# Compressing ciphertexts on the GPU

This document explains how to compress ciphertexts using the GPU. Compression can be applied on freshly encrypted ciphertexts or on ciphertexts that are the result of FHE integer operations. The syntax for ciphertext compression is identical to the one for the [CPU backend](../../fhe-computation/data-handling/compress.md#compression-ciphertexts-after-some-homomorphic-computation), but cryptographic parameters specific to the GPU must be configured for compression.

## API elements discussed in this document

- [tfhe::shortint::parameters](https://docs.rs/tfhe/latest/tfhe/shortint/parameters/index.html): this module provides the structure containing the cryptographic parameters required for the homomorphic evaluation of integer circuits as well as a list of secure cryptographic parameter sets.
- [tfhe::ConfigBuilder::with_custom_parameters](https://docs.rs/tfhe/latest/tfhe/struct.ConfigBuilder.html#method.with_custom_parameters): initializes a configuration builder with a user-specified parameter set
- [tfhe::ConfigBuilder::enable_compression](https://docs.rs/tfhe/latest/tfhe/struct.ConfigBuilder.html#method.enable_compression): enables the compression feature in the configuration builder 

## Cryptographic parameter setting

When using compression, the [`ConfigBuilder`](https://docs.rs/tfhe/latest/tfhe/struct.ConfigBuilder.html) class must be initialized with the `enable_compression` calls. This requires that the caller sets both the cryptographic PBS parameters and the compression cryptographic parameters.

The [`PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS`](https://docs.rs/tfhe/latest/tfhe/shortint/parameters/aliases/constant.PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS.html) parameter set corresponds to the default PBS parameter set instantiated by `ConfigBuilder::default()` when the `"gpu"` feature enabled. The [`COMP_PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS`](https://docs.rs/tfhe/latest/tfhe/shortint/parameters/aliases/constant.COMP_PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS.html) parameters are the corresponding compression cryptographic parameters.

```Rust
    let config =
        tfhe::ConfigBuilder::with_custom_parameters(PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS)
            .enable_compression(COMP_PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS)
            .build();
```

## GPU compression Example

The following example shows how to compress and decompress a list containing 4 messages: one 32-bit integer, one 64-bit integer, one Boolean, and one 2-bit integer.

```rust
use tfhe::prelude::*;
use tfhe::shortint::parameters::{
    COMP_PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS, PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS,
};
use tfhe::{
    set_server_key, CompressedCiphertextList, CompressedCiphertextListBuilder, FheBool,
    FheInt64, FheUint2, FheUint32,
};

fn main() {
    let config =
        tfhe::ConfigBuilder::with_custom_parameters(PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS)
            .enable_compression(COMP_PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS)
            .build();

    let ck = tfhe::ClientKey::generate(config);
    let compressed_server_key = tfhe::CompressedServerKey::new(&ck);
    let gpu_key = compressed_server_key.decompress_to_gpu();

    set_server_key(gpu_key);

    let ct1 = FheUint32::encrypt(17_u32, &ck);

    let ct2 = FheInt64::encrypt(-1i64, &ck);

    let ct3 = FheBool::encrypt(false, &ck);

    let ct4 = FheUint2::encrypt(3u8, &ck);

    let compressed_list = CompressedCiphertextListBuilder::new()
        .push(ct1)
        .push(ct2)
        .push(ct3)
        .push(ct4)
        .build()
        .unwrap();

    let serialized = bincode::serialize(&compressed_list).unwrap();

    println!("Serialized size: {} bytes", serialized.len());

    let compressed_list: CompressedCiphertextList = bincode::deserialize(&serialized).unwrap();

    let a: FheUint32 = compressed_list.get(0).unwrap().unwrap();
    let b: FheInt64 = compressed_list.get(1).unwrap().unwrap();
    let c: FheBool = compressed_list.get(2).unwrap().unwrap();
    let d: FheUint2 = compressed_list.get(3).unwrap().unwrap();

    let a: u32 = a.decrypt(&ck);
    assert_eq!(a, 17);
    let b: i64 = b.decrypt(&ck);
    assert_eq!(b, -1);
    let c = c.decrypt(&ck);
    assert!(!c);
    let d: u8 = d.decrypt(&ck);
    assert_eq!(d, 3);

}
```
