TechnitiumSoftware/DnsServer · error · ArgumentException

The ECDSA ({(keyType == DnssecPrivateKeyType.KeySigningKey ?

Error message

The ECDSA ({(keyType == DnssecPrivateKeyType.KeySigningKey ? "KSK" : "ZSK")}) private key must have key size of 384 bits.

What it means

Thrown by DnssecPrivateKey.Create (PEM overload) for DnssecAlgorithm.ECDSAP384SHA384 when the imported ECDsa key's KeySize is not exactly 384 bits. As with the P-256 case, the curve fixes the size and any mismatch indicates a PEM for a different curve.

Source

Thrown at DnsServerCore/Dns/Dnssec/DnssecPrivateKey.cs:255

                case DnssecAlgorithm.ECDSAP256SHA256:
                    using (ECDsa ecdsa = ECDsa.Create())
                    {
                        ecdsa.ImportFromPem(pemPrivateKey);

                        if (ecdsa.KeySize != 256)
                            throw new ArgumentException($"The ECDSA ({(keyType == DnssecPrivateKeyType.KeySigningKey ? "KSK" : "ZSK")}) private key must have key size of 256 bits.", nameof(pemPrivateKey));

                        return new DnssecEcdsaPrivateKey(algorithm, keyType, ecdsa.ExportParameters(true));
                    }

                case DnssecAlgorithm.ECDSAP384SHA384:
                    using (ECDsa ecdsa = ECDsa.Create())
                    {
                        ecdsa.ImportFromPem(pemPrivateKey);

                        if (ecdsa.KeySize != 384)
                            throw new ArgumentException($"The ECDSA ({(keyType == DnssecPrivateKeyType.KeySigningKey ? "KSK" : "ZSK")}) private key must have key size of 384 bits.", nameof(pemPrivateKey));

                        return new DnssecEcdsaPrivateKey(algorithm, keyType, ecdsa.ExportParameters(true));
                    }

                case DnssecAlgorithm.ED25519:
                    using (PemReader pemReader = new PemReader(new StringReader(pemPrivateKey)))
                    {
                        if (pemReader.ReadObject() is not Ed25519PrivateKeyParameters privateKey)
                            throw new ArgumentException($"The EdDSA ({(keyType == DnssecPrivateKeyType.KeySigningKey ? "KSK" : "ZSK")}) private key must be for Ed25519 curve.", nameof(pemPrivateKey));

                        return new DnssecEddsaPrivateKey(keyType, privateKey);
                    }

                case DnssecAlgorithm.ED448:
                    using (PemReader pemReader = new PemReader(new StringReader(pemPrivateKey)))
                    {
                        if (pemReader.ReadObject() is not Ed448PrivateKeyParameters privateKey)
                            throw new ArgumentException($"The EdDSA ({(keyType == DnssecPrivateKeyType.KeySigningKey ? "KSK" : "ZSK")}) private key must be for Ed448 curve.", nameof(pemPrivateKey));

View on GitHub (pinned to d0484b6c1e)

Solutions

Regenerate a P-384 key: openssl ecparam -name secp384r1 -genkey -noout -out key.pem.
If the PEM is genuinely P-256, use DnssecAlgorithm.ECDSAP256SHA256 instead.
Align the DnssecAlgorithm with the PEM's curve.

Example fix

// before: pem is a P-256 key
var key = DnssecPrivateKey.Create(DnssecAlgorithm.ECDSAP384SHA384, kt, p256Pem); // throws

// after
var key = DnssecPrivateKey.Create(DnssecAlgorithm.ECDSAP256SHA256, kt, p256Pem);

Defensive patterns

Strategy: validation

Validate before calling

using var ecdsa = ECDsa.Create();
ecdsa.ImportFromPem(pem);
if (ecdsa.KeySize != 384)
    throw new InvalidOperationException($"ECDSA key is {ecdsa.KeySize} bits; ECDSAP384SHA384 requires 384 (P-384).");
var key = DnssecPrivateKey.Create(DnssecAlgorithm.ECDSAP384SHA384, keyType, pem);

Type guard

static bool PemIsP384(string pem)
{
    try { using var e = ECDsa.Create(); e.ImportFromPem(pem); return e.KeySize == 384; }
    catch { return false; }
}

Try / catch

try { return DnssecPrivateKey.Create(DnssecAlgorithm.ECDSAP384SHA384, keyType, pem); }
catch (ArgumentException ex) when (ex.ParamName == nameof(pem))
{ throw new InvalidOperationException("PEM is not a P-384 key; check the curve or use ECDSAP256SHA256.", ex); }

Prevention

Generate keys on secp384r1 for the P-384 algorithm.
Keep the DnssecAlgorithm and the PEM curve in lockstep.
Name key files by curve to prevent cross-use.

When it happens

Trigger: Calling Create(ECDSAP384SHA384, keyType, pem) where the PEM decodes to a non-P-384 EC key (e.g. P-256).

Common situations: Using a P-256 PEM with the P-384 algorithm; mislabeling key files during rotation.

Related errors

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/5b2f5afcc083323a. Report an issue: GitHub.