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You use the command line client, but you already have to have an account to do so (so someone must make your account for you, and they must spend money doing so). The command line client comes with steemd I believe.

Try @anonsteem.

I know about anonsteam already. But that's not what I need.

So basically any account on the network has to be registered by another account? Is it possible to generate the keys for the new account seperatly?

I'm not sure how the key registration works, but I presume that the account creator doesn't need to know the private keys being fed (public keys should be enough).

Ok, so what I imagine now is this:

  1. new user creates a keypair
  2. new user gives public key to existing user
  3. existing user registers public key and pays fee (BY paying the fee?)
  4. one of them (who?) registers steem user name to the public key
  5. new user can now start to use steem with the client of his or her choice

Do you know if there are any technical prerequisites to run the client (I'm presuming it's a linux program)?

It's Linux, you have to download and sync the blockchain to do it. I don't really know much else as I haven't tried in practice.

ah ok, the need to download the whole chain obviously is at least a small hurdle. :)

You don't need to download the whole chain to do that.

You need the cli_wallet.

https://steemit.com/steemhelp/@felixxx/5-sbd-bounty-how-to-use-and-set-up-cliwallet-to-change-my-passwords

In the comment section of my post, you can find an easy method.

You need 4 keys to create an account.

The command you are looking for is: create_account_with_keys

create_account will create an account with the keys stored in your wallet.