Recover

Implement a program that recovers JPEGs from a forensic image, per the below.

$ ./recover card.raw

Background

In anticipation of this problem, we spent the past several days taking photos of people we know, all of which were saved on a digital camera as JPEGs on a memory card. (Okay, it’s possible we actually spent the past several days on Facebook instead.) Unfortunately, we somehow deleted them all! Thankfully, in the computer world, “deleted” tends not to mean “deleted” so much as “forgotten.” Even though the camera insists that the card is now blank, we’re pretty sure that’s not quite true. Indeed, we’re hoping (er, expecting!) you can write a program that recovers the photos for us!

Even though JPEGs are more complicated than BMPs, JPEGs have “signatures,” patterns of bytes that can distinguish them from other file formats. Specifically, the first three bytes of JPEGs are

0xff 0xd8 0xff

from first byte to third byte, left to right. The fourth byte, meanwhile, is either 0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, or 0xef. Put another way, the fourth byte’s first four bits are 1110.

Odds are, if you find this pattern of four bytes on media known to store photos (e.g., my memory card), they demarcate the start of a JPEG. To be fair, you might encounter these patterns on some disk purely by chance, so data recovery isn’t an exact science.

Fortunately, digital cameras tend to store photographs contiguously on memory cards, whereby each photo is stored immediately after the previously taken photo. Accordingly, the start of a JPEG usually demarks the end of another. However, digital cameras often initialize cards with a FAT file system whose “block size” is 512 bytes (B). The implication is that these cameras only write to those cards in units of 512 B. A photo that’s 1 MB (i.e., 1,048,576 B) thus takes up 1048576 ÷ 512 = 2048 “blocks” on a memory card. But so does a photo that’s, say, one byte smaller (i.e., 1,048,575 B)! The wasted space on disk is called “slack space.” Forensic investigators often look at slack space for remnants of suspicious data.

The implication of all these details is that you, the investigator, can probably write a program that iterates over a copy of my memory card, looking for JPEGs’ signatures. Each time you find a signature, you can open a new file for writing and start filling that file with bytes from my memory card, closing that file only once you encounter another signature. Moreover, rather than read my memory card’s bytes one at a time, you can read 512 of them at a time into a buffer for efficiency’s sake. Thanks to FAT, you can trust that JPEGs’ signatures will be “block-aligned.” That is, you need only look for those signatures in a block’s first four bytes.

Realize, of course, that JPEGs can span contiguous blocks. Otherwise, no JPEG could be larger than 512 B. But the last byte of a JPEG might not fall at the very end of a block. Recall the possibility of slack space. But not to worry. Because this memory card was brand-new when I started snapping photos, odds are it’d been “zeroed” (i.e., filled with 0s) by the manufacturer, in which case any slack space will be filled with 0s. It’s okay if those trailing 0s end up in the JPEGs you recover; they should still be viewable.

Now, I only have one memory card, but there are a lot of you! And so I’ve gone ahead and created a “forensic image” of the card, storing its contents, byte after byte, in a file called card.raw. So that you don’t waste time iterating over millions of 0s unnecessarily, I’ve only imaged the first few megabytes of the memory card. But you should ultimately find that the image contains 50 JPEGs.

Getting Started

Here’s how to download this problem’s “distribution code” (i.e., starter code) into your own CS50 IDE. Log into CS50 IDE and then, in a terminal window, execute each of the below.

  1. Execute cd to ensure that you’re in ~/ (i.e., your home directory).
  2. If you haven’t already, execute mkdir pset4 to make (i.e., create) a directory called pset4 in your home directory.
  3. Execute cd pset4 to change into (i.e., open) your pset4 directory.
  4. Execute wget http://cdn.cs50.net/2020/fall/psets/4/recover/recover.zip to download a (compressed) ZIP file with this problem’s distribution.
  5. Execute unzip recover.zip to uncompress that file.
  6. Execute rm recover.zip followed by yes or y to delete that ZIP file.
  7. Execute ls. You should see a directory called recover, which was inside of that ZIP file.
  8. Execute cd recover to change into that directory.
  9. Execute ls. You should see this problem’s distribution, including card.raw and recover.c.

Specification

Implement a program called recover that recovers JPEGs from a forensic image.

  • Implement your program in a file called recover.c in a directory called recover.
  • Your program should accept exactly one command-line argument, the name of a forensic image from which to recover JPEGs.
  • If your program is not executed with exactly one command-line argument, it should remind the user of correct usage, and main should return 1.
  • If the forensic image cannot be opened for reading, your program should inform the user as much, and main should return 1.
  • The files you generate should each be named ###.jpg, where ### is a three-digit decimal number, starting with 000 for the first image and counting up.
  • Your program, if it uses malloc, must not leak any memory.

Walkthrough

Usage

Your program should behave per the examples below.

$ ./recover
Usage: ./recover image

$ ./recover card.raw

Hints

Keep in mind that you can open card.raw programmatically with fopen, as with the below, provided argv[1] exists.

FILE *file = fopen(argv[1], "r");

When executed, your program should recover every one of the JPEGs from card.raw, storing each as a separate file in your current working directory. Your program should number the files it outputs by naming each ###.jpg, where ### is three-digit decimal number from 000 on up. (Befriend sprintf.) You need not try to recover the JPEGs’ original names. To check whether the JPEGs your program spit out are correct, simply double-click and take a look! If each photo appears intact, your operation was likely a success!

Odds are, though, the JPEGs that the first draft of your code spits out won’t be correct. (If you open them up and don’t see anything, they’re probably not correct!) Execute the command below to delete all JPEGs in your current working directory.

$ rm *.jpg

If you’d rather not be prompted to confirm each deletion, execute the command below instead.

$ rm -f *.jpg

Just be careful with that -f switch, as it “forces” deletion without prompting you.

If you’d like to create a new type to store a byte of data, you can do so via the below, which defines a new type called BYTE to be a uint8_t (a type defined in stdint.h, representing an 8-bit unsigned integer).

typedef uint8_t BYTE;

Keep in mind, too, that you can read data from a file using fread, which will read data from a file into a location in memory and return the number of items successfully read from the file.

Testing

Execute the below to evaluate the correctness of your code using check50. But be sure to compile and test it yourself as well!

check50 cs50/problems/2021/spring/recover

Execute the below to evaluate the style of your code using style50.

style50 recover.c

How to Submit

  1. Download your recover.c file by control-clicking or right-clicking on the file in CS50 IDE’s file browser and choosing Download.
  2. Go to CS50’s Gradescope page.
  3. Click “Problem Set 4: Recover”.
  4. Drag and drop your recover.c file to the area that says “Drag & Drop”. Be sure it has the correct filename, or the autograder will fail to run on it, and it will score no correctness points!
  5. Click “Upload”.

You should see a message that says “Problem Set 4: Recover submitted successfully!” You won’t see a score just yet, but if you see the message then we’ve received your submission!