key is the write key as a ByteArray: a P-256 private key in PKCS#8 DER form.
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A dongle ships with the KeyNub write key, keys/keynub-shipping-writeauth.key.der in the SDK repository; LicDongleRotateWriteKey replaces it with one you hold.
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The write role lasts until LicDongleSessionClose.
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The call needs a session; open one with LicDongleSessionOpen.
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On failure the result is Failure["LicDongleError", ...] with "Status" (the C status code), "Operation" and "Detail".