My understanding of the ECPA and other acts is that you can't intercept, decode, or receive by other intentional means any information or communications that ar

My understanding of the ECPA and other acts is that you can't intercept, decode, or receive by other intentional means any information or communications that aren't "generally accessible" without permission. It's pretty broad and doesn't care about the "how". Private keys are not "generally accessible" and my concern is that the authorities will see cracking the key itself as issue enough, and unlawful. If a security researcher triggers painful breach notifications, which could well happen for a compromised private key, I don't think it's unthinkable at all that an upset target will find a DA who is happy to take this interpretation. I don't think this specific DKIM case is particularly high-risk, but I still wouldn't do it without permission from the key holder.