JavaScript class SecureRandom() has a bug in it and doesn't generate really secure keys.
How is it related to cryptocurrencies?
There are numerous browser-based cryptocurrency products that still use popular SecureRandom() JS class. JavaScript is very popular for creating browser-based products but isn't a really good thing to use for cryptography purposes. The main problem is that JS is not a type-safe language.
Type safety is a complex topic and there's no one agreed definition of what exactly a "type-safe" language is, but by almost any definition of it, JavaScript is not type-safe. It means that JS doesn't really discourage or prevent type errors that are impermissible in cryptography.
The conclusion is, all crypto wallets generated by JS tools inside browsers had (and some - still have!) keys that are predictable enough to crack by brute-force attack. Yes, such keys have the proper length (cryptography-wise) but less than 48 bits of entropy due to the bug in JavaScript class.
Actually, it's not much to do about it. Like all good cryptocurrency bugs, this one isn’t new at all — here’s Greg Maxwell talking about it nearly three years ago (51:00 on):
This problem affects you if you:
use old cryptocurrency addresses
they were generated with JavaScript, i.e., in a web browser
Possibly affected:
BitAddress pre-2013;
bitcoinjs before 2014;
current software that uses outdated repos from Github.
What to do:
move your funds out of those addresses
don’t use them again
This will reduce the risk of your keys being cracked but, in general, this information should make you stop thinking that it will take ages for modern crypto keys to get cracked. It turns out, that it might be cracked in a week.
Some interesting facts:
JavaScript was originally called LiveScript. It wasn't developed by Sun Microsystems (as Java) and there was no good reason to rename LiveScript to JavaScript. This led to confusion that JavaScript is somehow related to Java, but JavaScript is a different language, it has more in common with functional languages like Lisp or Scheme than with Java.
JavaScript is an actual high-level, interpreted programming language, not the script, as the -Script suffix suggests.
