This document is a reference guide to the Advanced Forensics Format library. It is intended to be read by software developers seeking to use the library in their own programs. It is not intended for end users.

End users will be better off reading the "AFF Users Guide" that comes with the library. This guide gives an introduction to the AFF format and explains the tools included with the library. The default imager, aimage, is covered in this guide.

This version of documentation reflections version 2.3.0 of the library.

Introduction

Using the AFF is (hopefully) a rather transparent process. The library can handle plain files, such as those generated by dd, AFF files, and EnCase images. It is not necessary for your program to know which kind of file it is processing, the data is extracted and presented to the program transparently. All you need to do is change the basic file operations to their AFF equivalents. These simple substitutions allow you to use AFF or Encase files just like dd images.

Compiling AFF programs

The library currently supports the following compilers:

gcc
g++
MinGW
Microsoft Visual C++

When compiling an AFF program you must include the AFF header:

#include <afflib.h>

During linking you must include the AFF library itself and the SSL library. Additionally, if you're using a C compiler, you will need the C++ standard library. Some examples using several compilers are shown below:

Using g++

$ g++ file.c -lafflib -lssl

Using gcc

$ gcc file.c -lafflib -lssl -lstdc++

Using MinGW

C:\> gcc file.c -lafflib -lssl -lstdc++

Using Microsoft Visual C++

c:\> LINK file.obj afflib.lib

Testing for the AFF using Automake

If you're using the GNU Automake and other tools to check for the AFF, we recommend the following tests to determine if the AFF is present on the system:

###########################################################
## Test for AFF Support
###########################################################
AC_ARG_ENABLE([aff],
   AC_HELP_STRING([--enable-aff],[Use AFF for reading files (default yes)]),
                  [enable_aff=$enableval], [enable_aff=yes])
if test "${enable_aff}" = "yes" ; then
   AC_CHECK_HEADERS([afflib.h],,[enable_aff=no])
   AC_CHECK_LIB([aff],[af_open],,[enable_aff=no])
fi
if test "${enable_aff}" = "yes" ; then
   AC_DEFINE([USE_AFF],1,[Use the AFF library to read files])

   AC_MSG_RESULT([Found Advanced Forensics Format (AFF), good!])
   AC_MSG_RESULT([Checking for AFF specific features...])

   AC_CHECK_LIB([aff],[af_open_stdin],
     AC_DEFINE([USE_AFF_STDIN],1,[The AFF can be used to open standard input]))
fi

These tests will define two variables in the config.h file for you. First, the value USE_AFF is defined the AFF is found on the development system. Second, the value USE_AFF_STDIN is defined if the function af_open_stdin is supported. That function was not present in early versions of the library.

Your First AFF program

The first program we will examine the command line arguments to determine if they are valid AFF files. If so, it will attempt to open them and display the size of their uncompressed data. The complete source code is below.

#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#include <fcntl.h>
#include <afflib.h>

#define FALSE 0
#define TRUE  1

int process_file(char *fn)
{
  AFFILE *af = af_open(fn,O_RDONLY,0);
  if (NULL == af)
  {
    perror(fn);
    return TRUE;
  }

  printf ("The uncompressed size of this file is %"PRId64" bytes\n",
          af_get_imagesize(af));

  af_close(af);
  return FALSE;
}


int main(int argc, char **argv)
{
  if (argc < 2)
  {
    printf ("Usage: %s [FILES]\n", argv[0]);
    return EXIT_FAILURE;
  }

  argc++;
  argv--;

  while(*argv)
  {
    process_file(*argv);
    ++argv;
    --argc;
  }
  return EXIT_SUCCESS;
}

We will go through the important parts of the program.

#include <fcntl.h> #include <afflib.h>

These two lines include the necessary headers to use the AFF. Although fcntl.h is not actually required, it provides several macros that are highly recommended.

