I read an SF novel about 30 years ago, where one of the characters is killed early, and the rest of the novel is his friends trying to decrypt what is on his personal device as they knew it was significant. At the end of the novel they find the right passcode / phrase, and find the plans for a massive SETI array that would be built (I think) between Jupiter and the asteroid belt. Really want to read this again as looking back it seemed quite visionary, but I haven't been able to find it.

Edit after reading "how to ask a good story-id question meta-post": Written in English. Most likely read this in 1970's or maybe early 80's. Based on Earth, future but not distant future. Standalone not anthology. Targeted at adult audience. Don't remember any of characters names. Have looked through lists of published works for all the leading authors and those I recall reading at the time (Clarke, Asimov, Heinlein, Niven, Greg Bear, Gregory Benford, Samuel Delany, Philip K Dick, Harlan Ellison, Philip Jose Farmer). I believe it was single author, not a collaboration.