Add certificate validation and PGP/MIME encryption; rename to pro.thinhha.mailsender

- validation: CertificateValidator checks revocation, expiry, matching
  user ID, a valid communications encryption subkey and the algorithm
  policy; uses the earlier expiry when PGPainless and Bouncy Castle disagree
- crypto: PgpEncryptor (ASCII armor, hidden recipients for Bcc)
- mime: PgpMimeEncryptor builds RFC 3156 multipart/encrypted from the
  whole CRLF-canonical inner entity, keeping envelope headers in the clear
- autoconfigure: validator and encryptor beans
- demo: encrypts when a valid key is found, plaintext otherwise
- rename groupId and Java packages to pro.thinhha.mailsender

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G71TQ5jDETVwdUJxGAwaiT

README.md

@@ -14,16 +14,17 @@ mvn verify
 
 | Module | Contents |
 |---|---|
-| `mailsender-pgp-core` | WKD key discovery, caching, pinned keys (no Spring) |
+| `mailsender-pgp-core` | WKD key discovery, caching, pinned keys, certificate validation, OpenPGP encryption, PGP/MIME (no Spring) |
-| `mailsender-pgp-spring-boot-autoconfigure` | `mail.pgp.*` properties and discovery beans |
+| `mailsender-pgp-spring-boot-autoconfigure` | `mail.pgp.*` properties and beans |
 | `mailsender-pgp-spring-boot-starter` | dependency bundle for applications |
 | `mailsender-demo` | sample app that sends test mails |
 
 ## Demo app
 
-The demo app looks up the recipient's key via WKD, logs the result, and sends `DEMO_COUNT` test mails
+The demo app looks up and validates the recipient's key via WKD, then sends `DEMO_COUNT` test mails
-(default: 1 to `test@thinhha.pro`). Encryption is not wired in yet, so the mails are sent **in plaintext**,
+(default: 1 to `test@thinhha.pro`). With a valid key the mail is PGP/MIME-encrypted (RFC 3156); otherwise it is
-and the body says whether a key was found.
+sent in plaintext and a warning is logged. The policy layer (REQUIRE / OPPORTUNISTIC / PLAINTEXT, split
+delivery) comes in milestone 5.
 
 ```
 mvn -q install -DskipTests

docs/PLAN.md

@@ -164,12 +164,14 @@ mail.pgp:
 2. **WKD discovery + cache** — `WkdKeyDiscovery`, `HttpWkdFetcher`, `CachingKeyDiscovery`, static/composite discovery,
    properties + autoconfig for discovery beans. Tests: z-base-32 vector, advanced/direct fallback matrix,
    size limit, no redirects, cache TTLs. ✅
-3. **Validation** — generated test keys: expired, revoked, sign-only, wrong UID, valid.
+3. **Validation** — generated test keys: expired, revoked, revoked UID, revoked encryption subkey, wrong UID, valid. ✅
-4. **Encryptor + PGP/MIME** — round-trip decrypt test, golden structure test, CRLF canonicalization.
+   PGPainless and Bouncy Castle can disagree on a shortened expiry; the validator uses the earlier date.
+4. **Encryptor + PGP/MIME** — round-trip decrypt test, structure test, CRLF canonicalization, attachments. ✅
+   Verified live: the demo's mail to `test@thinhha.pro` is addressed to its cv25519 subkey `7CEE00AF05A08313`.
 5. **Policy + delivery planner** — table-driven tests over key/no-key × REQUIRE/OPPORTUNISTIC/PLAINTEXT × To/Cc/Bcc;
    `EnvelopeMimeMessage`.
 6. **SecureMailSender (BUILT_IN + RELAY)**, `SendReport`, sender keys, metrics, autoconfig switch.
 7. **Demo + E2E** with GreenMail; manual check against a real WKD domain, decrypt in Thunderbird.
-   Demo app scaffolded early (sends plaintext test mails + reports WKD result) ✅; verified live:
+   Demo app ✅ — encrypts when a valid key is found, otherwise sends plaintext (logged). Verified live:
    `test@thinhha.pro` → found via advanced method, fpr `ADA6EFA0CC1630D602889820112495670FF1B4F0`.
 8. **Hardening** — protected headers, actuator endpoint, docs.

mailsender-demo/pom.xml

@@ -5,7 +5,7 @@
     <modelVersion>4.0.0</modelVersion>
 
     <parent>
-        <groupId>io.github.sneakybugmaster.mailsender</groupId>
+        <groupId>pro.thinhha.mailsender</groupId>
         <artifactId>mailsender-parent</artifactId>
         <version>0.1.0-SNAPSHOT</version>
     </parent>
@@ -15,7 +15,7 @@
 
     <dependencies>
         <dependency>
-            <groupId>io.github.sneakybugmaster.mailsender</groupId>
+            <groupId>pro.thinhha.mailsender</groupId>
             <artifactId>mailsender-pgp-spring-boot-starter</artifactId>
         </dependency>
         <dependency>

mailsender-demo/src/main/java/io/github/sneakybugmaster/mailsender/demo/TestMailRunner.java

@@ -1,76 +0,0 @@
-package io.github.sneakybugmaster.mailsender.demo;
-
-import java.time.OffsetDateTime;
-import java.util.HexFormat;
-import java.util.stream.Collectors;
-
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-import org.springframework.boot.ApplicationArguments;
-import org.springframework.boot.ApplicationRunner;
-import org.springframework.mail.javamail.JavaMailSender;
-import org.springframework.mail.javamail.MimeMessageHelper;
-import org.springframework.stereotype.Component;
-
-import io.github.sneakybugmaster.mailsender.pgp.discovery.DiscoveryResult;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.KeyDiscovery;
-import jakarta.mail.MessagingException;
-import jakarta.mail.internet.MimeMessage;
-
-/**
- * Looks up the recipient's key via WKD, then sends {@code demo.count} test mails describing what was found.
- * <p>
- * Encryption is not wired in yet (milestones 3–6), so the mails go out in plaintext.
- */
-@Component
-class TestMailRunner implements ApplicationRunner {
-
-    private static final Logger log = LoggerFactory.getLogger(TestMailRunner.class);
-
-    private final DemoProperties properties;
-    private final KeyDiscovery keyDiscovery;
-    private final JavaMailSender mailSender;
-
-    TestMailRunner(DemoProperties properties, KeyDiscovery keyDiscovery, JavaMailSender mailSender) {
-        this.properties = properties;
-        this.keyDiscovery = keyDiscovery;
-        this.mailSender = mailSender;
-    }
-
-    @Override
-    public void run(ApplicationArguments args) throws MessagingException {
-        DiscoveryResult discovery = keyDiscovery.discover(properties.to());
-        String summary = describe(discovery);
-        log.info("Key discovery for {}: {}", properties.to(), summary);
-
-        for (int i = 1; i <= properties.count(); i++) {
-            MimeMessage message = mailSender.createMimeMessage();
-            MimeMessageHelper helper = new MimeMessageHelper(message, "UTF-8");
-            if (properties.from() != null) {
-                helper.setFrom(properties.from());
-            }
-            helper.setTo(properties.to());
-            helper.setSubject("mailsender test %d/%d".formatted(i, properties.count()));
-            helper.setText("""
-                    Test mail %d of %d from the mailsender demo, sent %s.
-
-                    Key discovery for %s: %s
-
-                    This message was sent in plaintext: PGP/MIME encryption is not implemented yet.
