[n2d4]

Arbitrary means: It's "easy" (square root) to find two numbers that resemble each other in a sufficiently large set, but neither of them will resemble anything meaningful. It's still "hard" to find a number that resembles a previously given different number, such as the bbcnews hash above. (The chance that any two kids in a room share a birthday is fairly high; the chance that a kid has their birthday on January 1st is much lower.)

> Although, realistically, I'd be very surprised if in a quintilion PETAFLOPS you found a single 128bit number that, after being hashed twice, starts with "face" and ends with "book"

We can just calculate it. "face" + "book" is 8 characters in base 64, for a total of 8*6=48 bits that need to be set a certain way. 2^48 is roughly 10^15. Hashing once or twice barely matters at this point (2*10^15 ~=~ 10^15). A quintillion petaflops is 10^33 flops, so unless your hashing algorithm takes 10^18 floating point operations, you have an incredibly high probability of finding such a number within a second.

[Jerrrry]

nerd.

Just kidding - this was a much more clearer response with some actual math behind it, thank you.

This (may) explain a facet of .onion phishing attempts at a certain point in time.