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So in some cases, the tunnel may fail to establish and return 'signature verification failed' errors if the sha1 phase1 proposal is not chosen (depending on whether the remote end derives the hash algorithm from the chosen proposals or not).
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FortiGate does not derive this hash algorithm from the phase1 proposals and by default uses SHA-1 to avoid interoperability problems. However, this is not the case with FortiOS. Some vendors acquire this hash algorithm from the phase1 proposal being used. It generally suggests that there is a mismatch in the hash algorithm used for this signature generation. This error is seen if FortiGate is unable to validate the contents of an IKE AUTH payload from a peer (which is signed by the peer certificate) and is indicated as 'signature' in the IKE debugs on FortiGate. This article describes issues that occur during VPN establishment due to 'signature verification failed' errors in IKE debug logs for an IKEv2 certificate based IPsec VPN.
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