-                    """.formatted(i, properties.count(), OffsetDateTime.now(), properties.to(), summary));
-            mailSender.send(message);
-            log.info("Sent test mail {}/{} to {}", i, properties.count(), properties.to());
-        }
-    }
-
-    private static String describe(DiscoveryResult result) {
-        return switch (result) {
-            case DiscoveryResult.Found found -> "found via " + found.source() + ", fingerprint(s) "
-                    + found.certificates().stream()
-                            .map(c -> HexFormat.of().withUpperCase().formatHex(c.getFingerprint()))
-                            .collect(Collectors.joining(", "));
-            case DiscoveryResult.NotFound ignored -> "no key published";
-            case DiscoveryResult.Unreachable u -> "lookup failed (" + u.reason() + ")";
-        };
-    }
-}

mailsender-demo/src/main/java/pro/thinhha/mailsender/demo/DemoApplication.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.demo;
+package pro.thinhha.mailsender.demo;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;

mailsender-demo/src/main/java/pro/thinhha/mailsender/demo/DemoProperties.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.demo;
+package pro.thinhha.mailsender.demo;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.boot.context.properties.bind.DefaultValue;

mailsender-demo/src/main/java/pro/thinhha/mailsender/demo/TestMailRunner.java

@@ -0,0 +1,123 @@
+package pro.thinhha.mailsender.demo;
+
+import java.io.IOException;
+import java.time.OffsetDateTime;
+import java.util.HexFormat;
+import java.util.List;
+import java.util.Optional;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.boot.ApplicationArguments;
+import org.springframework.boot.ApplicationRunner;
+import org.springframework.mail.javamail.JavaMailSender;
+import org.springframework.mail.javamail.MimeMessageHelper;
+import org.springframework.stereotype.Component;
+
+import jakarta.mail.MessagingException;
+import jakarta.mail.internet.MimeMessage;
+import pro.thinhha.mailsender.pgp.discovery.DiscoveryResult;
+import pro.thinhha.mailsender.pgp.discovery.KeyDiscovery;
+import pro.thinhha.mailsender.pgp.mime.PgpMimeEncryptor;
+import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
+import pro.thinhha.mailsender.pgp.validation.ValidationResult;
+
+/**
+ * Looks up and validates the recipient's key, then sends {@code demo.count} test mails: PGP/MIME-encrypted when a
+ * valid key was found, otherwise in plaintext (logged as a downgrade).
+ * <p>
+ * This calls the building blocks directly; the policy layer and {@code SecureMailSender} (milestones 5–6) will
+ * replace this wiring.
+ */
+@Component
+class TestMailRunner implements ApplicationRunner {
+
+    private static final Logger log = LoggerFactory.getLogger(TestMailRunner.class);
+
+    private final DemoProperties properties;
+    private final KeyDiscovery keyDiscovery;
+    private final CertificateValidator validator;
+    private final PgpMimeEncryptor mimeEncryptor;
+    private final JavaMailSender mailSender;
+
+    TestMailRunner(DemoProperties properties, KeyDiscovery keyDiscovery, CertificateValidator validator,
+            PgpMimeEncryptor mimeEncryptor, JavaMailSender mailSender) {
+        this.properties = properties;
+        this.keyDiscovery = keyDiscovery;
+        this.validator = validator;
+        this.mimeEncryptor = mimeEncryptor;
+        this.mailSender = mailSender;
+    }
+
+    @Override
+    public void run(ApplicationArguments args) throws MessagingException, IOException {
+        String to = properties.to();
+        DiscoveryResult discovery = keyDiscovery.discover(to);
+        Optional<OpenPGPCertificate> key = usableKey(discovery, to);
+
+        for (int i = 1; i <= properties.count(); i++) {
+            MimeMessage message = compose(i, discovery, key.isPresent());
+            if (key.isPresent()) {
+                message = mimeEncryptor.encrypt(message, List.of(key.get()), List.of(), mailSender::createMimeMessage);
+                log.info("Sending test mail {}/{} to {} ENCRYPTED to {}", i, properties.count(), to,
+                        fingerprint(key.get()));
+            } else {
+                log.warn("Sending test mail {}/{} to {} in PLAINTEXT: no usable key", i, properties.count(), to);
+            }
+            mailSender.send(message);
+        }
+    }
+
+    private Optional<OpenPGPCertificate> usableKey(DiscoveryResult discovery, String to) {
+        switch (discovery) {
+            case DiscoveryResult.Found found -> {
+                for (OpenPGPCertificate certificate : found.certificates()) {
+                    ValidationResult result = validator.validate(certificate, to);
+                    switch (result) {
+                        case ValidationResult.Valid valid -> {
+                            log.info("Key for {} found via {}: {}", to, found.source(), fingerprint(certificate));
+                            return Optional.of(valid.certificate());
+                        }
+                        case ValidationResult.Rejected rejected -> log.warn("Key {} for {} rejected: {} ({})",
+                                fingerprint(certificate), to, rejected.reason(), rejected.detail());
+                    }
+                }
+            }
+            case DiscoveryResult.NotFound ignored -> log.info("No key published for {}", to);
+            case DiscoveryResult.Unreachable unreachable ->
+                    log.warn("Key lookup for {} failed: {}", to, unreachable.reason());
+        }
+        return Optional.empty();
+    }
+
+    private MimeMessage compose(int index, DiscoveryResult discovery, boolean encrypted) throws MessagingException {
+        MimeMessage message = mailSender.createMimeMessage();
+        MimeMessageHelper helper = new MimeMessageHelper(message, "UTF-8");
+        if (properties.from() != null && !properties.from().isBlank()) {
+            helper.setFrom(properties.from());
+        }
+        helper.setTo(properties.to());
+        helper.setSubject("mailsender test %d/%d".formatted(index, properties.count()));
+        helper.setText("""
+                Test mail %d of %d from the mailsender demo, sent %s.
+
+                Key discovery for %s: %s
+                This message was sent %s.
