Introduce core-models module.

Needed to share some key components with feature modules without having
to depend on libsignal-service.

core-util-jvm/src/main/java/org/signal/core/util/RandomUtil.kt

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.util
+
+import java.security.SecureRandom
+
+object RandomUtil {
+
+  fun getSecureBytes(size: Int): ByteArray {
+    val secret = ByteArray(size)
+    SecureRandom().nextBytes(secret)
+    return secret
+  }
+}

core-util-jvm/src/main/java/org/signal/core/util/UuidExtensions.kt

@@ -0,0 +1,10 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.util
+
+import java.util.UUID
+
+fun UUID.toByteArray(): ByteArray = UuidUtil.toByteArray(this)

core-util-jvm/src/main/java/org/signal/core/util/UuidUtil.kt

@@ -0,0 +1,127 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+package org.signal.core.util
+
+import okio.ByteString
+import java.nio.ByteBuffer
+import java.util.Optional
+import java.util.UUID
+import java.util.regex.Pattern
+
+object UuidUtil {
+  @JvmField
+  val UNKNOWN_UUID: UUID = UUID(0, 0)
+  val UNKNOWN_UUID_STRING: String = UNKNOWN_UUID.toString()
+
+  private val UUID_PATTERN: Pattern = Pattern.compile("[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}", Pattern.CASE_INSENSITIVE)
+
+  fun parse(uuid: String?): Optional<UUID> {
+    return Optional.ofNullable(parseOrNull(uuid))
+  }
+
+  @JvmStatic
+  fun parseOrNull(uuid: String?): UUID? {
+    return uuid?.takeIf { isUuid(it) }?.let { parseOrThrow(it) }
+  }
+
+  fun parseOrUnknown(uuid: String?): UUID {
+    return parseOrNull(uuid) ?: UNKNOWN_UUID
+  }
+
+  @JvmStatic
+  fun parseOrThrow(uuid: String): UUID {
+    return UUID.fromString(uuid)
+  }
+
+  @JvmStatic
+  fun parseOrThrow(bytes: ByteArray): UUID {
+    val byteBuffer = ByteBuffer.wrap(bytes)
+    val high = byteBuffer.getLong()
+    val low = byteBuffer.getLong()
+
+    return UUID(high, low)
+  }
+
+  fun parseOrThrow(bytes: ByteString): UUID {
+    return parseOrNull(bytes.toByteArray())!!
+  }
+
+  fun isUuid(uuid: String?): Boolean {
+    return uuid != null && UUID_PATTERN.matcher(uuid).matches()
+  }
+
+  @JvmStatic
+  fun toByteArray(uuid: UUID): ByteArray {
+    val buffer = ByteBuffer.wrap(ByteArray(16))
+    buffer.putLong(uuid.mostSignificantBits)
+    buffer.putLong(uuid.leastSignificantBits)
+
+    return buffer.array()
+  }
+
+  @JvmStatic
+  fun toByteString(uuid: UUID): ByteString {
+    return ByteString.of(*toByteArray(uuid))
+  }
+
+  @JvmStatic
+  fun fromByteString(bytes: ByteString): UUID {
+    return parseOrThrow(bytes.toByteArray())
+  }
+
+  @JvmStatic
+  fun fromByteStringOrNull(bytes: ByteString?): UUID? {
+    if (bytes == null) {
+      return null
+    }
+
+    return parseOrNull(bytes.toByteArray())
+  }
+
+  @JvmStatic
+  fun getStringUUID(stringId: String?, bytes: ByteString?): String? {
+    val uuid = parseOrNull(bytes)
+    return uuid?.toString() ?: stringId
+  }
+
+  @JvmStatic
+  fun fromByteStringOrUnknown(bytes: ByteString?): UUID {
+    val uuid = fromByteStringOrNull(bytes)
+    return uuid ?: UNKNOWN_UUID
+  }
+
+  fun parseOrNull(byteArray: ByteArray?): UUID? {
+    return if (byteArray != null && byteArray.size == 16) parseOrThrow(byteArray) else null
+  }
+
+  fun parseOrNull(byteString: ByteString?): UUID? {
+    return if (byteString != null) parseOrNull(byteString.toByteArray()) else null
+  }
+
+  fun fromByteStrings(byteStringCollection: Collection<ByteString>): List<UUID> {
+    val result = ArrayList<UUID>(byteStringCollection.size)
+
+    for (byteString in byteStringCollection) {
+      result.add(fromByteString(byteString))
+    }
+
+    return result
+  }
+
+  /**
+   * Keep only UUIDs that are not the [.UNKNOWN_UUID].
+   */
+  fun filterKnown(uuids: MutableCollection<UUID?>): MutableList<UUID?> {
+    val result = ArrayList<UUID?>(uuids.size)
+
+    for (uuid in uuids) {
+      if (UNKNOWN_UUID != uuid) {
+        result.add(uuid)
+      }
+    }
+
+    return result
+  }
+}

core-util-jvm/src/test/java/org/signal/core/util/UuidUtilTest.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.util;
+
+import org.junit.Test;
+import org.signal.libsignal.protocol.util.Hex;
+
+import java.io.IOException;
+import java.util.UUID;
+
+import okio.ByteString;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+
+public final class UuidUtilTest {
+
+  @Test
+  public void toByteArray() throws IOException {
+    UUID uuid = UUID.fromString("67dfd496-ea02-4720-b13d-83a462168b1d");
+
+    byte[] serialized = UuidUtil.toByteArray(uuid);
+
+    assertArrayEquals(Hex.fromStringCondensed("67dfd496ea024720b13d83a462168b1d"), serialized);
+  }
+
+  @Test
+  public void toByteArray_alternativeValues() throws IOException {
+    UUID uuid = UUID.fromString("b70df6ac-3b21-4b39-a514-613561f51e2a");
+
+    byte[] serialized = UuidUtil.toByteArray(uuid);
+
+    assertArrayEquals(Hex.fromStringCondensed("b70df6ac3b214b39a514613561f51e2a"), serialized);
+  }
+
+  @Test
+  public void parseOrThrow_from_byteArray() throws IOException {
+    byte[] bytes = Hex.fromStringCondensed("3dc48790568b49c19bd6ab6604a5bc32");
+
+    UUID uuid = UuidUtil.parseOrThrow(bytes);
+
+    assertEquals("3dc48790-568b-49c1-9bd6-ab6604a5bc32", uuid.toString());
+  }
+
+  @Test
+  public void parseOrThrow_from_byteArray_alternativeValues() throws IOException {
+    byte[] bytes = Hex.fromStringCondensed("b83dfb0b67f141aa992e030c167cd011");
+
+    UUID uuid = UuidUtil.parseOrThrow(bytes);
+
+    assertEquals("b83dfb0b-67f1-41aa-992e-030c167cd011", uuid.toString());
+  }
+
+  @Test
+  public void byte_string_round_trip() {
+    UUID uuid = UUID.fromString("67dfd496-ea02-4720-b13d-83a462168b1d");
+
+    UUID result = UuidUtil.fromByteString(ByteString.of(UuidUtil.toByteArray(uuid)));
+
+    assertEquals(uuid, result);
+  }
+}