The question is back in the news: is using encryption a sign you are criminal?
In May of 2005, a Minnesota court filed a ruling that upheld a conviction in part based on the presence of encryption software (State v. Levie). The chilling sentence in the filing was: “We find that evidence of appellant’s internet use and the existence of an encryption program on his computer was at least somewhat relevant to the state’s case against him.” This was but one in a chain of legal cases involving cryptography. In fact, the software in question in the Levie case has, almost from its inception, been the subject of legal scrutiny. Yet the ruling set off a firestorm in part because it appeared to imply that encryption by itself was indicative of criminal activity.
The view was reinforced by Bruce Schneier, a prominent InfoSec analyst and cryptographer. “An appeals court in Minnesota has ruled that the presence of encryption software on a computer may be viewed as evidence of criminal intent.” As sometimes happens on the Internet, the resulting discussion involved many who did not read the ruling and many more who employed fallacy of extension arguments. For example, one commentator responded "Next I suppose they'll consider finding a knife in your kitchen is 'evidence' on criminal intent to commit some gruesome attack on an innocent bystander."
The actual ruling was significantly more balanced than it appeared from Schneier’s summary. When placed into context, it is clear that the presence of cryptography along with the existence of searches related to the crime were introduced to demonstrate the Levie’s state of mind. It was only in relation to the primary crime that they, in fact, became admissible. Other writers were quick to point this out. “The court did not hold that encryption is a signal of criminal activity. All it did was say that in one case, where a crucial witness testified about the presence of a computer file on a computer, that the presence of encryption software on the computer in early 2003 was "at least somewhat relevant" to the question of whether the defendant was a skilled computer user who had intentionally removed any traces of that file from the hard drive. (Kerr, 2005)”
The concern remained, however, that the ruling would be interpreted and used in future cases. This concern was best voiced by Jennifer Granick; the “hacker lawyer” and director of Stanford Law School's Center for Internet and Society. Granick repeated Kerr’s argument that the ruling suggested the presence of encryption software simply shows the defendant could have destroyed the evidence. She then repeated the argument that was most concerning: the ruling could demonstrate that encryption “suggests a consciousness of guilt.” That is, why encrypt if you have nothing to hide? While Granick is careful to say that both interpretations are valid, the primary concern should not be “what this opinion says or doesn't say, but how it could be used by courts looking at this issue in the future.”
Where do we stand some today? The legal opinions are still a mixed bag. Because encryption is such a wide field, let us take another example that deals with PGP. A federal judge ruled in 2007 that a decryption passphrase was protected under the Fifth Amendment (United States v. Boucher). This was celebrated at the time but the celebrations were short lived. In February of 2009, the court reversed its decision. At its heart was the definition of a PGP passphrase: was it speech or was it a key? The original ruling came down on the side of speech and thereby protected the passphrase. The reversal saw the passphrase like a key or combination, which prior rulings had established are unprotected by the Fifth Amendment.
A person can be legally required to open a safe and reveal incriminating documents or books. Likewise, according to States v. Boucher, a person can now be required to decrypt a folder to review digital documents. The issue is whether the presence of encrypting software or, for that matter, illicit digital materials is relevant to prosecution.
While this topic has not been specifically taken up in law journals, the related topic of suspicious materials has been covered in depth. See, for example, Swiss Cheese That's All Hole: How Using Reading Material To Prove Criminal Intent Threatens The Propensity Rule (Murphy, May 2008). Murphy details the legal precedence and evidentiary rules that allow for reading materials to be used by the prosecution. Such materials can be submitted if they are relevant in demonstrating the defendant’s mental state or aptitudes. But books cannot be used to demonstrate motive or intent. By itself, a book cannot be used to demonstrate a defendant had the inclination towards criminal acts (this is the propensity rule).
The same legal structures likely apply to digital materials. In State v. Levie, illicit web searches were relevant to Levie’s mental state at the time of the crime. The presence of PGP was relevant to Levie’s ability to obfuscate or remove digital evidence. Yet neither the web searches nor PGP were used to demonstrate his propensity to perform the criminal act. The digital materials, just like written materials would have been, were not used to define motive or intent. That proof was from the primary evidence: an eye witness testimony. People can continue to safely use PGP and other encryption technologies. The propensity rule prevents these from being admitted as evidence of criminal intent.