This approach is considered legacy, deprecated and is not recommended any longer by SAP. I just include this here as a reference for those that cannot update.

Tx: SM30
Table VUSREXTID

External ID Type DN

Add a new entry. Switch to edit mode.

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A new entry for external Id is added by inserting the CN information the certificate has in its Subject line.

The subject line of a certificate can be read using OpenSSL. The Subject in the given example is:

Subject: C=DE, ST=BW, L=Karlsruhe, O=blog, CN=tobias

External ID: CN=tobias
User: TOBIAS
Activated: check

Result

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