Post-Quantum Cryptography for DKIM, PGP, and S/MIME: Quantum Threat to Email Security
Home » Post-Quantum Cryptography for DKIM, PGP, and S/MIME: Quantum Threat to Email Security
Published: February 17, 2026
Quantum computers won’t break the internet tomorrow… but they will break your email security sooner than you think.
Post-Quantum Cryptography for DKIM, PGP, and S/MIME: Quantum Threat to Email Security
Home » Post-Quantum Cryptography for DKIM, PGP, and S/MIME: Quantum Threat to Email Security
Published: February 17, 2026
Quantum computers won’t break the internet tomorrow… but they will break your email security sooner than you think.
Today, cybercriminals and state-sponsored groups are quietly collecting encrypted emails in bulk. They know they can’t crack the encryption today. That’s fine. They don’t need to. They’re playing the long game. This tactic has a name: Store-Now-Decrypt-Later (SNDL).
Attackers capture your encrypted emails today and wait for the moment when quantum computers become powerful enough to crack RSA and ECC. And suddenly, everything you thought was secure becomes an open book.
Your business contracts, personal conversations, financial approvals, and customer data can all be exposed in one future wave of mass decryption.
Once your emails are stolen, you don’t get a second chance to protect them. You can patch a system. You can rotate a password. But you can’t “un-send” an email that’s already sitting in someone’s archive.
This is why the quantum threat to email isn’t a tomorrow problem, it’s a right now problem.
Also Read: What is PQC? How to Resist Post-quantum Computing Attacks?
Why Email Security Is About to Change Forever
Email feels old, but it’s still the backbone of every business on the planet. Contracts get approved over email. Credentials get shared over email. Invoices, legal letters, and internal plans all move through email every single day.
And the only reason this massive communication system works is because of three silent guardians:
DKIM — verifies that an email wasn’t tampered with.
PGP — encrypts messages so only the intended person can read them.
S/MIME — provides enterprise-grade encryption and digital signatures.
Also Read: What is DKIM, DMARC and SPF? The Ultimate Guide on Email Authentication Protocols
These tools make email usable in a hostile internet environment. They keep your messages authentic, private, and trustworthy.
All three depend on classical public-key cryptography, mainly RSA and ECC. And this is exactly what quantum computers are built to destroy.
Quantum machines running Shor’s algorithm can break RSA and ECC so fast that the security we rely on today becomes meaningless.
What currently takes thousands or millions of years for traditional computers will take a quantum computer hours… or even minutes.
Also Read: What is RSA Asymmetric Encryption? Basics, Principles and Applications
The Quantum Threat: What’s Actually Going to Break?
Quantum computers aren’t going to nibble at email security. They’re going to smash straight through the core systems that keep email trustworthy. Here’s what falls apart first.
DKIM: Domain Identity Becomes Fakeable
DKIM is what tells your inbox, “This email really came from Google, Microsoft, or your bank.”
The problem?
DKIM relies on RSA or ECC signatures, the exact algorithms quantum computers can tear apart. With a strong enough quantum machine, attackers can forge DKIM signatures and make a malicious email look 100% legitimate.
PGP: Your “Secure” Messages Become Readable
PGP is loved by privacy-focused users, security researchers, and anyone dealing with sensitive data.
But its strength depends entirely on one thing. Attackers cannot compute your private key.
Quantum computers change that. With Shor’s algorithm, a quantum machine can derive PGP private keys almost instantly. It’s like giving attackers a master skeleton key to your entire message history.
S/MIME: Enterprise Email Security Collapses
S/MIME is the corporate workhorse of email security. Enterprises, governments, and regulated industries rely on it for encryption, authentication, and compliance.
And it also relies on RSA and ECC. Quantum cracking makes S/MIME certificates worthless. The entire PKI chain collapses. Email confidentiality collapses with it.
Post-Quantum Cryptography (PQC)
Before you start worrying that email security is doomed forever, here’s the good news. We’re not walking into this quantum era blind.
NIST, the same organisation that standardised AES and modern encryption, has already been working on replacements for RSA and ECC for years. And they’ve officially selected the next generation of cryptography, Post-Quantum Cryptography (PQC).
These are not experimental algorithms. They have been proven in battle, peer-reviewed, and quantum-attack-resistant. These are the two giants you will see all around:
CRYSTALS-Kyber is used for key exchange and encryption.
CRYSTALS-Dilithium Used in digital signatures.
Both algorithms are designed around math problems that quantum computers can’t efficiently solve, unlike RSA and ECC.
Also Read: NIST Advances 14 Algorithms to Round 2 of the Post-Quantum Cryptography Standardization Process
How PQC Fits Into Email: The Real Upgrade Path
Now this question comes to everyone’s mind. How do we actually move email from “quantum-vulnerable” to “quantum-safe”? The answer isn’t ripping everything out and starting from scratch. It’s transitioning carefully, step by step, using hybrid cryptography.
DKIM + PQC: Double Signatures, Double Safety
DKIM needs a quantum-safe upgrade, but we can’t flip a switch overnight. That’s where hybrid signatures come in.
Here’s how it works:
Your mail server signs outbound emails with RSA/ECC (current standard)
And also signs them with PQC (Dilithium)
Both signatures sit inside the same DKIM DNS record
That even with the arrival of quantum computers tomorrow, your email is safe. Such standards organisations as IETF LAMPS are already specifying the appearance of these hybrid DKIM signatures.