AFFILE * af = af_open(fn,O_RDONLY,0); if (NULL == af) { perror(fn); return TRUE; }

This line attempts to open the given file using the af_open command. The O_RDONLY is one of many macros defined in fcntl.h that can be used when opening files. The final parameter specifies whether or not the file must be created. This normally only applies when writing AFF files, so it's safe to leave it as zero here. The function returns a pointer to an AFFILE structure. There are some public data fields in that structure defined in afflib.h.

printf ("The uncompressed size of this file is %"PRId64" bytes\n", af_get_imagesize(af));

This line uses one of the miscellaneous functions included with the AFF to display the size of the uncompressed image stored in the file. Compare the value displayed by this line to the size of the AFF image on the disk and you can see how much the library compresses. The example file zeros.aff shows an extreme case!

af_close(af);

This final line closes the file handle and destroys the AFFILE structure.

AFF Concepts

Segments

AFF files can be viewed as a series of name/value pairs.

Pages

AFF Return Values

The following status codes can be returned by AFF functions.

Code	Description
AF_ERROR_EOF	The end of the AFF file has been reached
AF_ERROR_DATASMALL	The buffer provided for the requested AFF segment is not large enough to hold the data that would be returned.
AF_ERROR_TAIL	There is no tail or an error occurred trying to read the tail of an AFF segment. This indicates that the file is corrupt (probably truncated).
AF_ERROR_SEGH	There is no head or an error occurred trying to read the head. This indicates that the AFF file is corrupt.
AF_ERROR_NAME	Invalid segment name
AF_ERROR_INVALID_ARG	The argument was invalid
AF_ERROR_NO_AES	This version of the library was compiled without AES support and the program is attempting to read an AES-encrypted segment.
AF_ERROR_AES_TOO_SMALL	The decrypted AES segment is too small for the amount of data that it was supposed to contain. The segment is probably corrupt.
AF_ERROR_KEY_SET	A key was previously set. (You can only have one AES encryption key per file.)

AFF Function Reference

Stream Functions

af_open

AFFILE * af_open(const char *filename, int flags, int mode);

The flags parameter controls what the program should be allowed to do with the file. There are a set of handy macros defined in fcntl.h that you should use. When reading files, using O_RDONLY is usually sufficient. When writing an AFF file, it is best to use O_CREAT|O_RDWR.

The mode parameter is only used when creating a new file (RBF – and only on *nix?). It controls the

It should be noted here that when writing AFF files, the af_open command does more than just open a file handle. When writing, the af_open command causes the default AFF header to be written to the file on the disk.

RBF – What happens, when opening for write, if the file already exists? RBF – Which parts of the header are written by default?

See open(3)

af_open_stdin

AFFILE * af_open_stdin(void);

A special case of af_open, this function an AFF handle on standard input. Note that this function did not appear until AFF version 2.3. Also note that if you are working on the Windows platform (or plan to support the Windows platform) you need to explicitly tell the compiler to treat standard input as a stream of binary data. For example, using MinGW you can set:

int _CRT_fmode = _O_BINARY;

somewhere in your code. See https://gnuwin32.sourceforge.net/compile.html for more

af_popen

AFFILE * af_popen(const char *command, const char *type)

This function is currently on supported on UNIX variants, Cygwin and MinGW (which are really both Unix variants).

See popen(3)

af_close

int af_close(AFFILE *af);

Closes an AFF file handle and frees all resources associated with it. You can use this function regardless if you opened the file with af_open or af_popen.

af_read

int af_read(AFFILE *af, unsigned char *buf, size_t count);

Similar to fread, this function reads count bytes from the file and stores them in buf. buf should already be allocated to hold this amount of data. The function returns the number of bytes read or -1 on error. Note the number of bytes read is size_t, not uint64_t, because size_t defines the maximum data segment size on the current platform.

RBF - This function is supposed to read a size_t number of bytes. Should it also return a size_t?

af_seek

uint64_t af_seek(AFFILE *af, int64 pos, int whence);

Sets the file pointer into the decompressed data. The new position, given in bytes, is obtained by adding pos bytes to the position specified by whence. If whence is set to SEEK_SET, SEEK_CUR, or SEEK_END, the offset is relative to the start of the file, the current position, or the end of the file, respectively. On success this function returns zero. RBF - On error this function returns minus one (uint64_t) ?? and sets errno appropriately.

