add function to return a Certificate from DER format

rcgen/src/certificate.rs

@@ -29,6 +29,28 @@ pub struct Certificate {
 }
 
 impl Certificate {
+
+	/// Create a `Certificate` from a DER encoded certificate.
+	/// Make sure the certificate match the format of x509-parser in rcgen,
+	/// or the generated `Certificate` will be different.
+	/// A safe way is to load the DER certificate generated from rcgen.
+	#[cfg(feature = "x509-parser")]
+	pub fn from_der(der: &[u8]) -> Result<Self, Error> {
+    	use x509_parser::prelude::{FromDer, X509Certificate};
+
+		let der = der.to_owned().into();
+		let params = CertificateParams::from_ca_cert_der(&der)?;
+		let (_, x509_cert) = X509Certificate::from_der(&der).unwrap();
+
+		let x509_spki_der = x509_cert.public_key().raw.to_vec();
+
+		Ok(Certificate {
+			params,
+			subject_public_key_info: x509_spki_der,
+			der,
+		})
+	}
+
 	/// Returns the certificate parameters
 	pub fn params(&self) -> &CertificateParams {
 		&self.params