I seem to remember that the original cargo of the Out of Band II in A Fire Upon the Deep was part of a one time pad - the ship held data that had to be xor-ed with data being carried by other ships (presumably so they couldn't keep a copy and read all of the messages).

Wasn't Pham also regarded as terribly naive for believing that public key encyrption would be secure? When they are discussing things in the bar scene where they first meet the skroderiders....