> For the complete documentation index, see [llms.txt](https://docs.zama.org/concrete/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.zama.org/concrete/developers/contributing/backends.md).

# Compiler backend

The Concrete backends are implementations of the cryptographic primitives of the Zama variant of [TFHE](https://eprint.iacr.org/2018/421.pdf). The compiler emits code which combines call into these backends to perform more complex homomorphic operations.

There are client and server features.

Client features are:

* private (G)LWE key generation (currently random bits)
* encryption of ciphertexts using a private key
* public key generation from private keys for keyswitch, bootstrap or private packing
* (de)serialization of ciphertexts and public keys (also needed server side)

Server features are homomorphic operations on ciphertexts:

* linear operations (multisums with plain weights)
* keyswitch
* simple PBS
* WoP PBS

There are currently 2 backends:

* `concrete-cpu` which implements both client and server features targeting the CPU.
* `concrete-cuda` which implements only server features targeting GPUs to accelerate homomorphic circuit evaluation.

The compiler uses `concrete-cpu` for the client and can use either `concrete-cpu` or `concrete-cuda` for the server.


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