+                """.formatted(index, properties.count(), OffsetDateTime.now(), properties.to(),
+                describe(discovery), encrypted ? "PGP/MIME-encrypted" : "in plaintext"));
+        return message;
+    }
+
+    private static String describe(DiscoveryResult result) {
+        return switch (result) {
+            case DiscoveryResult.Found found -> "found via " + found.source();
+            case DiscoveryResult.NotFound ignored -> "no key published";
+            case DiscoveryResult.Unreachable u -> "lookup failed (" + u.reason() + ")";
+        };
+    }
+
+    private static String fingerprint(OpenPGPCertificate certificate) {
+        return HexFormat.of().withUpperCase().formatHex(certificate.getFingerprint());
+    }
+}

mailsender-demo/src/main/resources/application.yml

@@ -20,4 +20,4 @@ demo:
 
 logging:
   level:
-    io.github.sneakybugmaster.mailsender: DEBUG
+    pro.thinhha.mailsender: DEBUG

mailsender-demo/src/test/java/pro/thinhha/mailsender/demo/EncryptedTestMailTest.java

@@ -0,0 +1,85 @@
+package pro.thinhha.mailsender.demo;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import org.bouncycastle.openpgp.api.OpenPGPKey;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.PGPainless;
+import org.pgpainless.decryption_verification.ConsumerOptions;
+import org.pgpainless.decryption_verification.DecryptionStream;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.context.DynamicPropertyRegistry;
+import org.springframework.test.context.DynamicPropertySource;
+
+import com.icegreen.greenmail.util.GreenMail;
+import com.icegreen.greenmail.util.ServerSetupTest;
+
+import jakarta.mail.Multipart;
+import jakarta.mail.internet.ContentType;
+import jakarta.mail.internet.MimeBodyPart;
+import jakarta.mail.internet.MimeMessage;
+
+/** The recipient has a (pinned) key, so the demo must deliver a decryptable PGP/MIME message. */
+@SpringBootTest(properties = {
+        "spring.mail.properties.mail.smtp.auth=false",
+        "spring.mail.properties.mail.smtp.starttls.enable=false",
+        "spring.mail.properties.mail.smtp.starttls.required=false",
+        "spring.mail.username=",
+        "demo.from=demo@mailsender.test",
+        "mail.pgp.wkd.enabled=false"
+})
+class EncryptedTestMailTest {
+
+    static final PGPainless API = PGPainless.getInstance();
+    static final OpenPGPKey RECIPIENT = API.generateKey().modernKeyRing("Test Account <test@thinhha.pro>");
+    static GreenMail greenMail;
+    static Path keyFile;
+
+    @BeforeAll
+    static void start() throws Exception {
+        keyFile = Files.createTempFile("recipient", ".asc");
+        Files.writeString(keyFile, RECIPIENT.toCertificate().toAsciiArmoredString());
+        greenMail = new GreenMail(ServerSetupTest.SMTP.dynamicPort());
+        greenMail.start();
+    }
+
+    @AfterAll
+    static void stop() throws Exception {
+        greenMail.stop();
+        Files.deleteIfExists(keyFile);
+    }
+
+    @DynamicPropertySource
+    static void properties(DynamicPropertyRegistry registry) {
+        registry.add("spring.mail.host", () -> "127.0.0.1");
+        registry.add("spring.mail.port", () -> greenMail.getSmtp().getPort());
+        registry.add("mail.pgp.static-keys[test@thinhha.pro]", () -> "file:" + keyFile);
+    }
+
+    @Test
+    void deliversDecryptablePgpMime() throws Exception {
+        MimeMessage[] received = greenMail.getReceivedMessages();
+        assertThat(received).hasSize(1);
+        MimeMessage message = received[0];
+
+        assertThat(new ContentType(message.getContentType()).getBaseType()).isEqualTo("multipart/encrypted");
+        assertThat(message.getSubject()).isEqualTo("mailsender test 1/1");
+        Multipart parts = (Multipart) message.getContent();
+        byte[] armored = parts.getBodyPart(1).getInputStream().readAllBytes();
+
+        ByteArrayOutputStream inner = new ByteArrayOutputStream();
+        try (DecryptionStream in = API.processMessage().onInputStream(new ByteArrayInputStream(armored))
+                .withOptions(ConsumerOptions.get(API).addDecryptionKey(RECIPIENT))) {
+            in.transferTo(inner);
+        }
+        MimeBodyPart entity = new MimeBodyPart(new ByteArrayInputStream(inner.toByteArray()));
+        assertThat(entity.getContent().toString()).contains("found via STATIC", "PGP/MIME-encrypted");
+    }
+}

mailsender-demo/src/test/java/pro/thinhha/mailsender/demo/PlaintextTestMailTest.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.demo;
+package pro.thinhha.mailsender.demo;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -25,7 +25,7 @@ import jakarta.mail.internet.MimeMessage;
         // keep the test offline: no real WKD lookup
         "mail.pgp.wkd.enabled=false"
 })
-class TestMailRunnerTest {
+class PlaintextTestMailTest {
 
     // Started before the Spring context, because the runner sends while the context starts.
     static GreenMail greenMail;
@@ -54,6 +54,6 @@ class TestMailRunnerTest {
         assertThat(received).hasSize(2);
         assertThat(received[0].getAllRecipients()).extracting(Object::toString).containsExactly("test@thinhha.pro");
         assertThat(received[0].getSubject()).isEqualTo("mailsender test 1/2");
-        assertThat(GreenMailUtil.getBody(received[0])).contains("no key published");
+        assertThat(GreenMailUtil.getBody(received[0])).contains("no key published", "in plaintext");
     }
 }

mailsender-pgp-core/pom.xml

@@ -5,7 +5,7 @@
     <modelVersion>4.0.0</modelVersion>
 
     <parent>
-        <groupId>io.github.sneakybugmaster.mailsender</groupId>
+        <groupId>pro.thinhha.mailsender</groupId>
         <artifactId>mailsender-parent</artifactId>
         <version>0.1.0-SNAPSHOT</version>
     </parent>
@@ -30,6 +30,11 @@
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-api</artifactId>
         </dependency>
+        <dependency>
+            <!-- Jakarta Mail API + implementation (same as spring-boot-starter-mail) -->
+            <groupId>org.eclipse.angus</groupId>
+            <artifactId>angus-mail</artifactId>
+        </dependency>
 
         <dependency>
             <groupId>org.junit.jupiter</groupId>

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/crypto/PgpEncryptor.java

@@ -0,0 +1,56 @@
+package pro.thinhha.mailsender.pgp.crypto;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.util.Collection;
+import java.util.Objects;
+
+import org.bouncycastle.openpgp.PGPException;
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+import org.pgpainless.PGPainless;
+import org.pgpainless.encryption_signing.EncryptionOptions;
+import org.pgpainless.encryption_signing.EncryptionStream;
+import org.pgpainless.encryption_signing.ProducerOptions;
+
+/** Encrypts data to OpenPGP certificates and returns the ASCII-armored message. */
+public final class PgpEncryptor {
+
+    private final PGPainless api;
+
+    public PgpEncryptor() {
+        this(PGPainless.getInstance());
+    }
+
+    public PgpEncryptor(PGPainless api) {
+        this.api = Objects.requireNonNull(api, "api");
+    }
+
+    /**
+     * @param recipients       certificates whose key IDs appear in the message
+     * @param hiddenRecipients certificates encrypted to with a wildcard key ID, so other recipients cannot tell
+     *                         they received it (use for Bcc)
+     * @return the ASCII-armored message, without armor headers
+     * @throws IllegalArgumentException if there is no recipient at all
+     */
+    public byte[] encrypt(byte[] plaintext, Collection<OpenPGPCertificate> recipients,
+            Collection<OpenPGPCertificate> hiddenRecipients) throws IOException {
+        if (recipients.isEmpty() && hiddenRecipients.isEmpty()) {
+            throw new IllegalArgumentException("At least one recipient is required");
+        }
+        EncryptionOptions options = EncryptionOptions.encryptCommunications(api);
+        recipients.forEach(options::addRecipient);
+        hiddenRecipients.forEach(options::addHiddenRecipient);
+
+        ByteArrayOutputStream out = new ByteArrayOutputStream(plaintext.length + 1024);
+        try (EncryptionStream stream = api.generateMessage()
+                .onOutputStream(out)
+                .withOptions(ProducerOptions.encrypt(options)
+                        .setAsciiArmor(true)
+                        .setHideArmorHeaders(true))) {
+            stream.write(plaintext);
+        } catch (PGPException e) {
+            throw new IOException("OpenPGP encryption failed", e);
+        }
+        return out.toByteArray();
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/CachingKeyDiscovery.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.time.Clock;
 import java.time.Duration;

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/Certificates.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.io.IOException;
 import java.util.List;

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/CompositeKeyDiscovery.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.util.List;
 

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/DiscoveryResult.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.util.List;
 import java.util.Objects;

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/DiscoverySource.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 /** Where a certificate came from. */
 public enum DiscoverySource {

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/HostResolver.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.net.InetAddress;
 import java.net.UnknownHostException;

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/HttpWkdFetcher.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.io.IOException;
 import java.io.InputStream;

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/KeyDiscovery.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 /**
  * Finds the OpenPGP certificate(s) published for an e-mail address.

