Uwe Hermann | A slightly paranoid Debian developer

jhead is a very nice and very powerful command line utility to mess with JPEG headers (esp. EXIF fields).

  $ apt-get install jhead

It can display/extract a great amount of metadata fields from JPEG files and also extract the thumbnails stored in JPEG files (if any). The following will list all known metadata fields from a sample photo:

  $ wget http://farm4.static.flickr.com/3173/3061542361_60acb0904b_o.jpg
  $ jhead *.jpg
  File name    : 3061542361_60acb0904b_o.jpg
  File size    : 1074172 bytes
  File date    : 2008:11:26 23:38:04
  Camera make  : Panasonic
  Camera model : DMC-FZ18
  Date/Time    : 2008:03:05 15:45:52
  Resolution   : 3264 x 2448
  Flash used   : No
  Focal length : 4.6mm  (35mm equivalent: 28mm)
  Exposure time: 0.0100 s  (1/100)
  Aperture     : f/3.6
  ISO equiv.   : 100
  Whitebalance : Auto
  Metering Mode: matrix
  Exposure     : program (auto)
  GPS Latitude : N %:.7fd %;.8fm %;.8fs
  GPS Longitude: E %;.8fd %:.7fm %;.8fs
  GPS Altitude : 174.00m
  Comment      : Aufgenommen auf dem <a href="http://www.froutes.de/TT00000014_Ars_Natura">Kunstweg Ars Natura</a>.
  ======= IPTC data: =======
  Record vers.  : 4
  Headline      : Felsburg auf dem Felsberg
  (C)Notice     : www.froutes.de
  Caption       : Aufgenommen auf dem <a href="http://www.froutes.de/TT00000014_Ars_Natura">Kunstweg Ars Natura</a>.

As you can see there's a huge amount of potentially privacy-sensitive metadata in your typical JPEG as generated by your camera (including camera type, settings, date/time, maybe even GPS coordinates of your location, etc).

You can extract the thumbnail stored in all JPEGs in the current directory with:

  $ jhead -st "&i_t.jpg" *.jpg
  Created: '3061542361_60acb0904b_o.jpg_t.jpg'

Note that the JPEG thumbnail does not necessarily show the same picture as the JPEG itself. Depending on the image manipulation software that was used to create the edited/fixed/cropped JPEG, the thumbnail may still reflect the original JPEG contents (see sample image on the right-hand side). This is a huge potential privacy issue. There have been a number of articles about this some years ago, in case you missed them:

• disLEXia: Hidden data in Document Formats
• disLEXia: EXIF Thumbnail in JPEG images (many sample JPEGs and their differing thumbnails)
• Kevin Cohen: Digital Still Camera Forensics (PDF)

Thus, an important jhead command line to know is the following, which removes all metadata (including any thumbnails) from all JPEG images in the current directory:

  $ jhead -purejpg *.jpg
  Modified: 3061542361_60acb0904b_o.jpg

As you can see the result is that only very basic information can be gathered from the file afterwards:

  $ jhead *.jpg
  File name    : 3061542361_60acb0904b_o.jpg
  File size    : 1052506 bytes
  File date    : 2008:11:26 23:38:04
  Resolution   : 3264 x 2448
  $ jhead -st "&i_t.jpg" *.jpg
  Image contains no thumbnail

I recommend doing this for most photos you make publically available on sites like flickr etc. (unless you have a good reason not to). Finally, see the jhead(1) manpage for lots more options that the tool supports.