You have to know how kangaroo program works. A single V100 can solve a 64 bit key using kangaroo in mere seconds. One cannot use kangaroo to crack #64 now because the pub key is not known. But once someone broadcasts to transfer the BTC from #64's address, the pub key will be exposed and someone can use a single GPU to solve for the private key in seconds. That is what the OP is saying. As others have stated, using the RBF with decent fee, will help from others "stealing" #64s key.
Do you have a reference for saying that a single V100 can solve a 64 bit (of entropy) key in seconds?
If what you are saying is true, it would be advisable to
not use RBF. The RBF would need to be set to False and a decent fee that is sufficient to be included in the next block should be used.
I looked at some of the documentation for kangaroo, but have not looked at the math closely.
Right, however the RBF works, you want to make sure that you tell it to not replace by higher fee.
As for the performance, here is a quick run of a 64 bit key using a much slower GTX 1060 6GB card:
Kangaroo v2.1
Start:8000000000000000
Stop :FFFFFFFFFFFFFFFF
Keys :1
Number of CPU thread: 0
Range width: 2^63
Jump Avg distance: 2^30.98
Number of kangaroos: 2^19.32
Suggested DP: 9
Expected operations: 2^32.86
Expected RAM: 84.5MB
DP size: 12 [0xFFF0000000000000]
GPU: GPU #0 NVIDIA GeForce GTX 1060 6GB (10x128 cores) Grid(20x256) (57.0 MB used)
SolveKeyGPU Thread GPU#0: creating kangaroos...
SolveKeyGPU Thread GPU#0: 2^19.32 kangaroos [4.5s]
[210.13 MK/s][GPU 210.13 MK/s][Count 2^30.96][Dead 0][12s (Avg 37s)][64.1/98.2MB]
Key# 0 [1S]Pub: 0x0311CEF632C14F4EF26CB1CE5D79B28E2988DC108F44EE0CDF9E6E6EFC7231C72C
Priv: 0x9CCE5EFDACCF6808
Done: Total time 18s
A V100 is at least 10x faster than the 1060 used in this test.
The new BSGS Cuda program can complete a 64 bit range using a RTX 3090 in 1-2 seconds as well.
Where did this value of 1.2 BTC come from?
The 1.2 BTC value came from the 120 bit challenge in this overall challenge. There is BTC in each range, from 1 bit to 160 bits. Every 5th range, 5, 10, 15, 20, ..., 100, 105, 110, 115, 120, etc., has the public key exposed, which allows for the use of programs such as BSGS or Kangaroo.
More info here:
https://bitcointalksearch.org/topic/bitcoin-challenge-transaction-1000-btc-total-bounty-to-solvers-updated-5218972