Nothing is 100% secure, at least nothing you can be certain of.

Your proceedure is a pretty good idea. I just played it through. Possible security implications:

1. The coin might not flip randomly enough. It's reported that some coins turn out more often heads than tails. Try a dice maybe (odds = 1, even = 0)?
2. If you use a machine to convert the binary number to hexadecimal, ensure it's offline und secure. Try a live medium.
3. The step saving it to disk (priv.txt) might leave a magnetic footprint on your harddisk. Again, use a live medium which operates in memory. But don't forget to backup your encrypted keystore.

I think it's easier and more safe than using geth directly to generate an encrypted keystore. Just make sure it's offline and operating in memory and well backed up.

Nothing is 100% secure, at least nothing you can be certain of.

Your procedure is a pretty good idea. I just played it through. Possible security implications:

1. The coin might not flip randomly enough. It's reported that some coins turn out more often heads than tails. Try a dice maybe (odds = 1, even = 0)?
2. If you use a machine to convert the binary number to hexadecimal, ensure it's offline and secure. Try to boot from a live medium before doing this.
3. The step saving it to disk (priv.txt) might leave a magnetic footprint on your harddisk. Again, boot from a live medium which operates in memory. But don't forget to backup your encrypted keystore.

I think it's easier and more safe than using geth directly to generate an encrypted keystore. Just make sure it's offline and operating in memory and well backed up.