Implement the kms.Decrypter with PKCS#11

This interface allows the use of SCEP with PKCS#11 modules.

kms/pkcs11/pkcs11.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"crypto"
 	"crypto/elliptic"
+	"crypto/rsa"
 	"crypto/x509"
 	"encoding/hex"
 	"fmt"
@@ -142,8 +143,7 @@ func (k *PKCS11) CreateKey(req *apiv1.CreateKeyRequest) (*apiv1.CreateKeyRespons
 	}, nil
 }
 
-// CreateSigner creates a signer using the key present in the PKCS#11 MODULE signature
-// slot.
+// CreateSigner creates a signer using a key present in the PKCS#11 module.
 func (k *PKCS11) CreateSigner(req *apiv1.CreateSignerRequest) (crypto.Signer, error) {
 	if req.SigningKey == "" {
 		return nil, errors.New("createSignerRequest 'signingKey' cannot be empty")
@@ -157,6 +157,27 @@ func (k *PKCS11) CreateSigner(req *apiv1.CreateSignerRequest) (crypto.Signer, er
 	return signer, nil
 }
 
+// CreateDecrypter creates a decrypter using a key present in the PKCS#11
+// module.
+func (k *PKCS11) CreateDecrypter(req *apiv1.CreateDecrypterRequest) (crypto.Decrypter, error) {
+	if req.DecryptionKey == "" {
+		return nil, errors.New("createDecrypterRequest 'decriptionKey' cannot be empty")
+	}
+
+	signer, err := findSigner(k.p11, req.DecryptionKey)
+	if err != nil {
+		return nil, errors.Wrap(err, "createDecrypterRequest failed")
+	}
+
+	// Only RSA keys will implement the Decrypter interface.
+	if _, ok := signer.Public().(*rsa.PublicKey); ok {
+		if dec, ok := signer.(crypto.Decrypter); ok {
+			return dec, nil
+		}
+	}
+	return nil, errors.New("createDecrypterRequest failed: signer does not implement crypto.Decrypter")
+}
+
 // LoadCertificate implements kms.CertificateManager and loads a certificate
 // from the YubiKey.
 func (k *PKCS11) LoadCertificate(req *apiv1.LoadCertificateRequest) (*x509.Certificate, error) {

kms/pkcs11/pkcs11_test.go

@@ -4,6 +4,7 @@
 package pkcs11
 
 import (
+	"bytes"
 	"context"
 	"crypto"
 	"crypto/ecdsa"
@@ -491,6 +492,86 @@ func TestPKCS11_CreateSigner(t *testing.T) {
 	}
 }
 
+func TestPKCS11_CreateDecrypter(t *testing.T) {
+	k := setupPKCS11(t)
+	data := []byte("buggy-coheir-RUBRIC-rabbet-liberal-eaglet-khartoum-stagger")
+
+	type args struct {
+		req *apiv1.CreateDecrypterRequest
+	}
+	tests := []struct {
+		name    string
+		args    args
+		wantErr bool
+	}{
+		{"RSA", args{&apiv1.CreateDecrypterRequest{
+			DecryptionKey: "pkcs11:id=7371;object=rsa-key",
+		}}, false},
+		{"RSA PSS", args{&apiv1.CreateDecrypterRequest{
+			DecryptionKey: "pkcs11:id=7372;object=rsa-pss-key",
+		}}, false},
+		{"ECDSA P256", args{&apiv1.CreateDecrypterRequest{
+			DecryptionKey: "pkcs11:id=7373;object=ecdsa-p256-key",
+		}}, true},
+		{"ECDSA P384", args{&apiv1.CreateDecrypterRequest{
+			DecryptionKey: "pkcs11:id=7374;object=ecdsa-p384-key",
+		}}, true},
+		{"ECDSA P521", args{&apiv1.CreateDecrypterRequest{
+			DecryptionKey: "pkcs11:id=7375;object=ecdsa-p521-key",
+		}}, true},
+		{"fail DecryptionKey", args{&apiv1.CreateDecrypterRequest{
+			DecryptionKey: "",
+		}}, true},
+		{"fail uri", args{&apiv1.CreateDecrypterRequest{
+			DecryptionKey: "https:id=7375;object=ecdsa-p521-key",
+		}}, true},
+		{"fail FindKeyPair", args{&apiv1.CreateDecrypterRequest{
+			DecryptionKey: "pkcs11:foo=bar",
+		}}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := k.CreateDecrypter(tt.args.req)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("PKCS11.CreateDecrypter() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if got != nil {
+				pub := got.Public().(*rsa.PublicKey)
+				// PKCS#1 v1.5
+				enc, err := rsa.EncryptPKCS1v15(rand.Reader, pub, data)
+				if err != nil {
+					t.Errorf("rsa.EncryptPKCS1v15() error = %v", err)
+					return
+				}
+				dec, err := got.Decrypt(rand.Reader, enc, nil)
+				if err != nil {
+					t.Errorf("PKCS1v15.Decrypt() error = %v", err)
+				} else if !bytes.Equal(dec, data) {
+					t.Errorf("PKCS1v15.Decrypt() failed got = %s, want = %s", dec, data)
+				}
+
+				// RSA-OAEP
+				enc, err = rsa.EncryptOAEP(crypto.SHA256.New(), rand.Reader, pub, data, []byte("label"))
+				if err != nil {
+					t.Errorf("rsa.EncryptOAEP() error = %v", err)
+					return
+				}
+				dec, err = got.Decrypt(rand.Reader, enc, &rsa.OAEPOptions{
+					Hash:  crypto.SHA256,
+					Label: []byte("label"),
+				})
+				if err != nil {
+					t.Errorf("RSA-OAEP.Decrypt() error = %v", err)
+				} else if !bytes.Equal(dec, data) {
+					t.Errorf("RSA-OAEP.Decrypt() RSA-OAEP failed got = %s, want = %s", dec, data)
+				}
+			}
+		})
+	}
+}
+
 func TestPKCS11_LoadCertificate(t *testing.T) {
 	k := setupPKCS11(t)