Distributed under the GNU GPL. See Copying section for details.
Copyright © 2004 Mario Juric
libencio is a library providing stdio-like interface for reading
and writing of encrypted files in MCrypt format. Additionally, through
creation of an "index", libencio provices full support for
fseek()-like random read access of encrypted data. This allows one to
operate on MCrypt-encrypted files as if they were ordinary, cleartext
files.
The main motivation for this library was to create code for easy random access of encrypted data for a project I'm working on. This is a preliminary, proof-of-concept release, able to seek/encrypt/decrypt to MCrypt format only and with symmetric encryption algorithms only. The final goal is to implement reading, writing and seeking of data in OpenPGP format, with support for public key algorithms. As it's an open question how much time will I have to continue working on it, I'm putting it out in case someone finds it useful. I will, however, accept patches and bugfixes.
A library like this one can be used to provide MUAs (mail readers) with a
layer to transparently handle encrypted attachments, or, more interestingly,
as a backend to software such as ffmpeg or mplayer to directly
play encrypted files without making temporary, decrypted copies. Another use would
be to combine it with tar archives for encrypted backups, like duplicity does.
A KDE IO-Slave can also be envisioned.
libencio uses libmcrypt and libmhash libraries for
encryption and hashing algorithms, respectively. I've chosen these two
mostly for their simple API. In the future, a transition to libgcrypt
is likely, due to its wider userbase.
libmcrypt
libmhash
This code has been poorly tested. Would I (the author) encrypt my most confidential files with it? No. I think it needs a more thorough checks before that, especially the random number generation routines. You have been warned.
However, it's great to use for reading files encrypted with other code!
libgcrypt
libencio conceptually follows stdio in the naming and function
of its function calls. This short example program demonstrates how to write
an encrypted file with default encryption settings (rijndael-128/cfb
mode/sha1 checksum).
#include <stdio.h>
#include "encio.h"
int main(int argc, char **argv)
{
ENCFILE *ef;
char buf[1000];
char *passphrase = "acomplicatedpassphrase";
/* open an file encrypted with passphrase */
ef = enc_fopen("test.txt.nc", "wb", passphrase);
while(!feof(stdio))
{
int nread = fread(stdio, 1, buf, 1000)
enc_fwrite(ef, 1, buf, nread);
}
enc_fclose(ef);
}
This short example program encrypts anything given to it from stdio into
a file named test.txt.nc. All enc_* calls function are
equivalent to their stdio counterparts, except they transparently
encrypt the data written to the output file. There is an additional third
parameter to enc_fopen(), compared to fopen(), through which
you supply the passphrase to use to encrypt the data.
Note that enc_fseek() is not available when a file is opened in write
mode – i.e., you cannot write 10 kb, then seek back to the beginning and
rewrite 1 kb. This is due to the properties of the encryption mode, where
each subsequent byte written depends on all of the preceeding bytes -
changing them would require reencryption of the whole file.
This short sample program should give you an idea what is and how to use libencio.
#include <stdio.h>
#include "encio.h"
int main(int argc, char **argv)
{
ENCFILE *ef;
char buf[100];
char *passphrase = "acomplicatedpassphrase";
/* open an file encrypted with passphrase */
ef = enc_fopen("test.txt.nc", "rb", passphrase);
enc_add_index(ef, "test.txt.ix", passphrase,
INDEX_LOAD | INDEX_CREATE | INDEX_SAVE);
/* read first 100 bytes of the encrypted data*/
enc_fread(ef, 1, buf, 100);
printf("First 100 bytes of the file: %s", ef);
/* read last 100 bytes of the encrypted data */
enc_fseek(ef, -100, SEEK_SET);
enc_fread(ef, 1, buf, 100);
printf("Last 100 bytes of the file: %s", ef);
enc_fclose(ef);
}
In the sample, we assume that we have a file named test.txt.nc which
has been symmetrically encrypted with passphrase "acomplicatedpassphrase".
enc_fopen() opens the file for reading, just as fopen() would
open a non-encrypted counterpart. The difference, compared to stdio
fopen() call, is a third parameter through which you supply the passphrase
for decryption. All other libencio functions in the example function
as their stdio counterparts.
