# No exception thrown in CKKS when firstModSize is smaller than scalingModSize

**URL:** <https://openfhe.discourse.group/t/no-exception-thrown-in-ckks-when-firstmodsize-is-smaller-than-scalingmodsize/2046>\
**Category:** Library Questions\
**Created:** [May 29, 2025, 10:35am UTC](https://openfhe.discourse.group/t/no-exception-thrown-in-ckks-when-firstmodsize-is-smaller-than-scalingmodsize/2046 "2025-05-29T10:35:11Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![hyerinpark](https://avatars.discourse-cdn.com/v4/letter/h/bc79bd/32.png) [@hyerinpark](https://openfhe.discourse.group/u/hyerinpark)\
**Post date:** [May 29, 2025, 10:35am UTC](https://openfhe.discourse.group/t/no-exception-thrown-in-ckks-when-firstmodsize-is-smaller-than-scalingmodsize/2046/1 "2025-05-29T10:35:11Z")

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Hello,

Sorry again for another report related to using an invalid context in OpenFHE. I understand these cases can be addressed under AAHE, but I wanted to share one more scenario in CKKS where an invalid context — **specifically, when `firstModSize` is smaller than `scalingModSize`** — does not throw any exceptions and instead leads to silently incorrect results.

Here are two cases of getting incorrect values under insufficient `firstModSize`.

### Case 1: Decrypting immediately after encryption gives slightly incorrect values

```auto
#include <iostream>
#include <chrono>

#include <iomanip>

#include "openfhe.h"
#include "../functional_units/functional_units.hpp"

using namespace lbcrypto;
using namespace std;

int main(void) {
  CCParams<CryptoContextCKKSRNS> parameters;
  parameters.SetRingDim(32768);
  parameters.SetMultiplicativeDepth(3);
  parameters.SetFirstModSize(54);
  parameters.SetScalingModSize(59);
  parameters.SetSecurityLevel(HEStd_256_classic);
  parameters.SetScalingTechnique(FLEXIBLEAUTO);
  
  CryptoContext<DCRTPoly> cc = GenCryptoContext(parameters);
  cc->Enable(PKE);
  cc->Enable(KEYSWITCH);
  cc->Enable(LEVELEDSHE);
  KeyPair<DCRTPoly> keyPair;
  keyPair = cc->KeyGen();
  cc->EvalMultKeyGen(keyPair.secretKey);
  size_t slots(7098);
  vector<complex<double>> tmp_vec_(7098);
  Plaintext tmp;
  Ciphertext<DCRTPoly> tmp_;

  Ciphertext<DCRTPoly> x, y;
  double yP;
  double yC;
  int c;
  vector<double> tmp_vec_1 = { 3612029277224623.500000, 2173980662796596.200000, 4324895368175417.000000 };
  tmp = cc->MakeCKKSPackedPlaintext(tmp_vec_1);
  x = cc->Encrypt(keyPair.publicKey, tmp);
  cc->Decrypt(keyPair.secretKey, x, &tmp);
  tmp->SetLength(20);
  tmp_vec_ = tmp->GetCKKSPackedValue();
  for (int64_t tmp_i = 0; tmp_i < 5; ++tmp_i) {
    cout << fixed << setprecision(5) << real(tmp_vec_[tmp_i]) << " ";
  }
  cout << endl;
  return 0;
}

```

- **Context issue** : `firstModSize` = 54, `scalingModSize` = 59
- **Observation** : Even without any operations, the decrypted data is already slightly different from the original.
  - Expected: `3612029277224623.500000 2173980662796596.200000 4324895368175417.000000 0.000000 0.000000`
  - Output: `3612029277224624.00000 2173980662796595.50000 4324895368175416.50000 0.02938 -0.04829`

- **Note** : Since the initial data is already corrupted, further operations only amplify the errors.

