Secure Generation of Private Keys Suppose you've created a random private key by rolling dices.
You've checked in the blockchain and nobody else has ever used the respective address of such private key.  

Issue
Is this process reasonably safe to generate private keys?
Or, is there a subset of private keys that you should avoid due to known vulnerabilities in Bitcoin ECDSA (ie: something faster than brute force)?
Any 256-bit number between 0x1 and 0xFFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4141 is a valid private key. Assuming your random number is in that range (and it's extremely likely that is the case), it should be just as fine of a private key as any other number.  In fact, you shouldn't even bother with checking if the address is already known because:


Probability tells us it isn't used
Its existence on a chain explorer is not proof that nobody else has generated that key
By checking, you may be telegraphing that you own that key


Be more concerned about how your key is generated, rather than is a particular number is vulnerable to some unknown attack.  Make sure your dice rolls are done in such a way that every byte is just as likely as every other byte.  I've used a D8 in conjunction with a coin flip in order to generate 4 bits at a time.  If the coin is heads, I use the D8 at face value.  If tails, I add 8.  I count 16 (tails + 8) as 0.  If you work it out, you'll see that this generates a perfectly random number between 0 and 15, or one hexit.  Do this 64 times, and you have a 256-bit key.