reactive-mail-server/backend/src/main/java/com/mailserver/smtp/SmtpTls.java

package com.mailserver.smtp;

import io.netty.handler.ssl.SslContext;
import io.netty.handler.ssl.SslContextBuilder;
import org.bouncycastle.asn1.x500.X500Name;
import org.bouncycastle.asn1.x509.Extension;
import org.bouncycastle.asn1.x509.GeneralName;
import org.bouncycastle.asn1.x509.GeneralNames;
import org.bouncycastle.cert.jcajce.JcaX509CertificateConverter;
import org.bouncycastle.cert.jcajce.JcaX509v3CertificateBuilder;
import org.bouncycastle.operator.jcajce.JcaContentSignerBuilder;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.attribute.PosixFilePermissions;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.SecureRandom;
import java.security.cert.X509Certificate;
import java.time.Clock;
import java.time.Duration;
import java.time.Instant;
import java.util.Base64;
import java.util.Date;

/**
 * Certificate for SMTP TLS (STARTTLS and implicit TLS):
 * <ul>
 *   <li>{@code cert} and {@code key} given (PEM: certificate chain and the key — PKCS#8, RSA or EC, e.g. from Let's Encrypt) —
 *       used as is and read again when the files change ({@link ReloadingKeyManager}, checked once a minute);</li>
 *   <li>otherwise a self-signed certificate for the server hostname, created once in {@code dir} and reused —
 *       enough for encryption between our own services; other servers deliver over it too (MX delivery does not
 *       check certificates, RFC 7435 opportunistic TLS).</li>
 * </ul>
 */
public final class SmtpTls {

    private static final Logger log = LoggerFactory.getLogger(SmtpTls.class);
    static final String CERT_FILE = "smtp-cert.pem";
    static final String KEY_FILE = "smtp-key.pem";
    static final Duration RELOAD_CHECK = Duration.ofMinutes(1);

    private SmtpTls() {
    }

    public static SslContext serverContext(String certFile, String keyFile, Path dir, String hostname) throws Exception {
        boolean givenCert = certFile != null && !certFile.isBlank();
        boolean givenKey = keyFile != null && !keyFile.isBlank();
        if (givenCert != givenKey) {
            throw new IllegalStateException("TLS: set both the certificate and the key file, or neither");
        }
        Path cert;
        Path key;
        if (givenCert) {
            cert = Path.of(certFile);
            key = Path.of(keyFile);
            log.info("SMTP TLS: certificate {}", cert);
        } else {
            cert = dir.resolve(CERT_FILE);
            key = dir.resolve(KEY_FILE);
            if (!Files.exists(cert) || !Files.exists(key)) {
                selfSigned(hostname, cert, key);
                log.warn("SMTP TLS: created a self-signed certificate for {} in {}", hostname, dir);
            } else {
                log.info("SMTP TLS: self-signed certificate {}", cert);
            }
        }
        // Read again when the files change (a renewed Let's Encrypt certificate) — no restart needed
        return SslContextBuilder.forServer(new ReloadingKeyManager(cert, key, RELOAD_CHECK, Clock.systemUTC()))
                .protocols("TLSv1.3", "TLSv1.2")
                .build();
    }

    static void selfSigned(String hostname, Path certFile, Path keyFile) throws Exception {
        KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
        generator.initialize(2048, new SecureRandom());
        KeyPair pair = generator.generateKeyPair();
        X500Name name = new X500Name("CN=" + hostname);
        Instant now = Instant.now();
        JcaX509v3CertificateBuilder builder = new JcaX509v3CertificateBuilder(name,
                new BigInteger(64, new SecureRandom()), Date.from(now.minus(Duration.ofDays(1))),
                Date.from(now.plus(Duration.ofDays(3650))), name, pair.getPublic());
        builder.addExtension(Extension.subjectAlternativeName, false,
                new GeneralNames(new GeneralName(GeneralName.dNSName, hostname)));
        X509Certificate certificate = new JcaX509CertificateConverter()
                .getCertificate(builder.build(new JcaContentSignerBuilder("SHA256withRSA").build(pair.getPrivate())));

        Files.createDirectories(keyFile.toAbsolutePath().getParent());
        write(keyFile, pem("PRIVATE KEY", pair.getPrivate().getEncoded()), true);
        write(certFile, pem("CERTIFICATE", certificate.getEncoded()), false);
    }

    private static String pem(String type, byte[] der) {
        return "-----BEGIN " + type + "-----\n"
                + Base64.getMimeEncoder(64, new byte[]{'\n'}).encodeToString(der)
                + "\n-----END " + type + "-----\n";
    }

    private static void write(Path file, String text, boolean secret) throws IOException {
        Files.writeString(file, text, StandardCharsets.US_ASCII);
        if (secret) {
            try {
                Files.setPosixFilePermissions(file, PosixFilePermissions.fromString("rw-------"));
            } catch (UnsupportedOperationException ignored) {
                // not a POSIX file system
            }
        }
    }
}