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/StaticKeyDiscovery.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.util.HashMap;
 import java.util.List;

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/WkdFallback.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 /** When to fall back from the WKD advanced method to the direct method. */
 public enum WkdFallback {

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/WkdFetcher.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.io.IOException;
 import java.net.URI;

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/discovery/WkdKeyDiscovery.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.io.IOException;
 import java.net.URI;

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/mime/EncryptedMultipart.java

@@ -0,0 +1,22 @@
+package pro.thinhha.mailsender.pgp.mime;
+
+import jakarta.mail.MessagingException;
+import jakarta.mail.internet.ContentType;
+import jakarta.mail.internet.MimeMultipart;
+
+/**
+ * {@code multipart/encrypted; protocol="application/pgp-encrypted"} (RFC 3156). A plain
+ * {@code new MimeMultipart("encrypted")} cannot carry the {@code protocol} parameter: {@code saveChanges()}
+ * regenerates the Content-Type from the multipart, so the parameter has to live in the multipart itself.
+ */
+final class EncryptedMultipart extends MimeMultipart {
+
+    static final String PROTOCOL = "application/pgp-encrypted";
+
+    EncryptedMultipart() throws MessagingException {
+        super("encrypted");
+        ContentType type = new ContentType(contentType);
+        type.setParameter("protocol", PROTOCOL);
+        contentType = type.toString();
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/mime/PgpMimeEncryptor.java

@@ -0,0 +1,138 @@
+package pro.thinhha.mailsender.pgp.mime;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.charset.StandardCharsets;
+import java.util.Collection;
+import java.util.Enumeration;
+import java.util.Locale;
+import java.util.Objects;
+import java.util.Properties;
+import java.util.function.Supplier;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+
+import pro.thinhha.mailsender.pgp.crypto.PgpEncryptor;
+import jakarta.activation.DataHandler;
+import jakarta.mail.Header;
+import jakarta.mail.MessagingException;
+import jakarta.mail.Session;
+import jakarta.mail.internet.MimeBodyPart;
+import jakarta.mail.internet.MimeMessage;
+import jakarta.mail.util.ByteArrayDataSource;
+
+/**
+ * Turns a ready-to-send {@link MimeMessage} into a PGP/MIME message (RFC 3156).
+ * <p>
+ * The whole inner MIME entity (its {@code Content-*} headers and body, including attachments) is encrypted, not
+ * just the text. Envelope headers (From, To, Cc, Subject, Date, ...) are copied to the outer message unencrypted.
+ */
+public final class PgpMimeEncryptor {
+
+    private static final Session PARSE_SESSION = Session.getInstance(new Properties());
+    private static final byte[] VERSION_PART = "Version: 1\r\n".getBytes(StandardCharsets.US_ASCII);
+
+    private final PgpEncryptor encryptor;
+
+    public PgpMimeEncryptor(PgpEncryptor encryptor) {
+        this.encryptor = Objects.requireNonNull(encryptor, "encryptor");
+    }
+
+    /**
+     * @param original         the message as it would be sent in plaintext; it is not modified
+     * @param recipients       certificates to encrypt to (recipients, plus the sender's own key for a readable
+     *                         sent copy)
+     * @param hiddenRecipients certificates encrypted to without revealing their key ID (Bcc)
+     * @param newMessage       creates the outer message, e.g. {@code JavaMailSender::createMimeMessage}
+     */
+    public MimeMessage encrypt(MimeMessage original, Collection<OpenPGPCertificate> recipients,
+            Collection<OpenPGPCertificate> hiddenRecipients, Supplier<MimeMessage> newMessage)
+            throws MessagingException, IOException {
+        MimeMessage source = snapshot(original);
+        byte[] ciphertext = encryptor.encrypt(innerEntity(source), recipients, hiddenRecipients);
+
+        MimeMessage outer = newMessage.get();
+        copyEnvelopeHeaders(source, outer);
+        outer.setContent(pgpMimeBody(ciphertext));
+        outer.saveChanges();
+        return outer;
+    }
+
+    /** Serializes and re-parses the message so headers are final and the encoded body is available. */
+    private static MimeMessage snapshot(MimeMessage original) throws MessagingException, IOException {
+        original.saveChanges();
+        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
+        original.writeTo(buffer);
+        return new MimeMessage(PARSE_SESSION, new ByteArrayInputStream(buffer.toByteArray()));
+    }
+
+    /** The {@code Content-*} headers plus the still-encoded body, with CRLF line endings. */
+    static byte[] innerEntity(MimeMessage source) throws MessagingException, IOException {
+        ByteArrayOutputStream entity = new ByteArrayOutputStream();
+        Enumeration<String> lines = source.getAllHeaderLines();
+        while (lines.hasMoreElements()) {
+            String line = lines.nextElement();
+            if (isContentHeader(line)) {
+                entity.write(line.getBytes(StandardCharsets.UTF_8));
+                entity.write('\r');
+                entity.write('\n');
+            }
+        }
+        entity.write('\r');
+        entity.write('\n');
+        try (InputStream body = source.getRawInputStream()) {
+            body.transferTo(entity);
+        }
+        return toCrlf(entity.toByteArray());
+    }
+
+    private static void copyEnvelopeHeaders(MimeMessage source, MimeMessage outer) throws MessagingException {
+        Enumeration<Header> headers = source.getAllHeaders();
+        while (headers.hasMoreElements()) {
+            Header header = headers.nextElement();
+            String name = header.getName().toLowerCase(Locale.ROOT);
+            if (!name.startsWith("content-") && !name.equals("mime-version") && !name.equals("message-id")) {
+                outer.addHeader(header.getName(), header.getValue());
+            }
+        }
+    }
+
+    private static EncryptedMultipart pgpMimeBody(byte[] ciphertext) throws MessagingException {
+        MimeBodyPart control = new MimeBodyPart();
+        control.setDataHandler(new DataHandler(new ByteArrayDataSource(VERSION_PART, EncryptedMultipart.PROTOCOL)));
+        control.setHeader("Content-Description", "PGP/MIME version identification");
+        control.setHeader("Content-Transfer-Encoding", "7bit");
+
+        MimeBodyPart encrypted = new MimeBodyPart();
+        encrypted.setDataHandler(new DataHandler(
+                new ByteArrayDataSource(ciphertext, "application/octet-stream; name=\"encrypted.asc\"")));
+        encrypted.setHeader("Content-Description", "OpenPGP encrypted message");
+        encrypted.setHeader("Content-Disposition", "inline; filename=\"encrypted.asc\"");
+        encrypted.setHeader("Content-Transfer-Encoding", "7bit");
+
+        EncryptedMultipart multipart = new EncryptedMultipart();
+        multipart.setPreamble("This is an OpenPGP/MIME encrypted message (RFC 4880 and 3156)");
+        multipart.addBodyPart(control);
+        multipart.addBodyPart(encrypted);
+        return multipart;
+    }
+
+    private static boolean isContentHeader(String headerLine) {
+        return headerLine.regionMatches(true, 0, "content-", 0, "content-".length());
+    }
+
+    /** RFC 3156 requires the encrypted entity in canonical form: every line ends in CRLF. */
+    static byte[] toCrlf(byte[] data) {
+        ByteArrayOutputStream out = new ByteArrayOutputStream(data.length + data.length / 50);
+        for (int i = 0; i < data.length; i++) {
+            byte b = data[i];
+            if (b == '\n' && (i == 0 || data[i - 1] != '\r')) {
+                out.write('\r');
+            }
+            out.write(b);
+        }
+        return out.toByteArray();
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/validation/CertificateValidator.java

@@ -0,0 +1,100 @@
+package pro.thinhha.mailsender.pgp.validation;
+
+import java.time.Clock;
+import java.util.Date;
+import java.util.List;
+import java.util.Locale;
+import java.util.Objects;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+import org.bouncycastle.openpgp.api.OpenPGPCertificate.OpenPGPComponentKey;
+import org.pgpainless.PGPainless;
+import org.pgpainless.algorithm.EncryptionPurpose;
+import org.pgpainless.key.info.KeyRingInfo;
+import org.pgpainless.policy.Policy;
+
+/**
+ * Decides whether a discovered certificate may be used to encrypt mail to an address. A certificate is
+ * {@link ValidationResult.Valid} only if its primary key is neither revoked nor expired, a valid User ID contains
+ * the address, and at least one valid subkey for encrypting communications passes the algorithm policy.
