Apple rolls out ‘PQ3,’ the most significant cryptographic security upgrade in iMessage history
Note: This comparison evaluates only the cryptographic aspect of messaging security, and therefore focuses on end-to-end encryption and quantum security. Such a comparison doesn’t include automatic key verification, which we believe is a critical protection for modern messaging apps. As of the time of this writing, only iMessage and WhatsApp provide automatic key verification. The iMessage implementation, called Contact Key Verification, is
the state of the art – it provides the broadest automatic protections and applies across all of a user’s devices.
Apple on Wednesday announced the most significant cryptographic security upgrade in iMessage history with the introduction of PQ3, a groundbreaking post-quantum cryptographic protocol that advances the state of the art of end-to-end secure messaging.
When iMessage launched in 2011, it was the first widely available messaging app to provide end-to-end encryption by default, and we have significantly upgraded its cryptography over the years. We most recently strengthened the iMessage cryptographic protocol in 2019 by switching from RSA to Elliptic Curve cryptography (ECC), and by protecting encryption keys on device with the Secure Enclave, making them significantly harder to extract from a device even for the most sophisticated adversaries. That protocol update went even further with an additional layer of defense: a periodic rekey mechanism to provide cryptographic self-healing even in the extremely unlikely case that a key ever became compromised. Each of these advances were formally verified by symbolic evaluation, a best practice that provides strong assurances of the security of cryptographic protocols.
Historically, messaging platforms have used classical public key cryptography, such as RSA, Elliptic Curve signatures, and Diffie-Hellman key exchange, to establish secure end-to-end encrypted connections between devices. All these algorithms are based on difficult mathematical problems that have long been considered too computationally intensive for computers to solve, even when accounting for Moore’s law. However, the rise of quantum computing threatens to change the equation. A sufficiently powerful quantum computer could solve these classical mathematical problems in fundamentally different ways, and therefore — in theory — do so fast enough to threaten the security of end-to-end encrypted communications.
Although quantum computers with this capability don’t exist yet, extremely well-resourced attackers can already prepare for their possible arrival by taking advantage of the steep decrease in modern data storage costs. The premise is simple: such attackers can collect large amounts of today’s encrypted data and file it all away for future reference. Even though they can’t decrypt any of this data today, they can retain it until they acquire a quantum computer that can decrypt it in the future, an attack scenario known as Harvest Now, Decrypt Later.
To mitigate risks from future quantum computers, the cryptographic community has been working on post-quantum cryptography (PQC): new public key algorithms that provide the building blocks for quantum-secure protocols but don’t require a quantum computer to run — that is, protocols that can run on the classical, non-quantum computers we’re all using today, but that will remain secure from known threats posed by future quantum computers.
To reason through how various messaging applications mitigate attacks, it’s helpful to place them along a spectrum of security properties. There’s no standard comparison to employ for this purpose, so we lay out our own simple, coarse-grained progression of messaging security levels in the image at the top of this post: we start on the left with classical cryptography and progress towards quantum security, which addresses current and future threats from quantum computers. Most existing messaging apps fall either into Level 0 — no end-to-end encryption by default and no quantum security — or Level 1 — with end-to-end encryption by default, but with no quantum security. A few months ago, Signal added support for the PQXDH protocol, becoming the first large-scale messaging app to introduce post-quantum security in the initial key establishment. This is a welcome and critical step that, by our scale, elevated Signal from Level 1 to Level 2 security.
