diff --git a/cms/db/__init__.py b/cms/db/__init__.py
index 3344ac10c7..1e7ee98152 100644
--- a/cms/db/__init__.py
+++ b/cms/db/__init__.py
@@ -81,7 +81,7 @@
 
 # Instantiate or import these objects.
 
-version = 44
+version = 45
 
 engine = create_engine(config.database, echo=config.database_debug,
                        pool_timeout=60, pool_recycle=120)
diff --git a/cms/db/submission.py b/cms/db/submission.py
index d262789828..6dacba9b7f 100644
--- a/cms/db/submission.py
+++ b/cms/db/submission.py
@@ -27,6 +27,7 @@
 """
 
 from datetime import datetime
+import random
 from sqlalchemy import Boolean
 from sqlalchemy.dialects.postgresql import ARRAY, JSONB
 from sqlalchemy.orm import relationship
@@ -46,6 +47,15 @@ class Submission(Base):
 
     """
     __tablename__ = 'submissions'
+    __table_args__ = (
+        UniqueConstraint("participation_id", "opaque_id",
+                         name="participation_opaque_unique"),
+    )
+
+    # Opaque ID to be used to refer to this submission.
+    opaque_id: int = Column(
+        BigInteger,
+        nullable=False)
 
     # Auto increment primary key.
     id: int = Column(
@@ -177,6 +187,25 @@ def tokened(self) -> bool:
         """
         return self.token is not None
 
+    @classmethod
+    def generate_opaque_id(cls, session, participation_id):
+        randint_upper_bound = 2**63-1
+
+        opaque_id = random.randint(0, randint_upper_bound)
+
+        # Note that in theory this may cause the transaction to fail by
+        # generating a non-actually-unique ID. This is however extremely
+        # unlikely (prob. ~num_parallel_submissions_per_contestant^2/2**63).
+        while (session
+               .query(Submission)
+               .filter(Submission.participation_id == participation_id)
+               .filter(Submission.opaque_id == opaque_id)
+               .first()
+               is not None):
+            opaque_id = random.randint(0, randint_upper_bound)
+
+        return opaque_id
+
 
 class File(Base):
     """Class to store information about one file submitted within a
diff --git a/cms/server/contest/handlers/contest.py b/cms/server/contest/handlers/contest.py
index 77e7f690b9..a79a53d0d7 100644
--- a/cms/server/contest/handlers/contest.py
+++ b/cms/server/contest/handlers/contest.py
@@ -250,7 +250,7 @@ def get_task(self, task_name: str) -> Task | None:
             .filter(Task.name == task_name) \
             .one_or_none()
 
-    def get_submission(self, task: Task, submission_num: str) -> Submission | None:
+    def get_submission(self, task: Task, opaque_id: str | int) -> Submission | None:
         """Return the num-th contestant's submission on the given task.
 
         task: a task for the contest that is being served.
@@ -265,8 +265,7 @@ def get_submission(self, task: Task, submission_num: str) -> Submission | None:
         return self.sql_session.query(Submission) \
             .filter(Submission.participation == self.current_user) \
             .filter(Submission.task == task) \
-            .order_by(Submission.timestamp) \
-            .offset(int(submission_num) - 1) \
+            .filter(Submission.opaque_id == int(opaque_id)) \
             .first()
 
     def get_user_test(self, task: Task, user_test_num: int) -> UserTest | None:
diff --git a/cms/server/contest/handlers/tasksubmission.py b/cms/server/contest/handlers/tasksubmission.py
index a05f68a116..c91e7eb75f 100644
--- a/cms/server/contest/handlers/tasksubmission.py
+++ b/cms/server/contest/handlers/tasksubmission.py
@@ -242,12 +242,12 @@ def add_task_score(self, participation: Participation, task: Task, data: dict):
     @tornado_web.authenticated
     @actual_phase_required(0, 1, 2, 3, 4)
     @multi_contest
-    def get(self, task_name, submission_num):
+    def get(self, task_name, opaque_id):
         task = self.get_task(task_name)
         if task is None:
             raise tornado_web.HTTPError(404)
 