Errors

EBADF: The stream is not seekable. (e.g. a stream that was opened with af_popen)

EINVAL: The whence argument was not one of SEEK_SET, SEEK_CUR, or SEEK_END.

af_tell

uint64 af_tell(AFFILE *af);

Similar to ftello, this function returns the current value of the file position indicator for the file pointed to by af. On an error, this function returns -1 and sets errno appropriately.

af_eof

int af_eof(AFFILE *af);

Returns TRUE if the file pointer into the decompressed data is at the end of the decompressed data. Otherwise, returns FALSE.

af_reopen

Writing AFF Files

af_enable_writing

void af_enable_writing(AFFILE *af, int flag);

This function must be executed before attempting to write data (using af_write) to an AFF file. You do not need to execute this function, however, before calling af_update_seg. If flag is TRUE, enables the user to write to the file AF.

af_set_callback

void af_set_callback(AFFILE *af, void (*wcb)(struct affcallback_info *));

Sets a callback function that is called during each of the four phases of the segment write operation.

af_enable_compression

void af_enable_compression(AFFILE *af, int type, int level);

Enables compression on the given AFF file. The type of compression must be an AFF compression algorithm as shown in Table 2. The compression level can be any integer between AF_COMPRESSION_MIN and AF_COMPRESSION_MAX. These values are currently these are one and nine, respectively, but could change in future versions.

For example, to enable maximum LZMA compression:

af_enable_compression(af,AF_COMPRESSION_ALG_LZMA,AF_COMPRESSION_MAX);

Flag	Algorithm
AF_COMPRESSION_ALG_NONE	No compression
AF_COMPRESSION_ALG_ZLIB	zlib
AF_COMPRESSION_ALG_LZMA	LZMA. Dramatically slower than zlib, this algorithm can produce image files that are significantly smaller.

af_compression_type

int af_compression_type(AFFILE *af);

Returns the compression algorithm being used by the file in question. The algorithms are given in Table 2.

af_write

int af_write(AFFILE *af, unsigned char *buf, size_t count);

Note that you must call af_enable_writing on af before using this function.

af_badflag

const unsigned char * af_badflag(AFFILE *af);

Returns the pattern used to identify bad sectors in this file.

af_is_badsector

af_is_badsector(AFFILE *af, unsigned char *buf);

Returns TRUE if buf is the pattern used to mark a bad sector, Otherwise, returns FALSE.

af_stats

void af_stats(AFFILE *af, FILE *f);

This function writes statistics about the current usage of the given AFF file. This includes the file's name and the number of pages read, pages written, pages compressed, pages decompressed, cache hits, cache misses, and bytes copied into the cache. This function is generally used only for debugging the AFF.

Sample output:

AFSTATS for filename.aff
Pages read: 2
Pages written: 0
Pages compressed: 0
Pages decompressed: 1
Cache hits: 0
Cache misses: 1
Bytes copied: 10848768

af_set_cache_size

Segment Functions

af_rewind_seg

af_rewind_seg(AFFILE *af);

Returns the segment pointer to the beginning the file. This function should be used before any segments are examined.

af_get_seg

int af_get_seg(AFFILE *af, const char *name, unsigned long *arg, unsigned char *data, size_t *datalen);

af_get_next_seg

int af_get_next_seg(AFFILE *af, char *segname, size_t segname_len, unsigned long *arg, unsigned char *data, size_t *datalen);

af_update_seg

int af_update_seg(AFFILE *af, const char *name, unsigned long arg, const void *value, unsigned int vallen);

RBF – Why aren't these arguments in C99 type? Arg, especically, needs to be denoted as being 32 bits

RBF – If the sizes of segments when read is returned in a size_t, why isn't the length size_t when being passed in for writing? And why the different name for the length parameter

Proposed new prototype:

int af_update_seg(AFFILE *af, const char *name, uin32_t arg, const void *value, size_t datalen);

This function is used both to set and change the values of segments in a file. The name of the segment to be updated is given in name. If the segment does not exist, it will be created. The value arg is 32-bits of application specific data. If your segment is only going to store 32 bits of data or less, you can easily store the value in the arg parameter. In this case, set data to NULL and datalen to zero.

