Quote from: mughat on March 06, 2013, 03:51:13 PM

Quote from: ffe on March 06, 2013, 03:49:00 PM

maybe 100 characters that are easy to remember like your email address plus 16 to 20 truly random alphanumeric characters.

The email address makes it specific to you so a general sweep of the block chain by a password cracker doesn't pick you up. The 16 to 20 truly random part gives you over 100 bits of entropy. Not ironclad but good.

You know you will forget the password unless you store it in a safe place. Why not just export a secret key generated by a good computer algorithm to paper?

I am making metal "brain" wallets. In copper and brass. 2 copies. one for backup. Fire proof Wink

Stamping the chars in the metal takes time so I want to keep it to a minimum.

Use a random selection of upper case, lower case, numbers, punctuation and other symbols. This will significantly decrease the odds of a rainbow table having your chosen string.

I think that gives you about 94 unique characters to select from. The number of possible combinations can then be computed as 94^x where x is the number of characters in your passphrase.

You'll have to make your own predictions about how fast brute forcing is likely to get in the future, but 350 billion attempts per second for the present is a good number to start with.

If you want to make sure that an attacker has less than a 1% chance of stumbling on your passphrase within 10 years, figure about 1.1x10²⁰ attempts in that time frame, so you'll want at least 1.1x10²² combinations.

94¹² = 4.76x10²³

If I haven't messed up my math anywhere (and that is definitely a possibility), it looks like you'll want at least 12 characters. Scale that up however you'd like to account for future increases in cracking speeds and reduction in chance of collision.