[client, management] Fix the edge cases left open in the certificate posture check (#8174)

shared/management/certposture/certposture_test.go

@@ -2,6 +2,8 @@ package certposture
 
 import (
 	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
 	"crypto/sha256"
@@ -86,6 +88,7 @@ func TestSignVerify_RoundTripPerKeyType(t *testing.T) {
 	ca := certtest.NewCA(t, "root")
 	keys := map[string]crypto.Signer{
 		SigAlgECDSASHA256:  certtest.ECDSAKey(t),
+		SigAlgECDSASHA384:  p384Key(t),
 		SigAlgRSAPSSSHA256: certtest.RSAKey(t),
 		SigAlgEd25519:      certtest.Ed25519Key(t),
 	}
@@ -103,6 +106,24 @@ func TestSignVerify_RoundTripPerKeyType(t *testing.T) {
 	}
 }
 
+func p384Key(t *testing.T) crypto.Signer {
+	t.Helper()
+	key, err := ecdsa.GenerateKey(elliptic.P384(), rand.Reader)
+	require.NoError(t, err)
+	return key
+}
+
+// TestSign_RefusesCurvesTheProofDoesNotName: a proof names its algorithm from the key, and
+// the verifier only knows P-256 and P-384, so P-521 is refused before anything is signed.
+func TestSign_RefusesCurvesTheProofDoesNotName(t *testing.T) {
+	key, err := ecdsa.GenerateKey(elliptic.P521(), rand.Reader)
+	require.NoError(t, err)
+
+	_, _, err = Sign(key, NewChallenger(secret).Nonce(peerKey, now), peerKey)
+
+	assert.ErrorIs(t, err, ErrUnsupportedKey)
+}
+
 func TestVerify_Rejections(t *testing.T) {
 	ca := certtest.NewCA(t, "root")
 	c := NewChallenger(secret)