What does Windows error code 13873 (ERROR_IPSEC_IKE_INVALID_ENCRYPT_ALG) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: The IKE peer proposed an invalid or unsupported encryption algorithm.

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ERROR_IPSEC_IKE_INVALID_HASH_SIZE	ERROR_IPSEC_IKE_INVALID_AUTH_ALG

ERROR_IPSEC_IKE_INVALID_ENCRYPT_ALG

ERROR_IPSEC_IKE_INVALID_ENCRYPT_ALG (0x00003631) The encryption algorithm in the IKE proposal is not supported or is disallowed by the local policy. The error must be resolved by finding a mutually supported cryptographic suite.

What to check

Compare Main Mode encryption proposals on both peers.
Identify whether a recent policy hardening change disabled the only common algorithm.
Prefer upgrading or reconfiguring the peer to a supported suite rather than restoring deprecated algorithms globally.

Get-NetIPsecMainModeRule

Microsoft: Get-NetIPsecMainModeRule

Microsoft: Audit IPsec Main Mode

Microsoft: IPsec/IKE system error codes

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