How To Compile And Use SEE
This file describes the SQLite Encryption Extension (SEE) for SQLite. The SEE allows SQLite to read and write encrypted database files. All database content, including the metadata, is encrypted so that to an outside observer the database appears to contain white noise.
A version of SQLite that includes SEE is also able to read and write normal database files created with a public domain version of SQLite. But the public version of SQLite will not be able to read or write an encrypted database file. Indeed, no version of any known software will be able to access an encrypted database file without knowing the encryption key.
The SEE is actually a set of extension employing various encryption algorithms. The following encryption algorithms are currently supported:
- RC4 with security enhancements
- AES-128 in OFB mode
- AES-128 in CCM mode
- AES-256 in OFB mode
There are five different variations on the SEE: one for each of the supported algorithms and a fifth variant that simultaneously supports all algorithms except AES-128 CCM. The fifth variant is recommended for new projects.
The core SQLite library is in the public domain. However, the extensions needed to read and write an encrypted database file are licensed software. You should only be able to see this software if you have a license. If you do not have a valid license you should delete the source code in this folder at once.
Your license is perpetual. You have paid a one-time fee that allows you to use and modify the software forever. You can ship as many copied of the software to your customers as you want so long as you insure that only compiled binaries are shipped (you cannot distribute source code) and that your customers cannot make additional copies of the software to use for other purposes.
You can create multiple products that use this software as long as all products are developed and maintained by the same team. For the purposes of this paragraph, a "team" is a work unit where everybody knows each others names. If you are in a large company where this product is used by multiple teams, then each team should acquire their own separate license.
How To Compile
The encryption extension is normally shipped as a ZIP archive that contains all source code to both the encryption extension and the public domain version of SQLite. The following files are provided:
This file contains the complete source code to the SEE variant that uses RC4.
This file contains the complete source code to the SEE variant that uses AES-128 in OFB mode.
This file contains the complete source code to the SEE variant that uses AES-128 in CCM mode. CCM mode includes a message authentication code which provides authentication in addition to confidentiality.
This file contains the complete source code to the SEE variant that uses AES-256 in OFB mode.
This file contains the complete source code to the SEE variant that uses any of the RC4, AES128-OFB, or AES258-OFB algorithms based on a prefix of the key. This variation of the encryption extension is recommended for all new projects.
This file contains the complete source code to the public-domain version of SQLite. All of the individual C-code files have been concatenated into this one convenient package. On the SQLite website, this file is called the "amalgamation".
This file contains the interface definitions for SQLite. Other programs that link against SQLite will need this file, and you will need this file in order to compile the CLI, but you do not need this file to compile SQLite itself.
This file contains source code for the "CLI", the Command Line Interface program named "sqlite3.exe" that you can use to access and control SQLite database files. This file is different from the "shell.c" file that comes with the public-domain version of SQLite. This shell.c has been enhanced to make use of the encryption extension.
To compile SEE into a static library, first append one of the see*.c files to the end of the sqlite3.c file, then compile the concatenation as a single source file. When compiling the library, you will need to add command-line options to your compiler to set the SQLITE_HAS_CODEC #define. On unix systems, the command sequence would be like this:
cat sqlite3.c see.c >see-sqlite3.c cc -c -DSQLITE_HAS_CODEC=1 see-sqlite3.c ar a see-sqlite3.a see-sqlite3.o
We strongly encourage you to statically link SQLite against your application. However, if you must use SQLite as a separate DLL or shared library, you can do so by adding the following compile-time option:
To compile the CLI, just hand the shell.c source file to your C compiler together with either the static library prepared above, or the original source code files. Here are some examples:
cc -o sqlite3 -DSQLITE_HAS_CODEC=1 shell.c see-sqlite3.c
For an added performance boost when building the CLI, consider adding the -DSQLITE_THREADSAFE=0 option. The CLI is single threaded and SQLite runs faster if it doesn't have to use its mutexes.