### Case 2: Values break after the last multiplication

```auto
#include <iostream>
#include <chrono>

#include <iomanip>

#include "openfhe.h"
#include "../functional_units/functional_units.hpp"

using namespace lbcrypto;
using namespace std;

int main(void) {
  CCParams<CryptoContextCKKSRNS> parameters;
  parameters.SetRingDim(16384);
  parameters.SetMultiplicativeDepth(1);
  parameters.SetFirstModSize(40);
  parameters.SetScalingModSize(59);
  parameters.SetSecurityLevel(HEStd_NotSet);
  parameters.SetScalingTechnique(FLEXIBLEAUTO);
  
  CryptoContext<DCRTPoly> cc = GenCryptoContext(parameters);
  cc->Enable(PKE);
  cc->Enable(KEYSWITCH);
  cc->Enable(LEVELEDSHE);
  KeyPair<DCRTPoly> keyPair;
  keyPair = cc->KeyGen();
  cc->EvalMultKeyGen(keyPair.secretKey);
  size_t slots(897);
  vector<complex<double>> tmp_vec_(897);
  Plaintext tmp;
  Ciphertext<DCRTPoly> tmp_;

  Ciphertext<DCRTPoly> x, y;
  double yP;
  double yC;
  int c;
  vector<double> tmp_vec_1 = { 1.177856 };
  tmp = cc->MakeCKKSPackedPlaintext(tmp_vec_1);
  x = cc->Encrypt(keyPair.publicKey, tmp);
  vector<double> tmp_vec_2 = { 1.759079 };
  tmp = cc->MakeCKKSPackedPlaintext(tmp_vec_2);
  y = cc->Encrypt(keyPair.publicKey, tmp);
  x = cc->EvalMultNoRelin(x, y);
  cc->RescaleInPlace(x);
  cc->RelinearizeInPlace(x);
  cc->Decrypt(keyPair.secretKey, x, &tmp);
  tmp->SetLength(5);
  tmp_vec_ = tmp->GetCKKSPackedValue();
  for (int64_t tmp_i = 0; tmp_i < 5; ++tmp_i) {
    cout << fixed << setprecision(5) << real(tmp_vec_[tmp_i]) << " ";
  }
  cout << endl;
  return 0;
}

```

- **Context issue** : `firstModSize` = 40, `scalingModSize` = 59
- **Observation** : Unlike Case 1, simple encryption and decryption don’t show a problem. But applying the allowed number of multiplication, the value breaks.
  - Expected: `2.071942 0.000000 0.000000 0.000000 0.000000`
  - Output: `-0.00009 -0.00009 -0.00020 0.00014 0.00002`

- **Note** : I tested with different multiplicative depths, and in every case, the result becomes incorrect **exactly at the n-th multiplication** , where _n_ is the multiplicative depth.

Again, I realize that this kind of invalid context may eventually be handled via AAHE. But if this particular check is already implemented and I simply missed it, I’d appreciate it if you could point me to the relevant reference.

\_\_  
Thanks as always for your support!

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<div class="post-metadata">

**Author:** ![ypolyakov](https://yyz1.discourse-cdn.com/flex031/user_avatar/openfhe.discourse.group/ypolyakov/32/47_2.png) [@ypolyakov](https://openfhe.discourse.group/u/ypolyakov)\
**Post date:** [June 3, 2025, 4:11pm UTC](https://openfhe.discourse.group/t/no-exception-thrown-in-ckks-when-firstmodsize-is-smaller-than-scalingmodsize/2046/2 "2025-06-03T16:11:25Z")

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Thank you reporting this. We can certainly throw an exception in this case. I’ve created an issue for this: [Improve exception handling for selected scenarios leading to incorrect results · Issue #995 · openfheorg/openfhe-development · GitHub](https://github.com/openfheorg/openfhe-development/issues/995)

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<div class="post-metadata">

**Author:** ![ypolyakov](https://yyz1.discourse-cdn.com/flex031/user_avatar/openfhe.discourse.group/ypolyakov/32/47_2.png) [@ypolyakov](https://openfhe.discourse.group/u/ypolyakov)\
**Post date:** [July 12, 2025, 12:35am UTC](https://openfhe.discourse.group/t/no-exception-thrown-in-ckks-when-firstmodsize-is-smaller-than-scalingmodsize/2046/3 "2025-07-12T00:35:22Z")

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The exception handling has been added in v1.3.1.