Add example of generating a cert chain

Generate two certificates and sign the second with the first.

Cargo.toml

@@ -26,6 +26,10 @@ required-features = ["pem"]
 name = "rsa-irc-openssl"
 required-features = ["pem"]
 
+[[example]]
+name = "sign-leaf-with-ca"
+required-features = ["pem", "x509-parser"]
+
 [dependencies]
 yasna = { version = "0.5.2", features = ["time", "std"] }
 ring = "0.17"

examples/sign-leaf-with-ca.rs

@@ -0,0 +1,58 @@
+use rcgen::{
+	BasicConstraints, Certificate, CertificateParams, DnType, DnValue::PrintableString,
+	ExtendedKeyUsagePurpose, IsCa, KeyUsagePurpose,
+};
+use time::{Duration, OffsetDateTime};
+
+/// Example demonstrating signing end-endity certificate with ca
+fn main() {
+	let ca = new_ca();
+	let end_entity = new_end_entity();
+
+	let end_entity_pem = end_entity.serialize_pem_with_signer(&ca).unwrap();
+	println!("directly signed end-entity certificate: {end_entity_pem}");
+
+	let ca_cert_pem = ca.serialize_pem().unwrap();
+	println!("ca certificate: {ca_cert_pem}",);
+}
+
+fn new_ca() -> Certificate {
+	let mut params = CertificateParams::new(Vec::default());
+	let (yesterday, tomorrow) = validity_period();
+	params.is_ca = IsCa::Ca(BasicConstraints::Unconstrained);
+	params
+		.distinguished_name
+		.push(DnType::CountryName, PrintableString("BR".into()));
+	params
+		.distinguished_name
+		.push(DnType::OrganizationName, "Crab widgits SE");
+	params.key_usages.push(KeyUsagePurpose::DigitalSignature);
+	params.key_usages.push(KeyUsagePurpose::KeyCertSign);
+	params.key_usages.push(KeyUsagePurpose::CrlSign);
+
+	params.not_before = yesterday;
+	params.not_after = tomorrow;
+	Certificate::from_params(params).unwrap()
+}
+
+fn new_end_entity() -> Certificate {
+	let name = "entity.other.host";
+	let mut params = CertificateParams::new(vec![name.into()]);
+	let (yesterday, tomorrow) = validity_period();
+	params.distinguished_name.push(DnType::CommonName, name);
+	params.use_authority_key_identifier_extension = true;
+	params.key_usages.push(KeyUsagePurpose::DigitalSignature);
+	params
+		.extended_key_usages
+		.push(ExtendedKeyUsagePurpose::ServerAuth);
+	params.not_before = yesterday;
+	params.not_after = tomorrow;
+	Certificate::from_params(params).unwrap()
+}
+
+fn validity_period() -> (OffsetDateTime, OffsetDateTime) {
+	let day = Duration::new(86400, 0);
+	let yesterday = OffsetDateTime::now_utc().checked_sub(day).unwrap();
+	let tomorrow = OffsetDateTime::now_utc().checked_add(day).unwrap();
+	(yesterday, tomorrow)
+}