+ */
+public final class CertificateValidator {
+
+    private final PGPainless api;
+    private final Clock clock;
+
+    public CertificateValidator() {
+        this(PGPainless.getInstance(), Clock.systemUTC());
+    }
+
+    public CertificateValidator(PGPainless api, Clock clock) {
+        this.api = Objects.requireNonNull(api, "api");
+        this.clock = Objects.requireNonNull(clock, "clock");
+    }
+
+    public ValidationResult validate(OpenPGPCertificate certificate, String email) {
+        Date now = Date.from(clock.instant());
+        try {
+            KeyRingInfo info = api.inspect(certificate, now);
+
+            // PGPainless and Bouncy Castle can disagree on expiry after it was changed; trust the earlier one.
+            Date expiration = earliest(info.getPrimaryKeyExpirationDate(), certificate.getExpirationTime(now));
+            if (expiration != null && !expiration.after(now)) {
+                return reject(certificate, ValidationResult.Reason.EXPIRED, "expired " + expiration.toInstant());
+            }
+            if (!info.getRevocationState().isNotRevoked() || !certificate.getPrimaryKey().isBoundAt(now)) {
+                return reject(certificate, ValidationResult.Reason.REVOKED, "primary key revoked");
+            }
+            if (!hasUserIdFor(info.getValidUserIds(), email)) {
+                return reject(certificate, ValidationResult.Reason.USER_ID_MISMATCH,
+                        "no valid user ID for " + email + " in " + info.getUserIds());
+            }
+
+            List<OpenPGPComponentKey> encryptionKeys = info.getEncryptionSubkeys(EncryptionPurpose.COMMUNICATIONS);
+            if (encryptionKeys.isEmpty()) {
+                return reject(certificate, ValidationResult.Reason.NO_ENCRYPTION_SUBKEY,
+                        "no valid subkey for encrypting communications");
+            }
+            Policy.PublicKeyAlgorithmPolicy algorithmPolicy = api.getAlgorithmPolicy().getPublicKeyAlgorithmPolicy();
+            boolean acceptable = encryptionKeys.stream().anyMatch(key ->
+                    algorithmPolicy.isAcceptable(key.getAlgorithm(), key.getPGPPublicKey().getBitStrength()));
+            if (!acceptable) {
+                return reject(certificate, ValidationResult.Reason.WEAK_ALGORITHM,
+                        "encryption subkeys rejected by algorithm policy");
+            }
+            return new ValidationResult.Valid(certificate);
+        } catch (RuntimeException e) {
+            return reject(certificate, ValidationResult.Reason.INVALID, String.valueOf(e.getMessage()));
+        }
+    }
+
+    /**
+     * Matches {@code email} against the address in each User ID, either the {@code <...>} part of
+     * {@code Name <addr>} or the whole User ID. Comparison ignores case: WKD already lowercases the local part.
+     */
+    static boolean hasUserIdFor(List<String> userIds, String email) {
+        String wanted = email.strip().toLowerCase(Locale.ROOT);
+        return userIds.stream()
+                .map(CertificateValidator::addressOf)
+                .anyMatch(wanted::equals);
+    }
+
+    private static String addressOf(String userId) {
+        int open = userId.lastIndexOf('<');
+        int close = userId.lastIndexOf('>');
+        String address = open >= 0 && close > open ? userId.substring(open + 1, close) : userId;
+        return address.strip().toLowerCase(Locale.ROOT);
+    }
+
+    private static Date earliest(Date a, Date b) {
+        if (a == null) {
+            return b;
+        }
+        return b == null || a.before(b) ? a : b;
+    }
+
+    private static ValidationResult reject(OpenPGPCertificate certificate, ValidationResult.Reason reason,
+            String detail) {
+        return new ValidationResult.Rejected(certificate, reason, detail);
+    }
+}

mailsender-pgp-core/src/main/java/pro/thinhha/mailsender/pgp/validation/ValidationResult.java

@@ -0,0 +1,40 @@
+package pro.thinhha.mailsender.pgp.validation;
+
+import java.util.Objects;
+
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+
+/** Outcome of {@link CertificateValidator#validate}. */
+public sealed interface ValidationResult {
+
+    OpenPGPCertificate certificate();
+
+    /** The certificate can be used to encrypt mail to the address it was validated for. */
+    record Valid(OpenPGPCertificate certificate) implements ValidationResult {
+        public Valid {
+            Objects.requireNonNull(certificate, "certificate");
+        }
+    }
+
+    record Rejected(OpenPGPCertificate certificate, Reason reason, String detail) implements ValidationResult {
+        public Rejected {
+            Objects.requireNonNull(certificate, "certificate");
+            Objects.requireNonNull(reason, "reason");
+        }
+    }
+
+    enum Reason {
+        /** The primary key is revoked. */
+        REVOKED,
+        /** The primary key is expired. */
+        EXPIRED,
+        /** No valid User ID carries the address. */
+        USER_ID_MISMATCH,
+        /** No valid, non-expired subkey is marked for encrypting communications. */
+        NO_ENCRYPTION_SUBKEY,
+        /** Encryption subkeys exist, but their algorithm or size is rejected by the algorithm policy. */
+        WEAK_ALGORITHM,
+        /** The certificate could not be evaluated (bad self-signatures, unsupported packets, ...). */
+        INVALID
+    }
+}

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/crypto/PgpEncryptorTest.java

@@ -0,0 +1,57 @@
+package pro.thinhha.mailsender.pgp.crypto;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+
+import org.bouncycastle.openpgp.api.OpenPGPKey;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.PGPainless;
+import org.pgpainless.decryption_verification.ConsumerOptions;
+import org.pgpainless.decryption_verification.DecryptionStream;
+
+public class PgpEncryptorTest {
+
+    private static final PGPainless API = PGPainless.getInstance();
+    private static final OpenPGPKey ALICE = API.generateKey().modernKeyRing("Alice <alice@example.org>");
+    private static final OpenPGPKey BOB = API.generateKey().modernKeyRing("Bob <bob@example.org>");
+    private static final OpenPGPKey CAROL = API.generateKey().modernKeyRing("Carol <carol@example.org>");
+
+    private final PgpEncryptor encryptor = new PgpEncryptor();
+
+    @Test
+    void everyRecipientCanDecrypt() throws Exception {
+        byte[] plaintext = "Hello, PGP/MIME\r\n".getBytes(StandardCharsets.UTF_8);
+
+        byte[] armored = encryptor.encrypt(plaintext, List.of(ALICE.toCertificate(), BOB.toCertificate()),
+                List.of(CAROL.toCertificate()));
+
+        String text = new String(armored, StandardCharsets.US_ASCII);
+        assertThat(text).startsWith("-----BEGIN PGP MESSAGE-----").doesNotContain("Version:");
+        assertThat(decrypt(armored, ALICE)).isEqualTo(plaintext);
+        assertThat(decrypt(armored, BOB)).isEqualTo(plaintext);
+        assertThat(decrypt(armored, CAROL)).isEqualTo(plaintext);
+    }
+
+    @Test
+    void requiresARecipient() {
+        assertThatIllegalArgumentException().isThrownBy(() -> encryptor.encrypt(new byte[1], List.of(), List.of()));
+    }
+
+    public static byte[] decrypt(byte[] message, OpenPGPKey key) throws Exception {
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        try (DecryptionStream in = API.processMessage()
+                .onInputStream(new ByteArrayInputStream(message))
+                .withOptions(ConsumerOptions.get(API).addDecryptionKey(key))) {
