# AWS CloudHSM key concepts
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The following are concepts to be aware of when working with keys in AWS CloudHSM.

**Token keys**  
Persistent keys that you create during key generate, import or unwrap operations. AWS CloudHSM synchronizes token keys across a cluster.

**Session keys**  
Ephemeral keys that exist only on one hardware security module (HSM) in the cluster. AWS CloudHSM does *not* synchronize session keys across a cluster.

**Client-side key synchronization**  
A client-side process that clones token keys you create during key generate, import or unwrap operations. You can make token keys more durable by running a cluster with a minimum of two HSMs.

**Server-side key synchronization**  
Periodically clones keys to every HSM in the cluster. Requires no management.

**Client key durability settings**  
Settings you configure on the client that impact key durability. These settings work differently in Client SDK 5 and Client SDK 3.   
+ In Client SDK 5, use this setting to run a single HSM cluster. 
+ In Client SDK 3, use this setting to specify the number of HSMs required for key creation operations to succeed.