Programmers do not need to call af_enable_writing before using this fucntion. af_update_seg returns zero on success and -1 on error. When an error occurs the value errno is set to indicate the problem. Possible values for errno include: ENOTSUP The operation is not supported by the filesystem. This happens when the programmer attempts to set segments in a file that doesn't support metadata (e.g. AF_IDENTIFY_RAW).

Examples:

Updating a small value:

int status = af_update_seg(af,"Luggage Combination",12345,NULL,0);
if (status)
{
  perror(af_filename(af));
  return -1;
}

Updating a large value:

status = af_update_seg(af,"List of problems solved by MacGuyver", 0, buf_macguyver,strlen(buf_macguyver));
if (status)
{
  perror(af_filename(af));
  return -1;
}

af_del_seg

int af_del_seg (AFFILE *af, const char *name);

Deletes the first segment of the given name from the file. Returns zero on success, -1 on error.

Miscellaneous Functions

af_filename

const char * af_filename(AFFILE *af);

Returns the filename associated with an opened file. Sample:

printf ("The current file being read is %s.\n", af_filename(af));

af_identify

int af_identify(AFFILE *af);

Returns the type of file associated with the given handle. The types are defined in Table 3.

Type	Description
AF_IDENTIFY_RAW	A raw file, sometimes also called a dd image or flat file
AF_IDENTIFY_AFF	A standard AFF archive
AF_IDENTIFY_AFD	A directory of AFD files, or a split AFF file
AF_IDENTIFY_EVF	An EnCase file read with the original AFF EnCase implementation; now deprecated.
AF_IDENTIFY_EVD	An EnCase file read with the original AFF EnCase implementation; now deprecated.
AF_IDENTIFY_SPLIT_RAW	A split raw file
AF_IDENTIFY_AFM	A raw file with metadata
AF_IDENTIFY_EWF	An EnCase file read with libewf.

Sample usage:

const char * af_identity_to_str(int id)
{
  switch(id) {
  case AF_IDENTIFY_RAW: return "Raw file";
  case AF_IDENTIFY_AFF: return "Standard AFF image";
  case AF_IDENTIFY_AFD: return "Split AFF image";
  case AF_IDENTIFY_EVF:
  case AF_IDENTIFY_EVD: return "Deprecated EnCase image";
  case AF_IDENTIFY_SPLIT_RAW: return "Split Raw file";
  case AF_IDENTIFY_AFM: return "Raw file with metadata";
  case AF_IDENTIFY_EWF: return "EnCase image";
  default: return "Unknown";
  }
}

int identify_filetype(AFFILE *af)
{
  printf ("The file is type: %s\n", af_identify_to_str(af_identify(af)));
  return FALSE;
}

af_get_imagesize

int64 af_get_imagesize(AFFILE *af);

Returns the number of bytes in the decompressed data. On an error, returns -1. Sample:

int64_t data_size = af_get_imagesize(af);
if (-1 == data_size)
{
  fprintf (stderr,"%s: Unable to determine size of decompressed data", af_filename(af));
  return TRUE;
}
printf ("The decompressed data is 0x%"PRIx64" bytes\n", data_size);

Segment Names

Although segment names and values can be defined by the user at any time, here is a list of the segments that are included in the AFF library. Those values set by aimage are noted.

RBF – Which segment names are required for the file to be a valid AFF?

RBF – How does the programmer enumerate all of the segment names?

Name	Aimage	Description
AF_IGNORE		Ignore segments with a zero length name
AF_PAGESIZE	Y	The size of each page, in bytes
AF_BADFLAG	Y	Data that is inserted into the image file to mark a bad sector.
AF_DMESG	Y	The results of the 'dmesg' command are stored in the AFF file.
AF_ACQUISITION_COMMAND_LINE	Y	The command line used to run the disk imaging program.
AF_ACQUISITION_ISO_COUNTRY	Y	The ISO country code of the computer used to create the image; set manually at imaging time.
AF_PAGE_MD5	Y	The MD5 hash of the current page
AF_ACQUISITION_SECONDS	Y	The number of seconds used to create the image file.