> Source: [sk174803](https://support.checkpoint.com/results/sk/sk174803)

# sk174803 - Site-to-Site VPN using a certificate fails on Main Mode packet 5 with "Invalid certificate" error

| Property | Value |
|----------|-------|
| Solution ID | sk174803 |
| Date Created | 2021-07-29 |
| Last Modified | 2026-09-02 |
| Technical Level | Advanced |
| Products | Security Gateway |
| Versions | R81.20 |

## Symptoms

- * Site-to-Site VPN negotiation fails after packet 5 is received with the error: "Invalid certificate".

* VPN debug ([sk180488](https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk180488)) shows in the $FWDIR/log/vpnd.elg file:  

  `
  fwValidatePath: bc->IsCA() == 0`  
  `
  ...`  
  `
  [tunnel] < FWIKE_MM_PACKET_5_EPILOGUE2 > Id = 409861`  
  `
  [tunnel] extended_log_info_build_reason_from_list: list is empty,`  
  `
  [tunnel] isakmpd_log: calling isakmpd_log with original reason=(Issuer "CA Name" is not a CA).`

## Cause

The issue arises because the certificate in question was issued for a host (end entity) rather than for a Certificate Authority (CA).

**Verification**   
To confirm this, follow these steps:

1. Open the `.cer` file associated with the certificate.
2. Navigate to the **Details** tab and locate the **Basic Constraints** field.

Issue Identified:

* Basic Constraints:
  * Subject Type = **End Entity**
  * Path Length Constraint = None

Expected Configuration:

* Basic Constraints:
  * Subject Type = **CA**
  * Path Length Constraint = None

## Solution

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