Ripple wiki specifies that each key family originates with (presumably) 128 bit master seed
https://ripple.com/wiki/Account_FamilyHow much energy and time must be expended before ASICs custom hardware can brute force attack a 128 bit key ?
Good overview is given here:
http://en.wikipedia.org/wiki/Brute-force_attackTL;DR version is that it would require
at least 30 gigawatts of power for one year. perhaps too much to attack your Grandma's ripple key pair, but what about ripple network's own master root key or the master root key of OpenCoin, Inc. in say 10 - 20 years ?
Having read the above two articles, I would prefer the master key to be at least 160 bit of good entropy, although it would make for a longer base-58 encoded sXXXXX representation, but Bitcoins secret keys carry close to 256 bits of entropy and that hasn't proven to be too cumbersome.
So, can ripple network survive another 50 years of ASICs custom hardware evolution ?