# Unexpected result after CKKS-to-FHEW switching with multiplicativeDepth=1 and FIXEDAUTO

**URL:** https://openfhe.discourse.group/t/unexpected-result-after-ckks-to-fhew-switching-with-multiplicativedepth-1-and-fixedauto/2380
**Category:** Bug Reports
**Created:** [September 8, 2026, 3:55pm UTC](https://openfhe.discourse.group/t/unexpected-result-after-ckks-to-fhew-switching-with-multiplicativedepth-1-and-fixedauto/2380 "2026-09-08T15:55:41Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![takataka](https://avatars.discourse-cdn.com/v4/letter/t/f04885/32.png) [@takataka](https://openfhe.discourse.group/u/takataka)
#### Post date: [September 8, 2026, 3:55pm UTC](https://openfhe.discourse.group/t/unexpected-result-after-ckks-to-fhew-switching-with-multiplicativedepth-1-and-fixedauto/2380/1 "2026-09-08T15:55:41Z")

</div>

”Unexpected result after CKKS-to-FHEW switching with multiplicativeDepth=1 and FIXEDAUTO”

Hello OpenFHE developers,

I am testing CKKS-to-FHEW scheme switching with the following configuration:

- OpenFHE version/commit: OpenFHE 1.5.1, commit ed361af22049007db2107e7c69bcff209e8c420d (git describe: v1.5.1-1-ged361af2)

- OS and compiler: Ubuntu 24.04.4 LTS (x86\_64, Linux kernel 6.8.0-138-generic), GCC/G++ 13.3.0

- CKKS security level: `HEStd_128_classic`

- FHEW security level: `STD128`

- `multiplicativeDepth = 1`

- `scalingTechnique = FIXEDAUTO`

- `firstModSize = 40`

- `scalingModSize = 20`

- Actual ring dimension: `8192`

- Batch size: `4096`

- Number of converted slots: `900`

- `logQ_ccLWE = 15`

- LWE plaintext modulus: `pLWE = 128`

- CKKS-to-FHEW precomputation scale: `1.0 / pLWE`

As a minimal test, I pack `10.0` into 900 slots of each of two CKKS ciphertexts and add them using `EvalAdd`. The expected value in each slot is therefore `20.0`.

CKKS decryption immediately after the addition gives approximately `20.0004`. However, after `EvalCKKStoFHEW` and FHEW decryption modulo 128, all 900 converted values in this run are `19` rather than `20`.

I have the following questions:

1. Is CKKS-to-FHEW scheme switching expected to support `multiplicativeDepth = 1` with `FIXEDAUTO` when the CKKS computation before switching contains only an addition?

2. Is a one-unit difference such as `19` instead of `20` an expected approximation or rounding effect, or does it indicate a scaling or modulus configuration problem?

3. Are there documented compatibility constraints or recommended ranges involving `firstModSize`, `scalingModSize`, the CKKS modulus, `logQ_ccLWE`, and `pLWE`?

4. If this result is not expected, which part of the configuration should I investigate first?

Thank you.