diff --git a/pkcs/cipher.go b/pkcs/cipher.go
index 6f8ce33..5f5e665 100644
--- a/pkcs/cipher.go
+++ b/pkcs/cipher.go
@@ -169,10 +169,13 @@ type gcmBlockCipher struct {
 	nonceSize int
 }
 
-// http://javadoc.iaik.tugraz.at/iaik_jce/current/index.html?iaik/security/cipher/GCMParameters.html
+// https://datatracker.ietf.org/doc/rfc5084/
+// GCMParameters ::= SEQUENCE {
+// 	aes-nonce        OCTET STRING, -- recommended size is 12 octets
+// 	aes-ICVlen       AES-GCM-ICVlen DEFAULT 12 }
 type gcmParameters struct {
-	Nonce  []byte `asn1:"tag:4"`
-	ICVLen int
+	Nonce  []byte
+	ICVLen int `asn1:"default:12,optional"`
 }
 
 func (c *gcmBlockCipher) Encrypt(key, plaintext []byte) (*pkix.AlgorithmIdentifier, []byte, error) {
@@ -222,7 +225,7 @@ func (c *gcmBlockCipher) Decrypt(key []byte, parameters *asn1.RawValue, encrypte
 		return nil, err
 	}
 	if params.ICVLen != aead.Overhead() {
-		return nil, errors.New("pkcs: invalid tag size")
+		return nil, errors.New("pkcs: we do not support non-standard tag size")
 	}
 
 	return aead.Open(nil, params.Nonce, encryptedKey, nil)
diff --git a/pkcs/cipher_test.go b/pkcs/cipher_test.go
index 01de947..c1c9746 100644
--- a/pkcs/cipher_test.go
+++ b/pkcs/cipher_test.go
@@ -1,9 +1,13 @@
 package pkcs
 
 import (
+	"bytes"
 	"crypto/x509/pkix"
 	"encoding/asn1"
 	"testing"
+
+	"golang.org/x/crypto/cryptobyte"
+	cryptobyte_asn1 "golang.org/x/crypto/cryptobyte/asn1"
 )
 
 func TestGetCipher(t *testing.T) {
@@ -57,3 +61,27 @@ func TestInvalidKeyLen(t *testing.T) {
 		t.Errorf("should be error")
 	}
 }
+
+func TestGcmParameters(t *testing.T) {
+	var b cryptobyte.Builder
+	b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) {
+		b.AddASN1OctetString([]byte("123456789012"))
+	})
+	pb1, _ := b.Bytes()
+	params := gcmParameters{}
+	_, err := asn1.Unmarshal(pb1, &params)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if params.ICVLen != 12 {
+		t.Errorf("should be 12, but got %v", params.ICVLen)
+	}
+	if !bytes.Equal([]byte("123456789012"), params.Nonce) {
+		t.Errorf("not expected nonce")
+	}
+
+	pb2, _ := asn1.Marshal(params)
+	if !bytes.Equal(pb1, pb2) {
+		t.Errorf("not consistent result")
+	}
+}