At Level 2, the application of post-quantum cryptography is limited to the initial key establishment, providing quantum security only if the conversation key material is never compromised. But today’s sophisticated adversaries already have incentives to compromise encryption keys, because doing so gives them the ability to decrypt messages protected by those keys for as long as the keys don’t change. To best protect end-to-end encrypted messaging, the post-quantum keys need to change on an ongoing basis to place an upper bound on how much of a conversation can be exposed by any single, point-in-time key compromise — both now and with future quantum computers. Therefore, we believe messaging protocols should go even further and attain Level 3 security, where post-quantum cryptography is used to secure both the initial key establishment and the ongoing message exchange, with the ability to rapidly and automatically restore the cryptographic security of a conversation even if a given key becomes compromised.
iMessage now meets this goal with a new cryptographic protocol that we call PQ3, offering the strongest protection against quantum attacks and becoming the only widely available messaging service to reach Level 3 security. Support for PQ3 will start to roll out with the public releases of iOS 17.4, iPadOS 17.4, macOS 14.4, and watchOS 10.4, and is already in the corresponding developer preview and beta releases. iMessage conversations between devices that support PQ3 are automatically ramping up to 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.
Designing PQ3
More than simply replacing an existing algorithm with a new one, we rebuilt the iMessage cryptographic protocol from the ground up to advance the state of the art in end-to-end encryption, and to deliver on the following requirements:
• Introduce post-quantum cryptography from the start of a conversation, so that all communication is protected from current and future adversaries.
• Mitigate the impact of key compromises by limiting how many past and future messages can be decrypted with a single compromised key.
• Use a hybrid design to combine new post-quantum algorithms with current Elliptic Curve algorithms, ensuring that PQ3 can can never be less safe than the existing classical protocol.
• Amortize message size to avoid excessive additional overhead from the added security.
• Use formal verification methods to provide strong security assurances for the new protocol.
PQ3 introduces a new post-quantum encryption key in the set of public keys each device generates locally and transmits to Apple servers as part of iMessage registration. For this application, we chose to use Kyber post-quantum public keys, an algorithm that received close scrutiny from the global cryptography community, and was selected by NIST as the Module Lattice-based Key Encapsulation Mechanism standard, or ML-KEM. This enables sender devices to obtain a receiver’s public keys and generate post-quantum encryption keys for the very first message, even if the receiver is offline. We refer to this as initial key establishment.
We then include — within conversations — a periodic post-quantum rekeying mechanism that has the ability to self-heal from key compromise and protect future messages. In PQ3, the new keys sent along with the conversation are used to create fresh message encryption keys that can’t be computed from past ones, thereby bringing the conversation back to a secure state even if previous keys were extracted or compromised by an adversary. PQ3 is the first large scale cryptographic messaging protocol to introduce this novel post-quantum rekeying property.
PQ3 employs a hybrid design that combines Elliptic Curve cryptography with post-quantum encryption both during the initial key establishment and during rekeying. Thus, the new cryptography is purely additive, and defeating PQ3 security requires defeating both the existing, classical ECC cryptography and the new post-quantum primitives. It also means the protocol benefits from all the experience we accumulated from deploying the ECC protocol and its implementations.
Rekeying in PQ3 involves transmitting fresh public key material in-band with the encrypted messages that devices are exchanging. A new public key based on Elliptic Curve Diffie-Hellman (ECDH) is transmitted inline with every response. The post-quantum key used by PQ3 has a significantly larger wire size than the existing protocol, so to meet our message size requirement we designed the quantum-secure rekeying to happen periodically rather than with every message. To determine whether a new post-quantum key is transmitted, PQ3 uses a rekeying condition that aims to balance the average size of messages on the wire, preserve the user experience in limited connectivity scenarios, and keep the global volume of messages within the capacity of our server infrastructure. Should the need arise, future software updates can increase the rekeying frequency in a way that’s backward-compatible with all devices that support PQ3.
With PQ3, iMessage continues to rely on classical cryptographic algorithms to authenticate the sender and verify the Contact Key Verification account key, because these mechanisms can’t be attacked retroactively with future quantum computers. To attempt to insert themselves in the middle of an iMessage conversation, an adversary would require a quantum computer capable of breaking one of the authentication keys before or at the time the communication takes place. In other words, these attacks cannot be performed in a Harvest Now, Decrypt Later scenario — they require the existence of a quantum computer capable of performing the attacks contemporaneously with the communication being attacked. We believe any such capability is still many years away, but as the threat of quantum computers evolves, we will continue to assess the need for post-quantum authentication to thwart such attacks.
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