District Court Holds that Running Hash Values on Computer Is A Search: The case is United States v. Crist, 2008 WL 4682806 (M.D.Pa. October 22 2008) (Kane, C.J.). It's a child pornography case involving a warrantless search that raises a very interesting and important question of first impression: Is running a hash a Fourth Amendment search? (For background on what a "hash" is and why it matters, see here).

  First, the facts. Crist is behind on his rent payments, and his landlord starts to evict him by hiring Sell to remove Crist's belongings and throw them away. Sell comes a cross Crist's computer, and he hands over the computer to his friend Hipple who he knows is looking for a computer. Hipple starts to look through the files, and he comes across child pornography: Hipple freaks out and calls the police. The police then conduct a warrantless forensic examination of the computer:

In the forensic examination, Agent Buckwash used the following procedure. First, Agent Buckwash created an "MD5 hash value" of Crist's hard drive. An MD5 hash value is a unique alphanumeric representation of the data, a sort of "fingerprint" or "digital DNA." When creating the hash value, Agent Buckwash used a "software write protect" in order to ensure that "nothing can be written to that hard drive." Supp. Tr. 88. Next, he ran a virus scan, during which he identified three relatively innocuous viruses. After that, he created an "image," or exact copy, of all the data on Crist's hard drive.

Agent Buckwash then opened up the image (not the actual hard drive) in a software program called EnCase, which is the principal tool in the analysis. He explained that EnCase does not access the hard drive in the traditional manner, i.e., through the computer's operating system. Rather, EnCase "reads the hard drive itself." Supp. Tr. 102. In other words, it reads every file-bit by bit, cluster by cluster-and creates a index of the files contained on the hard drive. EnCase can, therefore, bypass user-defined passwords, "break[ ] down complex file structures for examination," and recover "deleted" files as long as those files have not been written over. Supp. Tr. 102-03.

Once in EnCase, Agent Buckwash ran a "hash value and signature analysis on all of the files on the hard drive." Supp. Tr. 89. In doing so, he was able to "fingerprint" each file in the computer. Once he generated hash values of the files, he compared those hash values to the hash values of files that are known or suspected to contain child pornography.Agent Buckwash discovered five videos containing known child pornography. Attachment 5. He discovered 171 videos containing suspected child pornography.

  One of the interesting questions here is whether the search that resulted was within the scope of Hipple's private search; different courts have approached this question differently. But for now the most interesting question is whether running the hash was a Fourth Amendment search. The Court concluded that it was, and that the evidence of child pornography discovered had to be suppressed:

The Government argues that no search occurred in running the EnCase program because the agents "didn't look at any files, they simply accessed the computer." 2d Supp. Tr. 16. The Court rejects this view and finds that the "running of hash values" is a search protected by the Fourth Amendment.

Computers are composed of many compartments, among them a "hard drive," which in turn is composed of many "platters," or disks. To derive the hash values of Crist's computer, the Government physically removed the hard drive from the computer, created a duplicate image of the hard drive without physically invading it, and applied the EnCase program to each compartment, disk, file, folder, and bit.2d Supp. Tr. 18-19. By subjecting the entire computer to a hash value analysis-every file, internet history, picture, and "buddy list" became available for Government review. Such examination constitutes a search.

  I think this is generally a correct result: See my article Searches and Seizures in a Digital World, 119 Harv. L. Rev. 531 (2005), for the details. Still, given the lack of analysis here it's somewhat hard to know what to make of the decision. Which stage was the search — the creating the duplicate? The running of the hash? It's not really clear. I don't think it matters very much to this case, because the agent who got the positive hit on the hashes didn't then get a warrant. Instead, he immediately switched over to the EnCase "gallery view" function to see the images, which seems to be to be undoudtedly a search. Still, it's a really interesting question.

  Also, it seems that the Government failed to make the strongest argument that running the hash isn't a search: If the hash is for a known image of child pornography, then running a hash is a direct analog to a drug-sniffing dog in Illinois v. Caballes, 543 U.S. 405 (2005). Although Caballes is cited in the opinion for other reasons, it seems that the government didn't make the Caballes argument.

  It's possible that the argument wasn't raised because the agent made a hash of every file instead of running a search just for matches of known images. But I'm not sure that really makes a difference, and whether it does hinges on some interesting questions. Is the creation of the hash a search? Or is running a query that matches the hashes to known hashes and produces a positive hit a search? It might also break down based on how much the government saw of the machine while the hashes were being made: Perhaps the search occurred when the file structure was revealed to the officers (if it was in fact revealed). But if so, I'm not sure that the images themselves should be suppressed as compared to evidence more directly related to the revealing of the file structure.

  Either way, this is a fascinating computer crime law issue that gets debated from time to time without any case law; I believe this is the first case on the topic. Ah, more grist for the mill of the forthcoming second edition of my computer crime casebook. Thanks to FourthAmendment.com for the mention of the opinion, and Matt Caplan for the .pdf.