[Breaking change]: `CoseSigner.Key` may now be `null`

[PranavSenthilnathan]

Description

CoseSigner.Key may now be null. If CoseSigner is backed by an RSA or ECDSA key then CoseSigner.Key will continue to return the key and it will be non-null. However, when CoseSigner is backed by a key that doesn't derive from AsymmetricAlgorithm, like MLDsa (a new Post-Quantum Cryptography signing algorithm), CoseSigner.Key will be null.

Version

.NET 10 Preview 7

Previous behavior

CoseSigner.Key cannot be null. It had type AsymmetricAlgorithm.

New behavior

CoseSigner.Key can be null. It now has type AsymmetricAlgorithm?.

using RSA rsaKey = RSA.Create();

CoseSigner signer = new CoseSigner(rsaKey, RSASignaturePadding.Pss, HashAlgorithmName.SHA512);
// signer.Key is rsaKey here

// CoseKey is a new abstraction for all keys used in COSE
CoseKey coseKey = new CoseKey(rsaKey, RSASignaturePadding.Pss, HashAlgorithmName.SHA512);
signer = new CoseSigner(coseKey);
// signer.Key is rsaKey here

using MLDsa mldsa = MLDsa.GenerateKey(MLDsaAlgorithm.MLDsa44);

coseKey = new CoseKey(mldsa);
signer = new CoseSigner(coseKey);
// signer.Key is null here

Type of breaking change

• Binary incompatible: Existing binaries might encounter a breaking change in behavior, such as failure to load or execute, and if so, require recompilation.
• Source incompatible: When recompiled using the new SDK or component or to target the new runtime, existing source code might require source changes to compile successfully.
• Behavioral change: Existing binaries might behave differently at run time.

Reason for change

With the introduction of new signing algorithms such as ML-DSA, .NET has moved away from using AsymmetricAlgorithm as the universal base class for all asymmetric algorithms. Likewise, CoseSigner can now be constructed with a key that doesn't derive from AsymmetricAlgorithm. In this case CoseSigner.Key can't return a AsymmetricAlgorithm representing the underlying key and thus returns null instead.

This change was introduced in dotnet/runtime#115158.

Recommended action

CoseSigner.Key can still be used, but callers should handle null values. The new abstraction for COSE keys is CoseKey, so a more flexible approach is to use the new CoseSigner.CoseKey property to retrieve the key. This will work with all key types.

Feature area

Cryptography

Affected APIs

namespace System.Security.Cryptography.Cose
{
    public partial class CoseSigner
    {
-        public System.Security.Cryptography.AsymmetricAlgorithm Key { get; }
+        public System.Security.Cryptography.AsymmetricAlgorithm? Key { get; }
    }
}

[PranavSenthilnathan]

/cc @dotnet/area-system-security