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Virtual File System

In this section only those aspects of SQLite’s Virtual File System will be discussed which are relevant for the implementation of the new encryption extension. A full description of SQLite’s Virtual File System can be found here.

SQLite’s Virtual File System is used to access various types of database files:

  • Main database files (Open flag SQLITE_OPEN_MAIN_DB)
  • Temporary database files (Open flag SQLITE_OPEN_TEMP_DB)
  • Transient database files (Open flag SQLITE_OPEN_TRANSIENT_DB)
  • Main rollback journal files (Open flag ) SQLITE_OPEN_MAIN_JOURNAL)
  • Temporary rollback journal files (Open flag SQLITE_OPEN_TEMP_JOURNAL)
  • Subjournal files (Open flag SQLITE_OPEN_SUBJOURNAL)
  • Master journal files (Open flag SQLITE_OPEN_MASTER_JOURNAL)
  • WAL journal files (Open flag SQLITE_OPEN_WAL)

In accordance with the previous implementation of the encryption extension only main database files and journal files (rollback as well as WAL) will be handled (marked in bold in the above list). Maybe support for handling encryption for temporary files will be added in the future, if there is a demand. However, based on SQLite’s Virtual File System encryption can be supported only for ordinary file based databases, not for memory based databases.

Only the content of database pages will be encrypted. Just as in the previous implementation data structures used for housekeeping of journals will not be encrypted. Details of SQLite’s file formats can be found here.

An important information item for the encryption is the page number of a database page. Unfortunately the page number is not passed into the VFS methods, but has to be deduced from the given information, offset and size of the file chunk to be read or written.

Main database files

A main database file consists of database pages of fixed size. Therefore the page number can be deduced from the offset of the file chunk.

On writing only full database pages are written. On reading there are a few special cases where a database page is read only partially. This affects only the first database page, which contains the database header.

Rollback journal files

Rollback journal files mainly consist of a file header and page records. A page record consists of the page number, the page content, and a checksum. Page number, page content, and checksum are read and written separately in the given order. Therefore the page number can be deduced from the read or write operation preceeding the read or write operation for the page content.

WAL journal files

WAL journal files consist of a file header and WAL frames. Each frame consists of a frame header and the page content. The frame header contains the page number and various checksums.

Unfortunately, the frame header and the page content are not read or written in a particular order. Therefore on reading or writing the content of a page an additional read operation is required to read the page number from the frame header.

Checksums in journal files

For rollback journals as well as for WAL journals SQLite uses checksums to verify the integrity of a journal file. SQLite3 Multiple Ciphers handles the various journal types differently.

Rollback journal

In the previous implementation SQLite calculated checksums after the page content was encrypted. In the new implementation SQLite uses the unencrypted page content to calculate checksums. In the transition from the previous to the new implementation this could possibly cause problems, if journal files written by the previous implementation have to be processed by the new implementation. Therefore it is strongly recommended to do the transition for intact databases without active journal files.

WAL journal

Up to version 1.2.5 SQLite3 Multiple Ciphers encrypted the WAL journal frames within the VFS implementation. As a consequence WAL journal files were not compatible with the previous implementation. Since active journal files are a common case for WAL journal mode, this could cause problems not only in the transition phase, but also when concurrent access from legacy applications (for example, System.Data.SQLite or SQLCipher) has to be supported.

Beginning with version 1.3.0 the encryption implementation for WAL journals was modified to be compatible with legacy applications. That is, checksums are now calculated based on the encrypted page content.

Unfortunately the new implementation is not compatible with that used up to version 1.2.5. To be able to access WAL journals created by prior versions, the configuration parameter mc_legacy_wal was introduced. If the parameter is set to 1, then the prior WAL journal encryption mode is used. The default of this parameter can be set at compile time by setting the symbol SQLITE3MC_LEGACY_WAL accordingly, but the actual value can also be set at runtime using the pragma or the URI parameter mc_legacy_wal.

In principle, operating generally in WAL legacy mode is possible, but it is strongly recommended to use the WAL legacy mode only to recover WAL journals left behind by prior versions without data loss.

Copyright © 2020-2022 Ulrich Telle. Distributed under an MIT license.