-        submission = self.get_submission(task, submission_num)
+        submission = self.get_submission(task, opaque_id)
         if submission is None:
             raise tornado_web.HTTPError(404)
 
@@ -284,7 +284,7 @@ def get(self, task_name, submission_num):
                     round(score_type.max_score, task.score_precision)
                 if data["status"] == SubmissionResult.SCORED \
                         and (submission.token is not None
-                            or self.r_params["actual_phase"] == 3):
+                             or self.r_params["actual_phase"] == 3):
                     data["score"] = \
                         round(sr.score, task.score_precision)
                     data["score_message"] = score_type.format_score(
@@ -302,12 +302,12 @@ class SubmissionDetailsHandler(ContestHandler):
     @tornado_web.authenticated
     @actual_phase_required(0, 1, 2, 3, 4)
     @multi_contest
-    def get(self, task_name, submission_num):
+    def get(self, task_name, opaque_id):
         task = self.get_task(task_name)
         if task is None:
             raise tornado_web.HTTPError(404)
 
-        submission = self.get_submission(task, submission_num)
+        submission = self.get_submission(task, opaque_id)
         if submission is None:
             raise tornado_web.HTTPError(404)
 
@@ -343,7 +343,7 @@ class SubmissionFileHandler(FileHandler):
     @tornado_web.authenticated
     @actual_phase_required(0, 1, 2, 3, 4)
     @multi_contest
-    def get(self, task_name, submission_num, filename):
+    def get(self, task_name, opaque_id, filename):
         if not self.contest.submissions_download_allowed:
             raise tornado_web.HTTPError(404)
 
@@ -351,7 +351,7 @@ def get(self, task_name, submission_num, filename):
         if task is None:
             raise tornado_web.HTTPError(404)
 
-        submission = self.get_submission(task, submission_num)
+        submission = self.get_submission(task, opaque_id)
         if submission is None:
             raise tornado_web.HTTPError(404)
 
@@ -389,12 +389,12 @@ class UseTokenHandler(ContestHandler):
     @tornado_web.authenticated
     @actual_phase_required(0)
     @multi_contest
-    def post(self, task_name, submission_num):
+    def post(self, task_name, opaque_id):
         task = self.get_task(task_name)
         if task is None:
             raise tornado_web.HTTPError(404)
 
-        submission = self.get_submission(task, submission_num)
+        submission = self.get_submission(task, opaque_id)
         if submission is None:
             raise tornado_web.HTTPError(404)
 
diff --git a/cms/server/contest/submission/workflow.py b/cms/server/contest/submission/workflow.py
index 32922f5eed..4e14458d2c 100644
--- a/cms/server/contest/submission/workflow.py
+++ b/cms/server/contest/submission/workflow.py
@@ -221,10 +221,12 @@ def accept_submission(
                 participation.user.username)
 
     # Use the filenames of the contestant as a default submission comment
-    received_filenames_joined = ",".join([file.filename for file in received_files])
+    received_filenames_joined = ",".join(
+        [file.filename for file in received_files])
 
     submission = Submission(
         timestamp=timestamp,
+        opaque_id=Submission.generate_opaque_id(sql_session, participation.id),
         language=language.name if language is not None else None,
         task=task,
         participation=participation,
diff --git a/cms/server/contest/templates/macro/submission.html b/cms/server/contest/templates/macro/submission.html
index 22706cbf9e..e7832ebc4e 100644
--- a/cms/server/contest/templates/macro/submission.html
+++ b/cms/server/contest/templates/macro/submission.html
@@ -95,7 +95,7 @@
                     xsrf_form_html,
                     actual_phase,
                     s,
-                    submissions|length - loop.index0,
+                    s.opaque_id,
                     show_date,
                     can_use_tokens,
                     can_play_token,
@@ -109,7 +109,7 @@
 {%- endmacro %}
 
