Add PreSpecified(Vec<u8>) option to KeyIdMethod.

If using from_ca_cert_der/_pem, the key_identifier_method would always
be set to Sha256, which is not always true. If using OpenSSL for example
SHA1 would be used.

If the provided CA certificate contains a SubjectKeyIdentifier extension,
then this option will be automatically set.

Fixes #195.

rcgen/src/lib.rs

@@ -611,6 +611,16 @@ impl CertificateParams {
 		let name_constraints = Self::convert_x509_name_constraints(&x509)?;
 		let serial_number = Some(x509.serial.to_bytes_be().into());
 
+		let key_identifier_method = x509
+			.iter_extensions()
+			.find_map(|ext| match ext.parsed_extension() {
+				x509_parser::extensions::ParsedExtension::SubjectKeyIdentifier(key_id) => {
+					Some(KeyIdMethod::PreSpecified(key_id.0.into()))
+				},
+				_ => None,
+			})
+			.unwrap_or(KeyIdMethod::Sha256);
+
 		Ok(CertificateParams {
 			alg,
 			is_ca,
@@ -619,6 +629,7 @@ impl CertificateParams {
 			extended_key_usages,
 			name_constraints,
 			serial_number,
+			key_identifier_method,
 			distinguished_name: dn,
 			key_pair: Some(key_pair),
 			not_before: validity.not_before.to_datetime(),
@@ -1128,10 +1139,13 @@ impl CertificateParams {
 	/// This key identifier is used in the SubjectKeyIdentifier X.509v3 extension.
 	fn key_identifier<K: PublicKeyData>(&self, pub_key: &K) -> Vec<u8> {
 		// Decide which method from RFC 7093 to use
-		let digest_method = match self.key_identifier_method {
+		let digest_method = match &self.key_identifier_method {
 			KeyIdMethod::Sha256 => &digest::SHA256,
 			KeyIdMethod::Sha384 => &digest::SHA384,
 			KeyIdMethod::Sha512 => &digest::SHA512,
+			KeyIdMethod::PreSpecified(b) => {
+				return b.to_vec();
+			},
 		};
 		let digest = digest::digest(digest_method, pub_key.raw_bytes());
 		let truncated_digest = &digest.as_ref()[0..20];
@@ -1347,6 +1361,8 @@ pub enum KeyIdMethod {
 	Sha384,
 	/// RFC 7093 method 3
 	Sha512,
+	/// Pre-specified identifier.
+	PreSpecified(Vec<u8>),
 }
 
 /// Helper to obtain an `OffsetDateTime` from year, month, day values
@@ -1936,4 +1952,135 @@ mod tests {
 			assert_eq!(SanType::IpAddress(IpAddr::from(octets)), actual);
 		}
 	}
+
+	#[cfg(feature = "x509-parser")]
+	mod test_key_identifier_from_ca {
+		use super::*;
+
+		#[test]
+		fn load_ca_and_sign_cert() {
+			let ca_cert = r#"-----BEGIN CERTIFICATE-----
+MIIFDTCCAvWgAwIBAgIUVuDfDt/BUVfObGOHsM+L5/qPZfIwDQYJKoZIhvcNAQEL
+BQAwFjEUMBIGA1UEAwwLZXhhbXBsZS5jb20wHhcNMjMxMjA4MTAwOTI2WhcNMjQx
+MTI4MTAwOTI2WjAWMRQwEgYDVQQDDAtleGFtcGxlLmNvbTCCAiIwDQYJKoZIhvcN
+AQEBBQADggIPADCCAgoCggIBAKXyZsv7Zwek9yc54IXWjCkMwU4eDMz9Uw06WETF
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+L2Fl4tgIQ5tyaTpu9gpJZvPU0VZ/j+1Jdk1c9PJ6xhCjof4nzI9YsLbI8lPtu8K/
+Ng==
+-----END CERTIFICATE-----"#;
+
+			let ca_key = r#"-----BEGIN PRIVATE KEY-----
+MIIJQQIBADANBgkqhkiG9w0BAQEFAASCCSswggknAgEAAoICAQCl8mbL+2cHpPcn
+OeCF1owpDMFOHgzM/VMNOlhExYWbWrsA6OLrAnmCWv+8fEWWxnCPfbP7zjB4yHVc
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+PITGdT9dgN88nHPCle0B1+OY+OZ5
+-----END PRIVATE KEY-----"#;
+
+			let kp = KeyPair::from_pem(&ca_key).unwrap();
+			let params = CertificateParams::from_ca_cert_pem(ca_cert, kp).unwrap();
+
+			assert_eq!(
+				KeyIdMethod::PreSpecified(vec![
+					0x97, 0xD4, 0x76, 0xA1, 0x9B, 0x1A, 0x71, 0x35, 0x2A, 0xC7, 0xF4, 0xA1, 0x84,
+					0x12, 0x56, 0x06, 0xBA, 0x5D, 0x61, 0x84
+				]),
+				params.key_identifier_method
+			);
+
+			let ca_cert = Certificate::from_params(params).unwrap();
+
+			assert_eq!(
+				vec![
+					0x97, 0xD4, 0x76, 0xA1, 0x9B, 0x1A, 0x71, 0x35, 0x2A, 0xC7, 0xF4, 0xA1, 0x84,
+					0x12, 0x56, 0x06, 0xBA, 0x5D, 0x61, 0x84
+				],
+				ca_cert.get_key_identifier()
+			);
+
+			let ca_der = ca_cert.serialize_der().unwrap();
+
+			let (_remainder, x509) = x509_parser::parse_x509_certificate(&ca_der).unwrap();
+			assert_eq!(
+				vec![
+					0x97, 0xD4, 0x76, 0xA1, 0x9B, 0x1A, 0x71, 0x35, 0x2A, 0xC7, 0xF4, 0xA1, 0x84,
+					0x12, 0x56, 0x06, 0xBA, 0x5D, 0x61, 0x84
+				],
+				x509.iter_extensions()
+					.find_map(|ext| match ext.parsed_extension() {
+						x509_parser::extensions::ParsedExtension::SubjectKeyIdentifier(key_id) => {
+							Some(key_id.0.to_vec())
+						},
+						_ => None,
+					})
+					.unwrap()
+			);
+		}
+	}
 }