contains a list of the start position of each chunk in the encrypted file,
    // ordered as in the plaintext file. This allows us to recreate the original plaintext file
    // during decryption. We use this mode for full backups where the order of the data in the file
    // is important.
    EXPLICIT_STARTS = 1;
    // The chunk ordering does not contain any start positions, and instead each encrypted chunk in
    // the backup file is prefixed with its length. This allows us to decrypt each chunk but does
    // not give any information about the order. However, we use this mode for key value backups
    // where the order does not matter.
    INLINE_LENGTHS = 2;
}

// Chunk entry (for local state)
message Chunk {
    // SHA-256 MAC of the plaintext of the chunk
    optional bytes hash = 1;
    // Number of bytes in encrypted chunk
    optional int32 length = 2;
}

// List of the chunks in the blob, along with the length of each chunk. From this is it possible to
// extract individual chunks. (i.e., start position is equal to the sum of the lengths of all
// preceding chunks.)
//
// This is local state stored on the device. It is never sent to the backup server. See
// ChunkOrdering for how the device restores the chunks in the correct order.
// Next tag : 6
message ChunkListing {
    repeated Chunk chunks = 1;

    // Cipher algorithm with which the chunks are encrypted.
    optional CipherType cipher_type = 2;

    // Defines the type of chunk order used to encode the backup file on the server, so that we can
    // consistently use the same type between backups. If unspecified this backup file was created
    // before INLINE_LENGTHS was supported, thus assume it is EXPLICIT_STARTS.
    optional ChunkOrderingType chunk_ordering_type = 5;

    // The document ID returned from Scotty server after uploading the blob associated with this
    // listing. This needs to be sent when uploading new diff scripts.
    optional string document_id = 3;

    // Fingerprint mixer salt used for content defined chunking. This is randomly generated for each
    // package during the initial non-incremental backup and reused for incremental backups.
    optional bytes fingerprint_mixer_salt = 4;
}

// Ordering information about plaintext and checksum. This is used on restore to reconstruct the
// blob in its correct order. (The chunk order is randomized so as to give the server less
// information about which parts of the backup are changing over time.) This proto is encrypted
// before being uploaded to the server, with a key unknown to the server.
message ChunkOrdering {
    // For backups where ChunksMetadata#chunk_ordering_type = EXPLICIT STARTS:
    // Ordered start positions of chunks. i.e., the file is the chunk starting at this position,
    // followed by the chunk starting at this position, followed by ... etc. You can compute the
    // lengths of the chunks by sorting this list then looking at the start position of the next
    // chunk after the chunk you care about. This is guaranteed to work as all chunks are
    // represented in this list.
    //
    // For backups where ChunksMetadata#chunk_ordering_type = INLINE_LENGTHS:
    // This field is unused. See ChunkOrderingType#INLINE_LENGTHS.
    repeated int32 starts = 1 [packed = true];

    // Checksum of plaintext content. (i.e., in correct order.)
    //
    // Each chunk also has a MAC, as generated by GCM, so this is NOT Mac-then-Encrypt, which has
    // security implications. This is an additional checksum to verify that once the chunks have
    // been reordered, that the file matches the expected plaintext. This prevents the device
    // restoring garbage data in case of a mismatch between the ChunkOrdering and the backup blob.
    optional bytes checksum = 2;
}

// Additional metadata about a backup blob that needs to be synced to the server. This is used on
// restore to reconstruct the blob in its correct order. (The chunk order is randomized so as to
// give the server less information about which parts of the backup are changing over time.) This
// data structure is only ever uploaded to the server encrypted with a key unknown to the server.
// Next tag : 6
message ChunksMetadata {
    // Cipher algorithm with which the chunk listing and chunks are encrypted.
    optional CipherType cipher_type = 1;

    // Defines the type of chunk order this metadata contains. If unspecified this backup file was
    // created before INLINE_LENGTHS was supported, thus assume it is EXPLICIT_STARTS.
    optional ChunkOrderingType chunk_ordering_type = 5
    [default = CHUNK_ORDERING_TYPE_UNSPECIFIED];

    // Encrypted bytes of ChunkOrdering
    optional bytes chunk_ordering = 2;

    // The type of algorithm used for the checksum of the plaintext. (See ChunkOrdering.) This is
    // for forwards compatibility in case we change the algorithm in the future. For now, always
    // SHA-256.
    optional ChecksumType checksum_type = 3;

    // This used to be the plaintext tertiary key. No longer used.
    reserved 4;
}