 {% macro row(url, contest_url, translation, xsrf_form_html,
-             actual_phase, s, s_num, show_date,
+             actual_phase, s, opaque_id, show_date,
              can_use_tokens, can_play_token, can_play_token_now,
              submissions_download_allowed) -%}
 {#
@@ -121,8 +121,7 @@
 xsrf_form_html (str): input element for the XSRF protection.
 actual_phase (int): phase of the contest.
 s (Submission): the submission to display.
-s_num (int): 1-based position of the submission in the list of
-    submissions of the currently logged in participaiton on this task.
+opaque_id (int): opaque id of the submission.
 show_date (bool): whether to display only the time or also the date.
 can_use_tokens (bool): whether tokens are allowed for this task.
 can_play_token (bool): if can_use_tokens is true, whether the user has a
@@ -138,7 +137,7 @@
 {% set score_type = get_score_type(dataset=task.active_dataset) %}
 {% set sr = s.get_result(task.active_dataset) or undefined %}
 {% set status = sr.get_status() if sr is defined else SubmissionResult.COMPILING  %}
-<tr data-submission="{{ s_num }}" data-status="{{ status }}">
+<tr data-submission="{{ opaque_id }}" data-status="{{ status }}">
 {% if show_date %}
     <td class="datetime">{{ s.timestamp|format_datetime }}</td>
 {% else %}
@@ -198,7 +197,7 @@
             {% if s.language is not none %}
                 {% set filename = filename|replace(".%l", (s.language|to_language).source_extension) %}
             {% endif %}
-        <a class="btn" href="{{ contest_url("tasks", task.name, "submissions", s_num, "files", filename) }}">
+        <a class="btn" href="{{ contest_url("tasks", task.name, "submissions", opaque_id, "files", filename) }}">
             {% trans %}Download{% endtrans %}
         </a>
         {% else %}
@@ -216,7 +215,7 @@
                     {% endif %}
                 {% endif %}
                 <li>
-                    <a href="{{ contest_url("tasks", task.name, "submissions", s_num, "files", filename) }}">
+                    <a href="{{ contest_url("tasks", task.name, "submissions", opaque_id, "files", filename) }}">
                         {{ filename }}
                     </a>
                 </li>
@@ -233,7 +232,7 @@
         {% else %}
             {% if can_play_token_now %}
               {# Can play a token right now: show the button. #}
-        <form action="{{ contest_url("tasks", task.name, "submissions", s_num, "token") }}" method="POST">
+        <form action="{{ contest_url("tasks", task.name, "submissions", opaque_id, "token") }}" method="POST">
             {{ xsrf_form_html|safe }}
             <button type="submit" class="btn btn-warning">{% trans %}Play!{% endtrans %}</button>
         </form>
diff --git a/cmscontrib/AddSubmission.py b/cmscontrib/AddSubmission.py
index b58e417fa7..7410a2be94 100755
--- a/cmscontrib/AddSubmission.py
+++ b/cmscontrib/AddSubmission.py
@@ -145,8 +145,14 @@ def add_submission(
             return False
 
         # Create objects in the DB.
-        submission = Submission(make_datetime(timestamp), language_name,
-                                participation=participation, task=task)
+
+        submission = Submission(
+            timestamp=make_datetime(timestamp),
+            language=language_name,
+            participation=participation,
+            task=task,
+            opaque_id=Submission.generate_opaque_id(session, participation.id)
+        )
         for filename, digest in file_digests.items():
             session.add(File(filename, digest, submission=submission))
         session.add(submission)
@@ -188,7 +194,8 @@ def main() -> int:
         import time
         args.timestamp = time.time()
 
