Secure User Identification
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What is Secure User Identification (SUI) ?
Clients in the network are identified by a unique value called user hash. This user hash is stored in the preferences.dat file and is used to grant earned credits with other users. aMule (like eMule) can use an asymmetric encryption to avoid exploiting or manipulating other user hash values. The method uses a private and a public key to ensure a correct and unique identification on other clients.
Secure User Identification can be turned on in Preferences -> Connection (since aMule 2.x, version 2.0.3 has it under Preferences -> Security). It is recommended to use it.
The identification process
First aMule start
When you start aMule for the first time, it will create a 384-bit RSA private key (which will be stored in cryptkey.dat). This file should be kept forever, since losing it will make you lose all your credits.
When aMule connects to a client supporting SUI, it expects this client to send it's public key (it's user hash) along with a random number, while aMule will do right the same. If the public key it gets is unknown, it will store it in clients.met (only the public key, not the random number) so that it will be possible to identify that client next time it meets it.
When a client wants to identify to another client, it will send to this latter client a digital signature created from it's own private key, the public key of the other client and the random value the other client sent.
The other client will do the same.
Validating an identification
When the remote client get's this signature, it will check if it is created from the your public key and the random value it sent, and then check if it suits it's own private key. If both checks succeed, your client will be successfully identified on that client.