+            in.transferTo(out);
+        } catch (IOException e) {
+            throw e;
+        }
+        return out.toByteArray();
+    }
+}

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/discovery/CachingKeyDiscoveryTest.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import static org.assertj.core.api.Assertions.assertThat;
 

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/discovery/CompositeKeyDiscoveryTest.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import static org.assertj.core.api.Assertions.assertThat;
 

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/discovery/HttpWkdFetcherTest.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIOException;

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/discovery/TestKeys.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import java.io.IOException;
 import java.io.UncheckedIOException;

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/discovery/WkdAddressTest.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import static org.assertj.core.api.Assertions.assertThat;
 

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/discovery/WkdKeyDiscoveryTest.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.discovery;
+package pro.thinhha.mailsender.pgp.discovery;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/mime/PgpMimeEncryptorTest.java

@@ -0,0 +1,136 @@
+package pro.thinhha.mailsender.pgp.mime;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+import java.util.Properties;
+
+import org.bouncycastle.openpgp.api.OpenPGPKey;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.PGPainless;
+
+import pro.thinhha.mailsender.pgp.crypto.PgpEncryptor;
+import pro.thinhha.mailsender.pgp.crypto.PgpEncryptorTest;
+import jakarta.activation.DataHandler;
+import jakarta.mail.BodyPart;
+import jakarta.mail.Message;
+import jakarta.mail.Multipart;
+import jakarta.mail.Part;
+import jakarta.mail.Session;
+import jakarta.mail.internet.ContentType;
+import jakarta.mail.internet.InternetAddress;
+import jakarta.mail.internet.MimeBodyPart;
+import jakarta.mail.internet.MimeMessage;
+import jakarta.mail.internet.MimeMultipart;
+import jakarta.mail.util.ByteArrayDataSource;
+
+class PgpMimeEncryptorTest {
+
+    private static final OpenPGPKey ALICE =
+            PGPainless.getInstance().generateKey().modernKeyRing("Alice <alice@example.org>");
+    private static final Session SESSION = Session.getInstance(new Properties());
+
+    private final PgpMimeEncryptor mimeEncryptor = new PgpMimeEncryptor(new PgpEncryptor());
+
+    @Test
+    void producesRfc3156StructureWithEnvelopeHeadersInTheClear() throws Exception {
+        MimeMessage wire = sendAndReceive(mimeEncryptor.encrypt(textMessage("Hi Alice\nsecond line"),
+                List.of(ALICE.toCertificate()), List.of(), () -> new MimeMessage(SESSION)));
+
+        ContentType type = new ContentType(wire.getContentType());
+        assertThat(type.getBaseType()).isEqualTo("multipart/encrypted");
+        assertThat(type.getParameter("protocol")).isEqualTo("application/pgp-encrypted");
+
+        assertThat(wire.getSubject()).isEqualTo("Quarterly numbers");
+        assertThat(wire.getFrom()).extracting(Object::toString).containsExactly("sender@example.com");
+        assertThat(wire.getRecipients(Message.RecipientType.TO)).extracting(Object::toString)
+                .containsExactly("alice@example.org");
+        assertThat(wire.getRecipients(Message.RecipientType.CC)).extracting(Object::toString)
+                .containsExactly("bob@example.org");
+
+        Multipart parts = (Multipart) wire.getContent();
+        assertThat(parts.getCount()).isEqualTo(2);
+        BodyPart control = parts.getBodyPart(0);
+        assertThat(new ContentType(control.getContentType()).getBaseType()).isEqualTo("application/pgp-encrypted");
+        assertThat(readAll(control)).isEqualTo("Version: 1\r\n");
+        BodyPart encrypted = parts.getBodyPart(1);
+        assertThat(new ContentType(encrypted.getContentType()).getBaseType()).isEqualTo("application/octet-stream");
+        assertThat(readAll(encrypted)).startsWith("-----BEGIN PGP MESSAGE-----");
+        assertThat(wire.getContent().toString()).doesNotContain("Hi Alice");
+    }
+
+    @Test
+    void encryptsWholeInnerEntityIncludingAttachments() throws Exception {
+        MimeMessage original = textMessage(null);
+        MimeMultipart mixed = new MimeMultipart("mixed");
+        MimeBodyPart text = new MimeBodyPart();
+        text.setText("See attached, Zoë.\nBye", "UTF-8");
+        MimeBodyPart attachment = new MimeBodyPart();
+        byte[] pdf = {'%', 'P', 'D', 'F', 0, 1, 2, (byte) 0xFF, '\n'};
+        attachment.setDataHandler(new DataHandler(new ByteArrayDataSource(pdf, "application/pdf")));
+        attachment.setFileName("report.pdf");
+        mixed.addBodyPart(text);
+        mixed.addBodyPart(attachment);
+        original.setContent(mixed);
+
+        MimeMessage wire = sendAndReceive(mimeEncryptor.encrypt(original, List.of(ALICE.toCertificate()), List.of(),
+                () -> new MimeMessage(SESSION)));
+
+        byte[] armored = readAll(((Multipart) wire.getContent()).getBodyPart(1)).getBytes(StandardCharsets.US_ASCII);
+        byte[] inner = PgpEncryptorTest.decrypt(armored, ALICE);
+        assertThat(hasBareLineFeed(inner)).as("inner entity is CRLF-canonical").isFalse();
+
+        MimeBodyPart entity = new MimeBodyPart(new ByteArrayInputStream(inner));
+        assertThat(entity.isMimeType("multipart/mixed")).isTrue();
+        Multipart decrypted = (Multipart) entity.getContent();
+        assertThat(decrypted.getCount()).isEqualTo(2);
+        assertThat(decrypted.getBodyPart(0).getContent().toString()).isEqualTo("See attached, Zoë.\r\nBye");
+        assertThat(decrypted.getBodyPart(1).getFileName()).isEqualTo("report.pdf");
+        assertThat(decrypted.getBodyPart(1).getInputStream().readAllBytes()).isEqualTo(pdf);
+    }
+
+    @Test
+    void leavesOriginalUntouched() throws Exception {
+        MimeMessage original = textMessage("plain");
+
+        mimeEncryptor.encrypt(original, List.of(ALICE.toCertificate()), List.of(), () -> new MimeMessage(SESSION));
+
+        assertThat(original.isMimeType("text/plain")).isTrue();
+        assertThat(original.getContent()).isEqualTo("plain");
+    }
+
+    private static MimeMessage textMessage(String text) throws Exception {
+        MimeMessage message = new MimeMessage(SESSION);
+        message.setFrom(new InternetAddress("sender@example.com"));
+        message.setRecipient(Message.RecipientType.TO, new InternetAddress("alice@example.org"));
+        message.setRecipient(Message.RecipientType.CC, new InternetAddress("bob@example.org"));
+        message.setSubject("Quarterly numbers");
+        if (text != null) {
+            message.setText(text, "UTF-8");
+        }
+        return message;
+    }
+
+    /** Serializes like SMTP would and parses the result like a receiving client. */
+    private static MimeMessage sendAndReceive(MimeMessage message) throws Exception {
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        message.writeTo(out);
+        return new MimeMessage(SESSION, new ByteArrayInputStream(out.toByteArray()));
+    }
+
+    private static String readAll(Part part) throws Exception {
+        return new String(part.getInputStream().readAllBytes(), StandardCharsets.US_ASCII);
+    }
+
