Examining the SHA-1 collision files

SHA-1 is a cryptographic hash function. You give it a computer file, and it produces a 160-bit hash that is completely determined by the input file, but not in any obvious way. In early 2017, a group of researchers, using advanced mathematics and 6500 CPU-years of computer searching, found the first ever SHA-1 collision: two……

About that JPEG/ZIP/Shakespeare hybrid file

The other day, a Twitter user (David Buchanan, @David3141593) posted a message that gained some attention. It has an attached JPEG file, with an image of William Shakespeare. If you save a copy of the JPEG file, and unzip it as if it were a ZIP file, it unzips into the complete works of Shakespeare.……

How is a JPEG image’s color type determined?

In order to decode and display a JPEG image, you have to know what “color type” (or “color model”) it uses. There are several possibilities that your software probably ought to handle: Grayscale YCbCr (transformed from RGB) RGB YCCK (transformed from CMYK) CMYK Unfortunately, if you look at all the bits and bytes that make……

Preview images in old CorelDRAW files

CorelDRAW is a popular vector graphics program. Its native .CDR file format has gone through many different versions. I want to focus on the files written by CorelDRAW version 2. These are the .CDR files that begin with “WLl” or “WLm”. Even though it dates from 1989 or so, and has been obsolete for most……

Comments in WMF files

Windows Metafile (WMF) is a vector graphics format native to Microsoft Windows 3.x. It was succeeded by Enhanced Metafile (EMF) in 32-bit versions of Windows, but it seems to have remained in use, alongside EMF, long after Windows 3.x became obsolete. It was often embedded inside other file formats, such as PowerPoint (.ppt) or Write……

The finer points of TIFF “ASCII” fields

This post concerns the TIFF image file format, and some related formats: Exif, DNG, and JPEG XR. A TIFF file contains a set of “fields”, each of which contains some sort of data element. Each field has a “tag”: a code number that tells, logically, what is stored in the field. For example, tag #256……

JPEG thumbnail formats

How many ways are there to put an extra “thumbnail” or “preview” image inside a JPEG (.jpg) image file? This isn’t just trivia. One problem with thumbnails is the possibility that they might be neither updated nor deleted when the main image is edited. If you’re not careful, your edited JPEG file could contain a……