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.

Capture the rejected proposal before editing policy so the incompatible algorithm is known. This also shows whether an unexpected local or domain policy narrowed the algorithm set after the peer configuration was last tested.

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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