Apple Beefs Up iMessage With Quantum-Resistant Encryption

The revamped iMessage app uses Apples new PQ3 post-quantum cryptographic protocol, which its engineers say will make it the most secure messaging app — but Signals president begs to differ.

Apple is adding the quantum-computing resistant PQ3 protocol to its widely used iMessage, making it the most secure mainstream messaging app. The upgraded version of iMessage will start appearing in March in its monthly MacOS and iOS releases, according to Apples Security Engineering and Architecture (SEAR) team.
Apples PQ3 addition doesnt make iMessage the first messaging app with post-quantum cryptographic (PQC) encryption — the Signal secure messaging app added PQC encryption resilience in September 2023 with an upgrade to its
Signal Protocol, called PQXDH
. Apples engineers acknowledge Signals capabilities but say that iMessage with PQ3 leapfrogs the Signal Protocols post-quantum cryptographic capability.
Currently, iMessage offers end-to-end encryption by default using classical cryptography, which Apple describes as Level 1 security. Apple designated Signals PQC capability with PQXDH as having Level 2 security because its limited to PQC key establishment. The new iMessage with PQ3 is the first to achieve what Apple labels Level 3 security because its post-quantum cryptography secures not only the initial key establishment process, but also the continuous message exchange.
Apple says PQ3 quickly and automatically restores the cryptographic security of a message exchange, even if a specific key is compromised.
To our knowledge, PQ3 has the strongest security properties of any at-scale messaging protocol in the world, Apples SEAR team explained in
a blog post announcing
the new protocol.
The addition of PQ3 follows iMessages October 2023 enhancement featuring
Contact Key Verification
, designed to detect sophisticated attacks against Apples iMessage servers while letting users verify they are messaging specifically with their intended recipients.
IMessage with PQ3 is backed by mathematical validation from a team led by professor David Basin, head of the
Information Security Group at ETH Zürich
and co-inventor of
Tamarin
, a well-regarded security protocol verification tool. Basin and his research team at ETH Zürich used Tamarin to perform a
technical evaluation
of PQ3, published by Apple.
Also evaluating PQ3 was University of Waterloo professor Douglas Stebila, known for his research on post-quantum security for Internet protocols. According to Apples SEAR team, both research groups undertook divergent but complementary approaches, running different mathematical models to test the security of PQ3. Stebila noted that the evaluation the team performed and
the white paper it produced
was underwritten and published by Apple.
Signal president Meredith Whittaker dismisses Apples claims of post-quantum cryptographic superiority.
We dont have a comment on Apples novel hierarchical levels framework that they apply in their public-facing materials to rank various cryptographic approaches, Whitaker says. We recognize that companies struggle to market and describe these complex technological changes and that Apple chose this approach in service of such marketing.
Thanks to Signals own partnerships with the research community, a month after publishing PQXDH it became the first machine-checked post-quantum security proof of a real-world cryptographic protocol, Whitaker emphasizes.
Signal partnered with
Inria
and
Cryspen
and published machine-verified proofs in the formal model used for the analysis of PQ3, as well as in a more realistic computational model that includes passive quantum attacks on all aspects of the protocol, Whittaker says. In that sense, we believe that our verification goes beyond what Apple published today. Wed be interested to see the same formal verification tools used to validate PQ3 as well.
Apple says the beta version of PQ3 is already in the hands of developers; customers will start receiving it with the anticipated March releases of iOS 17.4, iPadOS 17.4, macOS 14.4, and watchOS 10.4. The Apple engineering team says iMessage communications between devices that support PQ3 are automatically ramping to enable the post-quantum encryption protocol.
As we gain operational experience with PQ3 at the massive global scale of iMessage, it will fully replace the existing protocol within all supported conversations this year, they stated in the post.
Instead of swapping out the current encryption algorithm in iMessage with a new one, the Apple engineers say they rebuilt the iMessage cryptographic protocol from scratch. Among their most important requirements were enabling post-quantum encryption from the beginning of a message exchange while mitigating the effect of a compromise to a key by restricting how many messages a single key that has been compromised can decrypt.
The new iMessage is based on a hybrid design that uses post-quantum algorithms and existing Elliptic Curve algorithms, which Apples engineers say ensures that PQ3 can never be less safe than the existing classical protocol.
The engineers also note that, with PQ3, each device will generate PQC keys locally and transmit them to Apple servers as part of the iMessage registration process. For this function, Apple says it is implementing Kyber, one of
the algorithms chosen
by the National Institute of Standards (NIST) in August 2023 as a proposed Module-Lattice-based Key-Encapsulation Mechanism (
ML-KEM
) standard.
Kyber enables devices to generate public keys and transmit them to Apple servers through the iMessage registration process.
Cryptographer
Bruce Schneier
credits Apple for adopting the NIST standard and for its agile approach to developing PQ3. But he warns that there are still many variables and unknowns to overcome before the first quantum computer is capable of breaking classical encryption.
I think their crypto agility is more important than what they are doing, Schneier says. Between us cryptographers, we have a lot to learn about the cryptanalysis of these algorithms. It is unlikely that they will be as resilient as RSA and other public-key algorithms have been, but theyre the standards. So if youre going to do it, you should use the standards.
About his skepticism of the long-term capabilities of PQC algorithms, Schneier says, Theres enormous amounts of mathematics to be discussed. And every year we are learning more and breaking more. But these are the standards. I mean, these are the best we have right now.
Indeed, quantum-resistant algorithms may be less critical today. Like many forecasts, Apple pointed to reports that the first quantum computer capable of breaking existing encryption isnt expected to appear before 2035, the year the Biden administration ordered federal agencies to
ensure their systems are quantum-resilient
.
Pegging the risk a decade later at just 50%, Apple, like many cybersecurity experts, is underscoring that threat actors are stealing data and holding onto it until they can acquire quantum computing resources. The practice, known as harvest now, decrypt later, is especially concerning to organizations such as health care providers, whose data will remain relevant for decades.

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Apple Beefs Up iMessage With Quantum-Resistant Encryption