The other difference from stdio is that a call to
enc_add_index() is required in order to support random seeking of the
file. This is needed because in most encryption modes considered secure (eg,
feedback modes - CFB), in order to decrypt n-th byte of a
file, one must decrypt all preceeding bytes as well. In the worst case
scenario – a seek to the last byte in the file – the complete file must be
decrypted. Such an implementation of seeking would be unacceptably
timeconsuming for large files.
libencio handles this by creating an "encryption state index"
(ENCINDEX) on the file - a snapshot of decryption engine state (usually, the
feedback register, FR), at predefined locations ("bookmarks") in the
file (usually, every blocksize bytes, where by default
blocksize = 1024). This allows libencio to quickly resume
decryption starting from the bookmark nearest to the requested seek
location. The worst case scenario in seeking with an index is a required
decryption of blocksize-1 bytes, which is usually acceptable.
Memory required for an index is proportional to
(fsize/blocksize)*FR_size where fsize is the encrypted file
size and FR_size is the size of the feedback register for the cipher mode.
For example, using 256 bit AES cipher requires 32 bytes per bookmark, which
for a 500MB file and one bookmark per kb (blocksize = 1024),
will generate a 16MB encryption state index. Increasing the blocksize
will reduce the ENCINDEX footprint, but will degrade the seek performance.
ENCINDEX can (and should, for frequently accessed files) be saved to speed
up construction on subsequent openings of the file. When and ENCINDEX is
saved, it is encrypted with the same passphrase as the file which it
indexes. The index is saved by adding an INDEX_SAVE flag as the
fourth parameter of the enc_add_index() call. It can be subsequently
loaded by specifying the INDEX_LOAD flag.
Default settings for encryption are to encrypt using rijndael-128 (AES)
algorithm, in cfb mode (with an IV), and attach a sha1 checksum to the
encrypted file to check for errors in transmission. There is a number of
global enc_* variables defined in seekcrypt_internal.h which
control this behavior. CAVEAT: THESE VARIABLES ARE INTERNAL TO THE
IMPLEMENTATION, AND WILL LIKELY CHANGE IN THE FUTURE, THUS BREAKING YOUR
CODE. So don't touch them unless you really need to change the encryption
details.
Any cipher from libmcrypt and any hash algorithm from libmhash can
be used. However, for now you can only use stream modes (such as cfb)
for encryption/decryption. Also, note the caveat from the previous
paragraph.
The files produced by encio will be readable by MCrypt, version
2.6. Similarly, encio can read files produced by MCrypt, version 2.6,
if they're in stream mode (eg., cfb).
ENCFILEtypedef struct ENCFILE_t ENCFILE;
ENCFILE is an opaque handle to an encrypted file, equivalent to
stdio's FILE.
enc_fopen()ENCFILE *enc_fopen(const char *filename, const char *rwmode, const char *passphrase);
enc_fopen() opens an encrypted file filename, encrypted with
passphrase, for reading or writing depending on rwmode. Other
than the passphrase parameter, it is equivalent to fopen().
enc_add_indexint enc_add_index(ENCFILE *ef, const char *indexfile, const char *passphrase, int index_mode);
Adds an "encryption state index" (ENCINDEX) to a file ef which has
been opened for reading. After the successfull completion of this call, you
can randomly access any location in the file using enc_fseek().
Parameter index_mode determines the source of the encryption state
index. If it contains the INDEX_LOAD flag, an attempt will be made to
load it from file indexfile, which is assumed to be encrypted with
passphrase. If successfull, the enc_add_index will return 0.
If index_mode has the INDEX_CREATE bit set, index will
be constructed by internally decrypting the whole file ef. Finally, if
index_mode has the INDEX_SAVE bit set, the index will
be encrypted with passphrase and saved to indexfile file.
INDEX_* flags can be OR-ed, in which case first the
INDEX_LOAD flag will be tested, then INDEX_CREATE and finally
INDEX_SAVE. A typical use would be:
enc_add_index(ef, "test.txt.ix", passphrase, INDEX_LOAD | INDEX_CREATE | INDEX_SAVE);
where encio will first attempt to load the index from
test.txt.ix and if that fails it will create the index, and store it
to test.txt.ix.
stdio-like functionsint enc_fread(void *out, int size, int count, ENCFILE *ef);
int enc_fwrite(void *data, int size, int count, ENCFILE *ef);
int enc_fseek(ENCFILE *ef, long at, int dir);
int enc_ftell(ENCFILE *ef);
void enc_fflush(ENCFILE *ef);
int enc_feof(ENCFILE *ef);
int enc_fclose(ENCFILE *ef);
void enc_info(ENCFILE *ef);
All these functions function exactly like their stdio counterparts.
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