USA and China with few billion dollars can bruteforce onion v2 address in days!
And any old non-curve cipher.

https://github.com/lachesis/scallion
>.onion Address | 2^(5*length-1) / hashspeed
https://en.bitcoin.it/wiki/Mining_Hardware_Comparison
> Ebit E10 | 18,000,000 MH/s
>Radeon 6850 301.4
yep, ~59722 times faster on a bitcoin double SHA-256.
for 'unit' it should be 35832780 MH/s.
(AMD Radeon HD6850 600 MH/s on scallion)

Quick calculations:
(2^79)/1000000=604462909807314587,353088
With 1 MH/s you need 6996098493141 days.
And...
With 35832780 MH/s you need 195243 days.
(For one unit with ~5500 price at start.)
With 5000 units and roughly ~27,2M $ you can do all job in 40 days.
https://qz.com/1055126/photos-china-has-one-of-worlds-largest-bitcoin-mines/
But there is 25,000 machines!
And states and china definitely can make something for ciphers bruteforce.
With 25K machines you can brute full 16 symbol onion in 8 days.
I think, bruteforce any old non-curve ciphers in ~week for 180M $ worth it
and governments can pay even some billions for that (for complex goals).

https://en.wikipedia.org/wiki/Military_budget_of_China
600 billion $ for US and ~166 billion $ for china
'Bruteforce facility' very plausible thing.

Thats why v3 address longer and based on elliptic curve cipher.