The CLI (also known as see.exe or ccm.exe) program takes an additional argument "-key" followed by text which is the encryption key to the database. Or for a binary key, you can use the -hexkey option and enter the key value as hexadecimal. Examples:
see -key secret database.db see -hexkey 73c563726574 database.db
If the key is omitted or is an empty string no encryption is performed. The dot-command ".rekey" can be used to change the encryption key:
.rekey OLD NEW NEW
The first argument to .rekey is the previous encryption key. Use an empty string if the database is not previously encrypted. The 2nd and 3rd arguments are the new encryption key. You have to enter the new key twice to check for typos - the rekey will not occur if both instances of the new key is not identical. To encrypt a previously unencrypted database, do this:
.rekey "" new-key new-key
Do decrypt a database do this:
.rekey old-key "" ""
The .rekey command only works with text keys. To rekey a database that contains a binary key use the ".hex-rekey" command instead. The .hex-rekey command works just like .rekey except that all keys are enters as hexadecimal instead of text.
5.0 C Interface
If you deploy the SQLite encryption extension as a DLL or shared library then you must first activate the library by invoking:
The argument is your product activation key. The activation key is available as plain-text in the source code so you can clearly see what it is. The purpose of the activation key is to prevent one of your customers from extracting the SQLite library and using it separately from your application. Without knowledge of the activation key, which only you should know, your users will be unable to access the encryption features.
Use the sqlite3_open() API to open an encrypted database or any database that you want to rekey. Immediately after opening, specify the key using sqlite3_key():
int sqlite3_key( sqlite3 *db, /* The connection from sqlite3_open() */ const void *pKey, /* The key */ int nKey /* Number of bytes in the key */ );
If the pKey argument is NULL or nKey is less than 1, then the database is assumed to be unencrypted. There is no upper bound on the size of the key, though only the first 256 bytes (RC4) or 16 bytes (AES) will be used.
If you specify an incorrect key, you will not get an error message right away. But the first time you try to access the database you will get an SQLITE_NOTADB error with a message of "file is encrypted or is not a database".
You can change the key on a database using the sqlite3_rekey() routine:
int sqlite3_rekey( sqlite *db, /* Database to be rekeyed */ const void *pKey, int nKey /* The new key */ );
A NULL key decrypts the database.
The sqlite3_rekey() routine must be called when no transactions are pending. If a transaction is open when sqlite3_rekey() is called, it will return SQLITE_BUSY. sqlite3_rekey() creates its own transaction so if the rekeying process is interrupted, the database will rollback to its old key the next time it is opened. A consequence of this is that a rekeying operation will produce a rollback journal file that is slightly larger than the original database.
Rekeying requires that every page of the database file be read, decrypted, reencrypted with the new key, then written out again. Consequently, rekeying can take a long time on a larger database.
Most SEE variants allow you to encrypt an existing database that was created using the public domain version of SQLite. This is not possible when using the authenticating version of the encryption extension in see-aes128-ccm.c. If you do encrypt a database that was created with the public domain version of SQLite, no nonce will be used and the file will be vulnerable to a chosen-plaintext attach. If you call sqlite3_key() immediately after sqlite3_open() when you are first creating the database, space will be reserved in the database for a nonce and the encryption will be much stronger. If you do not want to encrypt right away, call sqlite3_key() anyway, with a NULL key, and the space for the nonce will be reserved in the database even though no encryption is done initially.
A public domain version of the SQLite library can read and write an encrypted database with a NULL key. You only need the encryption extension if the key is non-NULL.
Using the "key" PRAGMA
As an alternative to calling sqlite3_key() to set the decryption key for a database, you can invoke a pragma:
You must invoke this pragma before trying to do any other interaction with the database. The key pragma only works with string keys. If you use a binary key, use the hexkey pragma instead:
Use the rekey or hexrekey pragmas to change the key. So, for example, to change the key to 'demo2' use one of:
PRAGMA rekey='demo2'; PRAGMA rehexkey='64656d6f32';
Through the use of these pragmas, it is never necessary to directly invoke the sqlite3_key() or sqlite3_rekey() interfaces. This means that SEE can be used with language wrappers that do not know about those interfaces.
Note that the hexkey, rekey and rehexkey pragmas only work with SQLite version 3.6.8 and later.
