Return the proper algorithm for ECDSA keys (go-jose#45)

Each ECDSA key type can only use a single signature algorithm. The
previous version returned ES256, ES384, and ES512 for all ECDSA
keys. Now we only return the one algorithm which matches the actual
key type.

This fixes the "This could be more precise" comment.

cryptosigner/cryptosigner.go

@@ -23,6 +23,7 @@ import (
 	"crypto"
 	"crypto/ecdsa"
 	"crypto/ed25519"
+	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
 	"encoding/asn1"
@@ -51,12 +52,20 @@ func (s *cryptoSigner) Public() *jose.JSONWebKey {
 }
 
 func (s *cryptoSigner) Algs() []jose.SignatureAlgorithm {
-	switch s.signer.Public().(type) {
+	switch key := s.signer.Public().(type) {
 	case ed25519.PublicKey:
 		return []jose.SignatureAlgorithm{jose.EdDSA}
 	case *ecdsa.PublicKey:
-		// This could be more precise
-		return []jose.SignatureAlgorithm{jose.ES256, jose.ES384, jose.ES512}
+		switch key.Curve {
+		case elliptic.P256():
+			return []jose.SignatureAlgorithm{jose.ES256}
+		case elliptic.P384():
+			return []jose.SignatureAlgorithm{jose.ES384}
+		case elliptic.P521():
+			return []jose.SignatureAlgorithm{jose.ES512}
+		default:
+			return nil
+		}
 	case *rsa.PublicKey:
 		return []jose.SignatureAlgorithm{jose.RS256, jose.RS384, jose.RS512, jose.PS256, jose.PS384, jose.PS512}
 	default:

cryptosigner/cryptosigner_test.go

@@ -24,7 +24,10 @@ import (
 	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
+	"errors"
 	"fmt"
+	"io"
+	"reflect"
 	"testing"
 
 	"github.com/go-jose/go-jose/v3"
@@ -135,3 +138,74 @@ func generateSigningTestKey(sigAlg jose.SignatureAlgorithm) (sig, ver interface{
 	}
 	return
 }
+
+type fakeSigner struct{}
+
+func (fakeSigner) Public() crypto.PublicKey {
+	return []byte("fake-key")
+}
+
+func (fakeSigner) Sign(rand io.Reader, digest []byte, opts crypto.SignerOpts) ([]byte, error) {
+	return nil, errors.New("not a signer")
+}
+
+func Test_cryptoSigner_Algs(t *testing.T) {
+	_, edKey, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	rsaKey, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	p224, err := ecdsa.GenerateKey(elliptic.P224(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	p256, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	p384, err := ecdsa.GenerateKey(elliptic.P384(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	p521, err := ecdsa.GenerateKey(elliptic.P521(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	type fields struct {
+		signer crypto.Signer
+	}
+
+	tests := []struct {
+		name   string
+		fields fields
+		want   []jose.SignatureAlgorithm
+	}{
+		{"EdDSA", fields{edKey}, []jose.SignatureAlgorithm{jose.EdDSA}},
+		{"ES256", fields{p256}, []jose.SignatureAlgorithm{jose.ES256}},
+		{"ES384", fields{p384}, []jose.SignatureAlgorithm{jose.ES384}},
+		{"ES512", fields{p521}, []jose.SignatureAlgorithm{jose.ES512}},
+		{"RSA", fields{rsaKey}, []jose.SignatureAlgorithm{jose.RS256, jose.RS384, jose.RS512, jose.PS256, jose.PS384, jose.PS512}},
+		{"fail P-224", fields{p224}, nil},
+		{"fail other", fields{fakeSigner{}}, nil},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cs := &cryptoSigner{
+				signer: tt.fields.signer,
+			}
+			if got := cs.Algs(); !reflect.DeepEqual(tt.want, got) {
+				t.Errorf("cryptoSigner.Algs() got = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}