+    private static boolean hasBareLineFeed(byte[] data) {
+        for (int i = 0; i < data.length; i++) {
+            if (data[i] == '\n' && (i == 0 || data[i - 1] != '\r')) {
+                return true;
+            }
+        }
+        return false;
+    }
+}

mailsender-pgp-core/src/test/java/pro/thinhha/mailsender/pgp/validation/CertificateValidatorTest.java

@@ -0,0 +1,102 @@
+package pro.thinhha.mailsender.pgp.validation;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.time.Clock;
+import java.time.Duration;
+import java.time.Instant;
+import java.time.ZoneOffset;
+import java.util.Date;
+import java.util.List;
+
+import org.bouncycastle.bcpg.KeyIdentifier;
+import org.bouncycastle.openpgp.api.OpenPGPCertificate;
+import org.bouncycastle.openpgp.api.OpenPGPKey;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.PGPainless;
+import org.pgpainless.algorithm.EncryptionPurpose;
+import org.pgpainless.key.protection.SecretKeyRingProtector;
+
+class CertificateValidatorTest {
+
+    private static final String ALICE = "alice@example.org";
+    private static final SecretKeyRingProtector NO_PASSPHRASE = SecretKeyRingProtector.unprotectedKeys();
+
+    private final PGPainless api = PGPainless.getInstance();
+    private final CertificateValidator validator = new CertificateValidator();
+
+    @Test
+    void acceptsFreshModernKey() {
+        assertThat(validator.validate(key("Alice <alice@example.org>").toCertificate(), ALICE))
+                .isInstanceOf(ValidationResult.Valid.class);
+    }
+
+    @Test
+    void matchesAddressCaseInsensitivelyAndBareUserIds() {
+        assertThat(validator.validate(key("Alice <Alice@Example.ORG>").toCertificate(), ALICE))
+                .isInstanceOf(ValidationResult.Valid.class);
+        assertThat(validator.validate(key("alice@example.org").toCertificate(), ALICE))
+                .isInstanceOf(ValidationResult.Valid.class);
+    }
+
+    @Test
+    void rejectsUserIdForAnotherAddress() {
+        assertRejected(validator.validate(key("Eve <eve@example.org>").toCertificate(), ALICE),
+                ValidationResult.Reason.USER_ID_MISMATCH);
+        // a look-alike must not match
+        assertRejected(validator.validate(key("Alice <alice@example.org.evil>").toCertificate(), ALICE),
+                ValidationResult.Reason.USER_ID_MISMATCH);
+    }
+
+    @Test
+    void rejectsRevokedUserId() throws Exception {
+        OpenPGPKey key = key("Alice <alice@other.example>");
+        key = api.modify(key).addUserId("Alice <alice@example.org>", NO_PASSPHRASE).done();
+        assertThat(validator.validate(key.toCertificate(), ALICE)).isInstanceOf(ValidationResult.Valid.class);
+
+        key = api.modify(key).revokeUserId("Alice <alice@example.org>", NO_PASSPHRASE).done();
+
+        assertRejected(validator.validate(key.toCertificate(), ALICE), ValidationResult.Reason.USER_ID_MISMATCH);
+    }
+
+    @Test
+    void rejectsRevokedKey() throws Exception {
+        OpenPGPKey key = api.modify(key("Alice <alice@example.org>")).revoke(NO_PASSPHRASE).done();
+
+        assertRejected(validator.validate(key.toCertificate(), ALICE), ValidationResult.Reason.REVOKED);
+    }
+
+    @Test
+    void rejectsExpiredKey() {
+        Instant now = Instant.now();
+        OpenPGPKey key = api.modify(key("Alice <alice@example.org>"))
+                .setExpirationDate(Date.from(now.plus(Duration.ofHours(1))), NO_PASSPHRASE)
+                .done();
+        CertificateValidator later = new CertificateValidator(api,
+                Clock.fixed(now.plus(Duration.ofHours(2)), ZoneOffset.UTC));
+
+        assertThat(validator.validate(key.toCertificate(), ALICE)).isInstanceOf(ValidationResult.Valid.class);
+        assertRejected(later.validate(key.toCertificate(), ALICE), ValidationResult.Reason.EXPIRED);
+    }
+
+    @Test
+    void rejectsKeyWhoseEncryptionSubkeyIsRevoked() throws Exception {
+        OpenPGPKey key = key("Alice <alice@example.org>");
+        List<KeyIdentifier> encryptionKeys = api.inspect(key).getEncryptionSubkeys(EncryptionPurpose.COMMUNICATIONS)
+                .stream().map(OpenPGPCertificate.OpenPGPComponentKey::getKeyIdentifier).toList();
+        for (KeyIdentifier id : encryptionKeys) {
+            key = api.modify(key).revokeSubKey(id, NO_PASSPHRASE).done();
+        }
+
+        assertRejected(validator.validate(key.toCertificate(), ALICE), ValidationResult.Reason.NO_ENCRYPTION_SUBKEY);
+    }
+
+    private OpenPGPKey key(String userId) {
+        return api.generateKey().modernKeyRing(userId);
+    }
+
+    private static void assertRejected(ValidationResult result, ValidationResult.Reason reason) {
+        assertThat(result).isInstanceOfSatisfying(ValidationResult.Rejected.class,
+                rejected -> assertThat(rejected.reason()).isEqualTo(reason));
+    }
+}

mailsender-pgp-spring-boot-autoconfigure/pom.xml

@@ -5,7 +5,7 @@
     <modelVersion>4.0.0</modelVersion>
 
     <parent>
-        <groupId>io.github.sneakybugmaster.mailsender</groupId>
+        <groupId>pro.thinhha.mailsender</groupId>
         <artifactId>mailsender-parent</artifactId>
         <version>0.1.0-SNAPSHOT</version>
     </parent>
@@ -14,7 +14,7 @@
 
     <dependencies>
         <dependency>
-            <groupId>io.github.sneakybugmaster.mailsender</groupId>
+            <groupId>pro.thinhha.mailsender</groupId>
             <artifactId>mailsender-pgp-core</artifactId>
         </dependency>
         <dependency>

mailsender-pgp-spring-boot-autoconfigure/src/main/java/pro/thinhha/mailsender/pgp/autoconfigure/PgpMailAutoConfiguration.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.autoconfigure;
+package pro.thinhha.mailsender.pgp.autoconfigure;
 
 import java.io.IOException;
 import java.io.InputStream;
@@ -18,19 +18,23 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Primary;
 import org.springframework.core.io.Resource;
 
-import io.github.sneakybugmaster.mailsender.pgp.discovery.CachingKeyDiscovery;
+import pro.thinhha.mailsender.pgp.crypto.PgpEncryptor;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.Certificates;
+import pro.thinhha.mailsender.pgp.discovery.CachingKeyDiscovery;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.CompositeKeyDiscovery;
+import pro.thinhha.mailsender.pgp.discovery.Certificates;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.HostResolver;
+import pro.thinhha.mailsender.pgp.discovery.CompositeKeyDiscovery;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.HttpWkdFetcher;
+import pro.thinhha.mailsender.pgp.discovery.HostResolver;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.KeyDiscovery;
+import pro.thinhha.mailsender.pgp.discovery.HttpWkdFetcher;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.StaticKeyDiscovery;
+import pro.thinhha.mailsender.pgp.discovery.KeyDiscovery;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.WkdFetcher;
+import pro.thinhha.mailsender.pgp.discovery.StaticKeyDiscovery;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.WkdKeyDiscovery;
+import pro.thinhha.mailsender.pgp.discovery.WkdFetcher;
+import pro.thinhha.mailsender.pgp.discovery.WkdKeyDiscovery;
+import pro.thinhha.mailsender.pgp.mime.PgpMimeEncryptor;
+import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
 
 /**
- * Key discovery beans: pinned keys first, then WKD behind a cache. The primary {@link KeyDiscovery} bean is the
+ * Key discovery (pinned keys first, then WKD behind a cache), certificate validation and PGP/MIME encryption beans.
