user1580: How is wallet at official client encrypted? What encryption it use? 
What encryption does Bitcoin-qt use and what encryption is used at Multibit?
And how strong that encryption is compared to other methods?

user9175: Both bitcoind and bitcoinj use AES-256 encryption for the private keys in their wallets (this does not mean that the wallet formats are compatible though).
Bitcoin-Qt is just a GUI on top of bitcoind and Multibit is using bitcoinj internally, so the same should apply to them.
References:

comment in the bitcoind source code
bitcoinj documentation


1580: <p>How is wallet at official client encrypted? What encryption it use? </p>

<p>What encryption does Bitcoin-qt use and what encryption is used at Multibit?
And how strong that encryption is compared to other methods?</p>
 9175: <p>Both bitcoind and bitcoinj use AES-256 encryption for the private keys in their wallets (this does not mean that the wallet formats are compatible though).</p>

<p>Bitcoin-Qt is just a GUI on top of bitcoind and Multibit is using bitcoinj internally, so the same should apply to them.</p>

<p>References:</p>

<ul>
<li><a href="https://github.com/bitcoin/bitcoin/blob/master/src/crypter.h" rel="nofollow">comment in the bitcoind source code</a></li>
<li><a href="http://code.google.com/p/bitcoinj/wiki/WorkingWithTheWallet#Encrypting_the_private_keys" rel="nofollow">bitcoinj documentation</a></li>
</ul>
