The ML-DSA private key is missing

A call with CryptoMode.Hybrid or CryptoMode.PostQuantum throws a CryptographicException with one of these messages:

The ML-DSA private key of issuer 'CN=Contoso Hybrid CA' is not available.
No ML-DSA private key is available for signer 'CN=Contoso Accounting'; Hybrid signing requires one.
A PQ export was requested, but no ML-DSA private key is available for any certificate.
No ML-DSA key pair is available for 'CN=www.example.com'; a PQ store operation needs one.

Or GetAlternativePrivateMLDsaKey or GetPrivateMLDsaKey returns null.

All targets. Operations that sign with a certificate's private ML-DSA key are affected: issuing certificates and CRLs, signing CMS messages, exporting PFX files with a PQ key, and storing key files.

The .NET X509Certificate2 class doesn't carry an ML-DSA key. The library keeps the key in a process table, where the certificate finds it through its thumbprint. The entry is missing in these cases:

• The certificate was issued in a different process and loaded from a PEM or DER file. Such files don't contain a private key.
• The certificate was imported with the .NET new X509Certificate2(pfx, password) constructor. That path skips the PQ key in the PFX file.
• A pure ML-DSA certificate was issued from a request with Create. At first, it only knows the public key.
• The process restarted. The table lives only as long as the process.

Associate the key before you call the operation. How you do that depends on where the key is:

Then check with GetAlternativePrivateMLDsaKey or GetPrivateMLDsaKey that the key is present. The .NET HasPrivateKey property only reflects the classical key. For details, see Keys and key storage.