feat(web): add HMAC token signer for artifact downloads

cameleer-server-app/src/main/java/com/cameleer/server/app/web/ArtifactDownloadTokenSigner.java

@@ -0,0 +1,69 @@
+package com.cameleer.server.app.web;
+
+import javax.crypto.Mac;
+import javax.crypto.spec.SecretKeySpec;
+import java.nio.charset.StandardCharsets;
+import java.time.Clock;
+import java.time.Duration;
+import java.util.Base64;
+import java.util.UUID;
+
+/**
+ * HMAC-SHA256 signed URL tokens for artifact downloads. Key derivation is
+ * deterministic from the JWT signing secret (HMAC-SHA256(jwtSecret,
+ * "cameleer-artifact-token-v1")) so server restarts don't invalidate fresh
+ * tokens. The loader container does NOT carry the JWT or the bootstrap token —
+ * it only carries the signed URL, which is scoped to one appVersionId and one
+ * short TTL.
+ */
+public class ArtifactDownloadTokenSigner {
+
+    private static final String DERIVATION = "cameleer-artifact-token-v1";
+
+    public record SignedToken(long exp, String sig) {}
+
+    private final byte[] key;
+    private final Clock clock;
+
+    public ArtifactDownloadTokenSigner(String jwtSecret, Clock clock) {
+        this.key = deriveKey(jwtSecret);
+        this.clock = clock;
+    }
+
+    private static byte[] deriveKey(String jwtSecret) {
+        try {
+            Mac mac = Mac.getInstance("HmacSHA256");
+            mac.init(new SecretKeySpec(jwtSecret.getBytes(StandardCharsets.UTF_8), "HmacSHA256"));
+            return mac.doFinal(DERIVATION.getBytes(StandardCharsets.UTF_8));
+        } catch (Exception e) {
+            throw new IllegalStateException("HMAC init failed", e);
+        }
+    }
+
+    public SignedToken sign(UUID appVersionId, Duration ttl) {
+        long exp = clock.instant().plus(ttl).getEpochSecond();
+        return new SignedToken(exp, signRaw(appVersionId, exp));
+    }
+
+    public String signRaw(UUID appVersionId, long exp) {
+        try {
+            Mac mac = Mac.getInstance("HmacSHA256");
+            mac.init(new SecretKeySpec(key, "HmacSHA256"));
+            String payload = appVersionId + ":" + exp;
+            byte[] tag = mac.doFinal(payload.getBytes(StandardCharsets.UTF_8));
+            return Base64.getUrlEncoder().withoutPadding().encodeToString(tag);
+        } catch (Exception e) {
+            throw new IllegalStateException("sign failed", e);
+        }
+    }
+
+    public boolean verify(UUID appVersionId, long exp, String sig) {
+        if (sig == null || sig.isBlank()) return false;
+        if (clock.instant().getEpochSecond() > exp) return false;
+        String expected = signRaw(appVersionId, exp);
+        // constant-time compare
+        return java.security.MessageDigest.isEqual(
+                expected.getBytes(StandardCharsets.UTF_8),
+                sig.getBytes(StandardCharsets.UTF_8));
+    }
+}

cameleer-server-app/src/test/java/com/cameleer/server/app/web/ArtifactDownloadTokenSignerTest.java

@@ -0,0 +1,50 @@
+package com.cameleer.server.app.web;
+
+import org.junit.jupiter.api.Test;
+import java.time.Clock;
+import java.time.Duration;
+import java.time.Instant;
+import java.time.ZoneOffset;
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class ArtifactDownloadTokenSignerTest {
+
+    private final String secret = "test-secret-do-not-use";
+    private final Instant now = Instant.parse("2026-04-27T10:00:00Z");
+    private final Clock clock = Clock.fixed(now, ZoneOffset.UTC);
+
+    @Test
+    void signedTokenVerifiesWithinTtl() {
+        var signer = new ArtifactDownloadTokenSigner(secret, clock);
+        UUID id = UUID.randomUUID();
+        var token = signer.sign(id, Duration.ofMinutes(5));
+        assertThat(signer.verify(id, token.exp(), token.sig())).isTrue();
+    }
+
+    @Test
+    void rejectsTamperedSignature() {
+        var signer = new ArtifactDownloadTokenSigner(secret, clock);
+        UUID id = UUID.randomUUID();
+        var token = signer.sign(id, Duration.ofMinutes(5));
+        assertThat(signer.verify(id, token.exp(), token.sig() + "x")).isFalse();
+    }
+
+    @Test
+    void rejectsMismatchedAppVersionId() {
+        var signer = new ArtifactDownloadTokenSigner(secret, clock);
+        UUID id = UUID.randomUUID();
+        var token = signer.sign(id, Duration.ofMinutes(5));
+        assertThat(signer.verify(UUID.randomUUID(), token.exp(), token.sig())).isFalse();
+    }
+
+    @Test
+    void rejectsExpired() {
+        var signer = new ArtifactDownloadTokenSigner(secret, clock);
+        UUID id = UUID.randomUUID();
+        long pastExp = now.minusSeconds(1).getEpochSecond();
+        String sig = signer.signRaw(id, pastExp);
+        assertThat(signer.verify(id, pastExp, sig)).isFalse();
+    }
+}