We're given an SSH private keys, but some of the characters in the base64 string are replaced with x's. Luckily, it looks like while the modulus was partially lost, all of p and q were intact. I used openssl asn1parse -in someotherkey.pem to give me the offsets of p and q in another key, and I extracted them with python. Then I used the following script to recreate a working SSH key:
#!/usr/bin/env python2
import gmpy
from Crypto.PublicKey import RSA
p = 160715260849342318931136112813341037345926969012288227225240875622403009493539093929333081548188459992247771680452063593583756278915740193557402138743266217376005578973188641800583345510266770139969709567420846366801788060791738229180205729066714584288249507088921482835100030743352147986722422517067206563539
q = 156522822773738162417254450203271175855220146400024771706084276654684994055624152101542626647589634389361232150411812572776336649201321449632016603858688896275125914484326556417817195311471437215701390750315213065194536381852437122083849274951300180499399546807140772435452395099516509211865918104434503784667
n = long(p*q)
phi = (p-1)*(q-1)
e = long(0x10001)
d = long(gmpy.invert(e, phi))
rsa = RSA.construct((n, e, d))
open('key', 'w').write(rsa.exportKey())
I was then able to run ssh -p 1504 -i key berlin@cthulhu.fluxfingers.net to get the flag: flag{thought_ssh_privkeys_are_secure?}