-    split_files: list[tuple[str, str]] = [file_.split(":", 1) for file_ in args.file]
+    split_files: list[tuple[str, str]] = [
+        file_.split(":", 1) for file_ in args.file]
     if any(len(file_) != 2 for file_ in split_files):
         parser.error("Invalid value for the file argument: format is "
                      "<name>:<file>.")
diff --git a/cmscontrib/updaters/update_45.py b/cmscontrib/updaters/update_45.py
new file mode 100644
index 0000000000..f77c175563
--- /dev/null
+++ b/cmscontrib/updaters/update_45.py
@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+
+# Contest Management System - http://cms-dev.github.io/
+# Copyright © 2025 Luca Versari <veluca93@gmail.com>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+"""A class to update a dump created by CMS.
+
+Used by DumpImporter and DumpUpdater.
+
+This adapts the dump to some changes in the model introduced in the
+commit that created this same file.
+
+"""
+
+
+import random
+
+
+class Updater:
+
+    def __init__(self, data):
+        assert data["_version"] == 44
+        self.objs = data
+
+    def run(self):
+        used_ids = set()
+        for k, v in self.objs.items():
+            if k.startswith("_"):
+                continue
+            if v["_class"] == "Submission":
+                while "opaque_id" not in v or v["opaque_id"] in used_ids:
+                    v["opaque_id"] = random.randint(0, 2**63-1)
+                used_ids.add(v["opaque_id"])
+
+        return self.objs
diff --git a/cmscontrib/updaters/update_from_1.5.sql b/cmscontrib/updaters/update_from_1.5.sql
index a66f0eaac4..d3ce3f3498 100644
--- a/cmscontrib/updaters/update_from_1.5.sql
+++ b/cmscontrib/updaters/update_from_1.5.sql
@@ -14,4 +14,10 @@ ALTER TABLE public.contests ALTER COLUMN allow_unofficial_submission_before_anal
 -- https://github.com/cms-dev/cms/pull/1393
 ALTER TABLE public.submission_results ADD COLUMN scored_at timestamp without time zone;
 
+-- https://github.com/cms-dev/cms/pull/1419
+ALTER TABLE submissions ADD COLUMN opaque_id BIGINT;
+UPDATE submissions SET opaque_id = id WHERE opaque_id IS NULL;
+ALTER TABLE submissions ADD CONSTRAINT participation_opaque_unique UNIQUE (participation_id, opaque_id);
+ALTER TABLE submissions ALTER COLUMN opaque_id SET NOT NULL;
+
 COMMIT;
diff --git a/cmstestsuite/unit_tests/cmscontrib/DumpImporterTest.py b/cmstestsuite/unit_tests/cmscontrib/DumpImporterTest.py
index c0451cb00b..d35a037e0d 100755
--- a/cmstestsuite/unit_tests/cmscontrib/DumpImporterTest.py
+++ b/cmstestsuite/unit_tests/cmscontrib/DumpImporterTest.py
@@ -92,6 +92,7 @@ class TestDumpImporter(DatabaseMixin, FileSystemMixin, unittest.TestCase):
         },
         "sub_key": {
             "_class": "Submission",
+            "opaque_id": 458958398291,
             "timestamp": 1_234_567_890.123,
             "participation": "part_key",
             "task": "task_key",
@@ -187,7 +188,7 @@ def assertContestInDb(self, name, description, task_names_and_titles,
         self.assertCountEqual([(t.name, t.title) for t in c.tasks],
                               task_names_and_titles)
         self.assertCountEqual([(u.user.username, u.user.last_name)
-                                for u in c.participations],
+                               for u in c.participations],
                               usernames_and_last_names)
 
     def assertContestNotInDb(self, name):
diff --git a/cmstestsuite/unit_tests/databasemixin.py b/cmstestsuite/unit_tests/databasemixin.py
index 94ef43daa5..f24c7b6e14 100644
--- a/cmstestsuite/unit_tests/databasemixin.py
+++ b/cmstestsuite/unit_tests/databasemixin.py
@@ -198,6 +198,7 @@ def get_submission(cls, task=None, participation=None, **kwargs):
         args = {
             "task": task,
             "participation": participation,
+            "opaque_id": unique_long_id(),
             "timestamp": (task.contest.start + timedelta(0, unique_long_id())),
         }
         args.update(kwargs)