- * combined chain; {@link CachingKeyDiscovery} is exposed separately so applications can evict entries.
+ * The primary {@link KeyDiscovery} bean is the combined chain; {@link CachingKeyDiscovery} is exposed separately so
+ * applications can evict entries.
  */
 @AutoConfiguration
 @EnableConfigurationProperties(PgpMailProperties.class)
@@ -75,6 +79,24 @@ public class PgpMailAutoConfiguration {
         return new CompositeKeyDiscovery(chain);
     }
 
+    @Bean
+    @ConditionalOnMissingBean
+    CertificateValidator pgpCertificateValidator() {
+        return new CertificateValidator();
+    }
+
+    @Bean
+    @ConditionalOnMissingBean
+    PgpEncryptor pgpEncryptor() {
+        return new PgpEncryptor();
+    }
+
+    @Bean
+    @ConditionalOnMissingBean
+    PgpMimeEncryptor pgpMimeEncryptor(PgpEncryptor encryptor) {
+        return new PgpMimeEncryptor(encryptor);
+    }
+
     private static List<OpenPGPCertificate> read(String email, Resource resource) {
         try (InputStream in = resource.getInputStream()) {
             List<OpenPGPCertificate> certificates = Certificates.parse(in.readAllBytes());

mailsender-pgp-spring-boot-autoconfigure/src/main/java/pro/thinhha/mailsender/pgp/autoconfigure/PgpMailProperties.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.autoconfigure;
+package pro.thinhha.mailsender.pgp.autoconfigure;
 
 import java.time.Duration;
 import java.util.Map;
@@ -8,7 +8,7 @@ import org.springframework.boot.context.properties.bind.DefaultValue;
 import org.springframework.core.io.Resource;
 import org.springframework.util.unit.DataSize;
 
-import io.github.sneakybugmaster.mailsender.pgp.discovery.WkdFallback;
+import pro.thinhha.mailsender.pgp.discovery.WkdFallback;
 
 /**
  * {@code mail.pgp.*} configuration.

mailsender-pgp-spring-boot-autoconfigure/src/main/resources/META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports

@@ -1 +1 @@
-io.github.sneakybugmaster.mailsender.pgp.autoconfigure.PgpMailAutoConfiguration
+pro.thinhha.mailsender.pgp.autoconfigure.PgpMailAutoConfiguration

mailsender-pgp-spring-boot-autoconfigure/src/test/java/pro/thinhha/mailsender/pgp/autoconfigure/PgpMailAutoConfigurationTest.java

@@ -1,4 +1,4 @@
-package io.github.sneakybugmaster.mailsender.pgp.autoconfigure;
+package pro.thinhha.mailsender.pgp.autoconfigure;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -13,12 +13,14 @@ import org.pgpainless.PGPainless;
 import org.springframework.boot.autoconfigure.AutoConfigurations;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 
-import io.github.sneakybugmaster.mailsender.pgp.discovery.CachingKeyDiscovery;
+import pro.thinhha.mailsender.pgp.discovery.CachingKeyDiscovery;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.DiscoveryResult;
+import pro.thinhha.mailsender.pgp.discovery.DiscoveryResult;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.DiscoverySource;
+import pro.thinhha.mailsender.pgp.discovery.DiscoverySource;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.KeyDiscovery;
+import pro.thinhha.mailsender.pgp.discovery.KeyDiscovery;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.WkdFallback;
+import pro.thinhha.mailsender.pgp.discovery.WkdFallback;
-import io.github.sneakybugmaster.mailsender.pgp.discovery.WkdFetcher;
+import pro.thinhha.mailsender.pgp.discovery.WkdFetcher;
+import pro.thinhha.mailsender.pgp.mime.PgpMimeEncryptor;
+import pro.thinhha.mailsender.pgp.validation.CertificateValidator;
 
 class PgpMailAutoConfigurationTest {
 
@@ -31,7 +33,8 @@ class PgpMailAutoConfigurationTest {
     @Test
     void createsDiscoveryChainWithDefaults() {
         runner.run(context -> {
-            assertThat(context).hasSingleBean(WkdFetcher.class).hasSingleBean(CachingKeyDiscovery.class);
+            assertThat(context).hasSingleBean(WkdFetcher.class).hasSingleBean(CachingKeyDiscovery.class)
+                    .hasSingleBean(CertificateValidator.class).hasSingleBean(PgpMimeEncryptor.class);
             assertThat(context.getBean(KeyDiscovery.class)).isNotInstanceOf(CachingKeyDiscovery.class);
 
             PgpMailProperties properties = context.getBean(PgpMailProperties.class);

mailsender-pgp-spring-boot-starter/pom.xml

@@ -5,7 +5,7 @@
     <modelVersion>4.0.0</modelVersion>
 
     <parent>
-        <groupId>io.github.sneakybugmaster.mailsender</groupId>
+        <groupId>pro.thinhha.mailsender</groupId>
         <artifactId>mailsender-parent</artifactId>
         <version>0.1.0-SNAPSHOT</version>
     </parent>
@@ -15,7 +15,7 @@
 
     <dependencies>
         <dependency>
-            <groupId>io.github.sneakybugmaster.mailsender</groupId>
+            <groupId>pro.thinhha.mailsender</groupId>
             <artifactId>mailsender-pgp-spring-boot-autoconfigure</artifactId>
         </dependency>
         <dependency>

pom.xml

@@ -4,7 +4,7 @@
          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
     <modelVersion>4.0.0</modelVersion>
 
-    <groupId>io.github.sneakybugmaster.mailsender</groupId>
+    <groupId>pro.thinhha.mailsender</groupId>
     <artifactId>mailsender-parent</artifactId>
     <version>0.1.0-SNAPSHOT</version>
     <packaging>pom</packaging>
@@ -40,17 +40,17 @@
                 <scope>import</scope>
             </dependency>
             <dependency>
-                <groupId>io.github.sneakybugmaster.mailsender</groupId>
+                <groupId>pro.thinhha.mailsender</groupId>
                 <artifactId>mailsender-pgp-core</artifactId>
                 <version>${project.version}</version>
             </dependency>
             <dependency>
-                <groupId>io.github.sneakybugmaster.mailsender</groupId>
+                <groupId>pro.thinhha.mailsender</groupId>
                 <artifactId>mailsender-pgp-spring-boot-autoconfigure</artifactId>
                 <version>${project.version}</version>
             </dependency>
             <dependency>
-                <groupId>io.github.sneakybugmaster.mailsender</groupId>
+                <groupId>pro.thinhha.mailsender</groupId>
                 <artifactId>mailsender-pgp-spring-boot-starter</artifactId>
                 <version>${project.version}</version>
             </dependency>