EFF Joins Coalition to Launch Canarywatch.org
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The EFF representatives (Kurt Opsahl & Nate Cardozo) discuss that warrant canaries are in a legally ambiguous state. Of the three outcomes two of them neuter the canary, either the government would compel speech (slightly unlikely) or that judges would see through the legal 'loophole' that warrant canaries provide.
The There main bit of advice for implementers was that only publish the warrant canaries once or twice a year. If the implementation was 'cute' or overly technical judges would rule against such uses as it would convey information that is explicitly forbidden in national security letters.