user257: It is possible to get it from here:
https://blockchain.info/de/q/pubkeyaddr
https://blockchain.info/q
Is it possible with the standard client? How?
I am aware that the blockchain only contains the pubkey after a tx from that address.

user30396: It is impossible.  Given an ECDSA (compressed 65Bytes or not 33Bytes) public-key K, a Bitcoin
address is generated using the cryptographic hash functions SHA-256 and RIPEMD-160.
The public-key is hashed twice: HASH160 = RIPEMD-160(SHA-256(K)). The Bitcoin address is computed directly from this HASH160 value as
base58(0x00 || HASH160 || bSHA-256(SHA-256(0x00 || HASH160))/2224c). "||=concatenation"


so it is impossible to reverse the hash to get the public key.

user52507: To be honest, it IS possible, but you need a signature made by that address. From that point, you can get the public key.
See this piece of code: https://github.com/bitcoinjs/bitcoinjs-message/blob/master/index.js#L57

user208: It is impossible to compute the public key of an address, as the address is computed from the hash of the public key. You can retrieve the public key from address with the reference client using the validateaddress RPC call (or in the debug window of Bitcoin-Qt), but that simply fetches it from the wallet, and only works if the address belongs to you.
Update: you need to use getaddressinfo now instead of validateaddress.

257: <p>It is possible to get it from here:
<a href="https://blockchain.info/de/q/pubkeyaddr">https://blockchain.info/de/q/pubkeyaddr</a>
<a href="https://blockchain.info/q">https://blockchain.info/q</a></p>

<p>Is it possible with the standard client? How?</p>

<p>I am aware that the blockchain only contains the pubkey after a tx from that address.</p>
 30396: <p>It is impossible.  Given an ECDSA (compressed 65Bytes or not 33Bytes) public-key K, a Bitcoin
address is generated using the cryptographic hash functions SHA-256 and RIPEMD-160.
The public-key is hashed twice: HASH160 = RIPEMD-160(SHA-256(K)). The Bitcoin address is computed directly from this HASH160 value as</p>

<pre><code>base58(0x00 || HASH160 || bSHA-256(SHA-256(0x00 || HASH160))/2224c). "||=concatenation"
</code></pre>

<p><a href="https://i.stack.imgur.com/AcXYt.png" rel="noreferrer"><img src="https://i.stack.imgur.com/AcXYt.png" alt="enter image description here"></a></p>

<p>so it is impossible to reverse the hash to get the public key.</p>
 52507: <p>To be honest, it IS possible, but you need a signature made by that address. From that point, you can get the public key.</p>

<p>See this piece of code: <a href="https://github.com/bitcoinjs/bitcoinjs-message/blob/master/index.js#L57" rel="noreferrer">https://github.com/bitcoinjs/bitcoinjs-message/blob/master/index.js#L57</a></p>
 208: <p>It is impossible to compute the public key of an address, as the address is computed from the hash of the public key. You can retrieve the public key from address with the reference client using the <code>validateaddress</code> RPC call (or in the debug window of Bitcoin-Qt), but that simply fetches it from the wallet, and only works if the address belongs to you.</p>
<p>Update: you need to use <code>getaddressinfo</code> now instead of <code>validateaddress</code>.</p>
