Increase min-depth for funding transactions
#2973
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We already use a minimum depth of 6 before announcing channels to protect against reorgs. However, we allowed using the channel for payments after only 3 confirmations (for small channels). A reorg of 3 blocks that invalidates the funding transaction would allow our peer to potentially steal funds. It's more consistent to use the same depth for announcing the channel and actually using it. Note that for wumbo channels, we already scaled the number of confirmations based on the size of the channel. For closing transaction, we don't need the same reorg safety, since we keep watching the funding output for any transaction that spends it, and concurrently spend any commitment transaction that we detect. We thus keep a minimum depth of 3 for closing transactions.
We were still using values from before the halving. We update those values and change the scaling factor to a reasonable scaling. This protects channels against attackers with significant mining power.
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We already use a minimum depth of 6 before announcing channels to protect against reorgs. However, we allowed using the channel for payments after only 3 confirmations (for small channels). A reorg of 3 blocks that invalidates the funding transaction would allow our peer to potentially steal funds. It's more consistent to use the same depth for announcing the channel and actually using it.
Note that for wumbo channels, we already scaled the number of confirmations based on the size of the channel. We update our scaling factor in the second commit, which hadn't been updated since the last halving.
For closing transaction, we don't need the same reorg safety, since we keep watching the funding output for any transaction that spends it, and concurrently spend any commitment transaction that we detect. We thus keep a minimum depth of 3 for closing transactions.
Related discussion: lightning/bolts#1215