Using The ATTACH Command
The key for an attached database is specified using the KEY clause at the end of the ATTACH statement. Like this:
ATTACH DATABASE 'file2.db' AS two KEY 'xyzzy';
If the KEY clause is omitted, the same key is used that is currently in use by the main database. If the attached database is not encrypted, specify an empty string as the key. The argument to the KEY keyword can be BLOB constant. For example:
ATTACH DATABASE 'file2.db' AS two KEY X'78797a7a79';
The amount of key material actually used by the encryption extension depends on which variant of SEE you are using. With see-rc4.c, the first 256 byte of key are used. With the see-aes128-ofb and and see-aes128-ccm variants, the first 16 bytes of the key are used. With see-aes256-ofb, the first 32 bytes of key are used.
If you specify a key that is shorter than the maximum key length, then the key material is repeated as many times as necessary to complete the key. If you specify a key that is larger than the maximum key length, then the excess key material is silently ignored.
For the see.c SEE variant, the key must begin with an ASCII prefix to specify which algorithm to use. The prefix must be one of "rc4:", "aes128:", or "aes256:". The prefix is not used as part of the key sent into the encryption algorithm. So the real key should begin on the first byte after the prefix.
Note that when using the see.c variant, you can change encryption algorithms using the sqlite3_rekey() interface or the .rekey command-line. For example, to convert a legacy RC4-encrypted database to use AES-256, one type:
.rekey rc4:mykey aes256:mykey aes256:mykey
The Importance of a Nonce
The encryption is much more secure if it has a random nonce value on each page of the database. Without a nonce, the encryption can be broken using a chosen-plaintext attack. Purists will argue that (rightly) the encryption is weak without a nonce.
The number of bytes of nonce on each page of the database is determined by byte 20 of the database file. This value is set to zero by default in databases created by the public-domain version of SQLite. And once a database has been created, this value cannot be changed. The nonce size can only be set when the database is first being created.
If you call sqlite3_key() immediately after sqlite3_open() when creating a new database, the value of byte20 in the new database will be set to 4 (RC4) or 12 (AES-128 and AES-256) or 32 (AES-128-CCM). (Technically, only 16 bytes of 32 bytes specified for AES-128-CCM is really nonce. The rest is the message authentication code or MAC.)
Because it is so important ot have a nonce for good encryption, it is highly recommended that you always call sqlite3_key() after sqlite3_open() even if the key is originally empty (indicating no encryption.)
Because of the extra space taken to store the nonce on each page of the database, your database file might be slightly larger when encryption is enabled. This size increase (if any) will be very small. But if this is still a problem for you and if your use of encryption is to more to prevent idle snooping that to block a determined hack attempt, then you can create one or more tables in the database first, then invoke sqlite3_rekey() to change the encryption key from NULL to whatever you want. No nonce will be used in that case. The encryption will be weaker, but no extra space will be used in the database file to store the nonce.
The option to omit the nonce, described in the previous paragraph, is not available for AES-128-CCM. The AES-128-CCM variant requires its 32-bit nonce and will not work without it.
Bytes 16 through 23 of the database are unencrypted. Thus, you can always check to see how much nonce is being used, even on an encrypted database file, just by looking at byte 20. It is recommended that any product that uses encryption check this byte to make sure it is being set to 4 or 12 or 32 and not 0.
- TEMP tables are not encrypted.
- In-memory (":memory:") databases are not encrypted.
- Bytes 16 through 23 of the database file contain header information which is not encrypted.
How SEE Works
Each page is encrypted separately. The key to encryption is a combination of the page number, the random nonce (if any) and the database key. The data is encrypted in both the main database and in the rollback journal but is unencrypted when held in memory. This means that if an adversary is able to view the memory used by your program, she will be able to see unencrypted data.
The nonce value is changed by a rollback.
The see-aes128-ccm.c variant uses AES in CCM mode with a 16-byte randomly choosen nonce on each page and and 16-byte message authentication code (MAC). Thus with crypto3ccm.c, 32 bytes of every database pages are taken up by encryption and authentication overhead. Consequently, database files created using crypto3ccm.c may be a little larger. Also, because the MAC is computed whenever a page is modified, and verified when a page is read, crypto3ccm.c will often be a little slower. Such is the cost of authentication.