diff --git a/pkg/crypto/key.go b/pkg/crypto/key.go
index d75d1ab..79e2046 100644
--- a/pkg/crypto/key.go
+++ b/pkg/crypto/key.go
@@ -4,14 +4,19 @@ import (
 	"crypto/rsa"
 	"crypto/x509"
 	"encoding/pem"
+	"errors"
 )
 
-func BytesToPrivateKey(priv []byte) (*rsa.PrivateKey, error) {
-	block, _ := pem.Decode(priv)
-	b := block.Bytes
-	key, err := x509.ParsePKCS1PrivateKey(b)
+func BytesToPrivateKey(b []byte) (*rsa.PrivateKey, error) {
+	block, _ := pem.Decode(b)
+	if block == nil {
+		return nil, errors.New("PEM decode failed")
+	}
+
+	key, err := x509.ParsePKCS1PrivateKey(block.Bytes)
 	if err != nil {
 		return nil, err
 	}
+
 	return key, nil
 }
diff --git a/pkg/crypto/key_test.go b/pkg/crypto/key_test.go
new file mode 100644
index 0000000..23ebdc0
--- /dev/null
+++ b/pkg/crypto/key_test.go
@@ -0,0 +1,62 @@
+package crypto_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/zitadel/oidc/v3/pkg/crypto"
+)
+
+func TestBytesToPrivateKey(tt *testing.T) {
+	tt.Run("PEMDecodeError", func(t *testing.T) {
+		_, err := crypto.BytesToPrivateKey([]byte("The non-PEM sequence"))
+		assert.EqualError(t, err, "PEM decode failed")
+	})
+
+	tt.Run("InvalidKeyFormat", func(t *testing.T) {
+		_, err := crypto.BytesToPrivateKey([]byte(`-----BEGIN PRIVATE KEY-----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+-----END PRIVATE KEY-----`))
+		assert.EqualError(t, err, "x509: failed to parse private key (use ParsePKCS8PrivateKey instead for this key format)")
+	})
+
+	tt.Run("Ok", func(t *testing.T) {
+		key, err := crypto.BytesToPrivateKey([]byte(`-----BEGIN RSA PRIVATE KEY-----
+MIIBOgIBAAJBAKj34GkxFhD90vcNLYLInFEX6Ppy1tPf9Cnzj4p4WGeKLs1Pt8Qu
+KUpRKfFLfRYC9AIKjbJTWit+CqvjWYzvQwECAwEAAQJAIJLixBy2qpFoS4DSmoEm
+o3qGy0t6z09AIJtH+5OeRV1be+N4cDYJKffGzDa88vQENZiRm0GRq6a+HPGQMd2k
+TQIhAKMSvzIBnni7ot/OSie2TmJLY4SwTQAevXysE2RbFDYdAiEBCUEaRQnMnbp7
+9mxDXDf6AU0cN/RPBjb9qSHDcWZHGzUCIG2Es59z8ugGrDY+pxLQnwfotadxd+Uy
+v/Ow5T0q5gIJAiEAyS4RaI9YG8EWx/2w0T67ZUVAw8eOMB6BIUg0Xcu+3okCIBOs
+/5OiPgoTdSy7bcF9IGpSE8ZgGKzgYQVZeN97YE00
+-----END RSA PRIVATE KEY-----`))
+		assert.NoError(t, err)
+		assert.NotNil(t, key)
+	})
+}