<p>It is all a matter of how many bits of entropy your passphrase has (how hard it would be to guess). Using a couple words provides a lot more randomness than a couple random characters.</p>

<p>As far as I remember, the algorithm used to encrypt the private keys that is used by BitcoinQT has about 100 bits of security (which is a lot). You would want to attain a similar amount of security with your passphrase.</p>

<p>The english language has <a href="http://oxforddictionaries.com/words/how-many-words-are-there-in-the-english-language" rel="nofollow">about 171000 words</a> in it. But lets face it, <a href="http://www.antimoon.com/forum/t10694.htm" rel="nofollow">most people won't use more than 60000</a>. Assuming that your phrase would consist of lets say only the 10000 most popular words, you would need log_10000(2^100) words to achieve 100 bits of security, or <a href="http://www.wolframalpha.com/input/?i=log_10000%282%5E100%29" rel="nofollow">about 7.5</a>.</p>

<p>So using 8 words should be enough to secure your coins for a couple decades at least (assuming the current rate of computational speed growth). However, once the quantum computers enter the picture and become powerful enough, all guesses made now can be tossed out.</p>

CC BY-SA 3.0
2013-01-07T11:34:23.900Z
Helpful. For reference, see [Thomas' post](http://crypto.stackexchange.com/a/5928/4690) too!
superuser
CC BY-SA 3.0
2013-01-07T10:22:32.013Z
2013-01-07T10:22:32.013Z
ThePiachu
<p>It seems to me that simply doubling your passphrase is sheer folly.  Programs that try to guess passwords don't have to be super-smart to know that humans will often choose passwords with repetitions in them.  In this brave new world, your old standards of password strength will just not be good enough.  If you really want to be secure for many years to come, then you should take the advice of Satoshi -- he gave us the 256-bit private key because he believed that it would be strong enough to last a lifetime.  If you use a key derived from a memorable pass-phrase ( even if it is fairly long ), then you are not taking advantage of the full 256-bits that Satoshi gave us.  A pass-phrase that you think is pretty strong might last for a few years, and give you a false sense of security, until one day your money just disappears.  That's the problem -- you will not have any warning until it is too late.  I would not trust any long-term money to any pass-phrase, no matter how strong it seems to be.  It's only a matter of time before you are hacked.</p>

CC BY-SA 3.0
2014-03-16T04:20:57.397Z
2014-03-16T04:20:57.397Z
Nick Perkins
<p>I made a cool 5 word passphrase back then using the old <a href="http://world.std.com/~reinhold/diceware.html" rel="nofollow">Diceware method</a> and use it as a master password. The question is: as computing power increases, will we need to add more and more words to our passphrases which we will eventually forget? I'm in my mid 30s, will passphrases be enough in my lifetime? :) So back then, Diceware suggested just 5 words, not the original Bitcoin QT client recommends 8 words. Will the original 5 words from the Diceware age serve me in the Bitcoin age?</p>

<p>I don't really want to mess around with a Yubikey. Heck, I Googled it and didn't really managed to find out what it is, what it does. Maybe I'm not the target market. I'm just a simple user. Carrying around a Yubikey would also be a red flag in my eyes. He has secrets!</p>

<p>How effective is doubling a passphrase? Like I have now [passphrase] and simply doule it: [passphrase][passphrase]. So I don't really have more stuff to remember, just more stuff to type. That might serve me for a time.</p>

CC BY-SA 3.0
2013-01-07T11:56:43.607Z
While yubikeys are not very common (but not uncommon either!), if you have a smartphone you could use a google authenticator, which is an app, so it could go unnoticed.
o0'.
CC BY-SA 3.0
2014-03-16T13:03:50.607Z
Obligatory XKCD reference
http://www.explainxkcd.com/wiki/index.php?title=936:_Password_Strength
Joe Pineda
<security><passphrase>
Ideal passphrase length: old diceware method (5 words) vs. your Bitcoin wallet.dat passphrase length (8 words) and doubling passwords?
superuser
# Ideal passphrase length: old diceware method (5 words) vs. your Bitcoin wallet.dat passphrase length (8 words) and doubling passwords?

(Asked by: username_0 on 2013-01-07)

I made a cool 5 word passphrase back then using the old [Diceware method](http://world.std.com/~reinhold/diceware.html) and use it as a master password. The question is: as computing power increases, will we need to add more and more words to our passphrases which we will eventually forget? I'm in my mid 30s, will passphrases be enough in my lifetime? :) So back then, Diceware suggested just 5 words, not the original Bitcoin QT client recommends 8 words. Will the original 5 words from the Diceware age serve me in the Bitcoin age?

I don't really want to mess around with a Yubikey. Heck, I Googled it and didn't really managed to find out what it is, what it does. Maybe I'm not the target market. I'm just a simple user. Carrying around a Yubikey would also be a red flag in my eyes. He has secrets!

How effective is doubling a passphrase? Like I have now [passphrase] and simply doule it: [passphrase][passphrase]. So I don't really have more stuff to remember, just more stuff to type. That might serve me for a time.

**Tags:** security, passphrase

> **Comments:**
> **username_1**: While yubikeys are not very common (but not uncommon either!), if you have a smartphone you could use a google authenticator, which is an app, so it could go unnoticed.  
> **username_2**: Obligatory XKCD reference http://www.explainxkcd.com/wiki/index.php?title=936:_Password_Strength  

---

## 2 answers

### Answer by username_3 on 2013-01-07. Score: 2

It is all a matter of how many bits of entropy your passphrase has (how hard it would be to guess). Using a couple words provides a lot more randomness than a couple random characters.

As far as I remember, the algorithm used to encrypt the private keys that is used by BitcoinQT has about 100 bits of security (which is a lot). You would want to attain a similar amount of security with your passphrase.

The english language has [about 171000 words](http://oxforddictionaries.com/words/how-many-words-are-there-in-the-english-language) in it. But lets face it, [most people won't use more than 60000](http://www.antimoon.com/forum/t10694.htm). Assuming that your phrase would consist of lets say only the 10000 most popular words, you would need log\_10000(2^100) words to achieve 100 bits of security, or [about 7.5](http://www.wolframalpha.com/input/?i=log_10000%282%5E100%29).

So using 8 words should be enough to secure your coins for a couple decades at least (assuming the current rate of computational speed growth). However, once the quantum computers enter the picture and become powerful enough, all guesses made now can be tossed out.

> **Comments:**
> **username_0**: Helpful. For reference, see [Thomas' post](http://crypto.stackexchange.com/a/5928/4690) too!  

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### Answer by username_4 on 2014-03-16. Score: 0

It seems to me that simply doubling your passphrase is sheer folly. Programs that try to guess passwords don't have to be super-smart to know that humans will often choose passwords with repetitions in them. In this brave new world, your old standards of password strength will just not be good enough. If you really want to be secure for many years to come, then you should take the advice of Satoshi -- he gave us the 256-bit private key because he believed that it would be strong enough to last a lifetime. If you use a key derived from a memorable pass-phrase ( even if it is fairly long ), then you are not taking advantage of the full 256-bits that Satoshi gave us. A pass-phrase that you think is pretty strong might last for a few years, and give you a false sense of security, until one day your money just disappears. That's the problem -- you will not have any warning until it is too late. I would not trust any long-term money to any pass-phrase, no matter how strong it seems to be. It's only a matter of time before you are hacked.


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