The military determines the admissibility of evidence from encrypted chat apps through a two-step process: first, lawfully obtaining the evidence, and second, authenticating it in court. The use of an encrypted app like Signal or WhatsApp does not make the contents of a chat legally undiscoverable. Law enforcement investigators (like CID or NCIS) can obtain the messages through several means. They can get a search warrant or authorization to seize the device of the sender or the recipient. A forensic examination of the device can often recover the chat logs directly from the phone’s memory. Alternatively, one of the participants in the chat can voluntarily consent to a search and provide the messages to investigators.
Once the messages are lawfully obtained, they must be authenticated in court under the Military Rules of Evidence to be admitted. The prosecutor must prove that the message is what they claim it to be. This is often done through the testimony of one of the participants in the chat, who can identify the message and testify that it is a true and accurate record of their conversation. The prosecutor can also use technical evidence, such as the phone numbers or user IDs associated with the chat, to link the messages to the accused.
A military defense attorney will challenge this evidence by attacking both its acquisition and its authentication. They will first file a motion to suppress the messages if there is any question about the lawfulness of the search that obtained them. If the search was legal, they will then challenge the authentication, arguing that the government cannot definitively prove their client was the one who sent the messages. They might suggest someone else had access to the phone or that the messages could have been altered. The judge must be convinced the evidence is authentic before it can be shown to the panel.