And the big mail companies, Google, Microsoft, and Fastmail, are conducting experimental background experiments.
PGP + PQC: Hybrid Keys for Real Privacy
PGP will also shift into hybrid mode. Instead of choosing between “old crypto” and “new crypto,” you combine both.
An ECC key for compatibility
A PQC key (Kyber + Dilithium) for quantum safety
Think of it like having two locks on your safe. If one fails, the other still protects you. The OpenPGP working group is actively developing this hybrid PGP design using Kyber for encryption and Dilithium for signatures.
The good part is that it’s Backward compatibility. Some older PGP clients will break. Some email tools won’t understand the new hybrid keys. And that means adoption will be slower, and privacy tools take time to evolve. PGP won’t survive without PQC.
S/MIME + PQC: The Enterprise Shake-Up
The real headache is S/MIME. Since S/MIME is based on certificates and complicated chains of PKI, this is not as simple as the addition of a new algorithm. PQC migration (of S/MIME) implies:
New quantum-resistant certificates
New PKI chains
New trust anchors
New certificate rotation policies.
Basically, every link in the chain must be upgraded before anything works. Enterprises also need to start rotating encryption keys early. If not, they risk storing years of emails that become instantly readable when quantum decryption becomes practical.
S/MIME adoption will move more slowly than DKIM or PGP simply because the certificate ecosystem is… complicated.
Think of it like replacing the foundation of a building instead of just swapping the front door. But the shift is already happening. The vendors and certificate authorities are testing PQC-enabled S/MIME certificates today.
The Biggest Migration Problems
Switching email to post-quantum cryptography sounds exciting… until you look under the hood.
The truth? This migration won’t be smooth. And almost nobody is talking about the real challenges coming our way.
Larger Key Sizes = Slower Everything
Quantum-safe algorithms are powerful, but they’re also big. We’re talking key sizes that are 5x to 20x larger than RSA or ECC.
Bigger keys mean:
Bigger emails
Bigger DNS records
More bandwidth
More processing time
It’s like replacing a small lock with a huge steel vault. You get better security, but everything becomes heavier and slower.
Old Email Clients Will Break
Not every email client is ready for PQC. Many of them were built decades ago and barely support modern TLS, let alone hybrid PQC signatures.
This will cause:
Broken signatures
Failed decryption attempts
“Unknown algorithm” errors
Lack of backward compatibility
S/MIME depends on certificate authorities (CAs). But most CAs today are built around RSA and ECC.
To make the jump, CAs will need to rethink and adopt PQC:
new signing algorithms
new certificate formats
new trust chains
new policy rules
new hardware to protect keys
What Businesses Should Start Doing Today
Quantum risks feel like one of those things everyone agrees are “important,” but no one wants to think about yet. The problem is that crypto transitions move more slowly than people expect.
By the time the danger feels real, it’s already too late to adapt.
The companies that start early won’t even notice the shift. The ones that don’t will eventually scramble in a way that looks embarrassing in hindsight.
Figure Out Where You’re Using RSA and ECC
Most teams don’t actually know. They assume they know, which is worse. Look at the parts of your email system that depend on these algorithms:
DKIM signatures
PGP keys
S/MIME certs
Whatever TLS your mail servers negotiate
Any glue code, cron jobs, or scripts still generating RSA/ECC keys
If you can’t describe where your encryption lives, start writing it down. Even a messy inventory is better than guessing. Everything else in your PQC plan depends on this step.
Start Playing With PQC Libraries
You don’t need to deploy anything for real yet. Just get your hands dirty. Make a small prototype. Even if it breaks, you’ll understand the shape of the future a bit more clearly. Better to have things fall apart during a test project than during a real migration.
Push Your Vendors
You’re not switching to PQC alone. Your stack is glued together with tools made by other people, and if they aren’t moving, neither are you. Most vendors only move when customers start asking the same questions in unison. Put “PQC-ready” as a requirement in new contracts. Vendors notice those lines.
Use Hybrid Crypto where possible.
The good fact with hybrid cryptography is that there is no need to make a choice: it uses classical and post-quantum cryptography simultaneously. It interacts with the current systems but does not subject you to the problems that will arise tomorrow. Consider it as putting a seatbelt on a prehistoric motor vehicle, not ideal, but at least safer.
Revise Your Policies Before somebody pushes you to.
Finance, healthcare, and government industries have already begun developing quantum-readiness rules. You can do so without having to wait till you are mandated to.
Zero Trust + PQC Will Be the Future of Email
Email isn’t going anywhere. Despite every “email is dead” prediction of the last decade, businesses rely on it more than ever. But the way we secure email is about to change big time. The future will combine two powerful ideas:
Quantum-safe cryptography (PQC)
Zero Trust email security
Together, they form a defence strong enough for the threats of the next 20+ years.
Also Read: NIST Publishes New Zero Trust Implementation Guidance
Conclusion
The threat of quantum is no longer science fiction. They’re a countdown. And when quantum computers come to the fore with a vengeance, all emails encrypted by RSA or ECC will be decrypted overnight. It is the organisations that begin preparing today that will remain safe, regulatory, and ahead of the curve. The ones that wait? They will be responding to a totally foreseeable crisis.
If you want to future-proof your email systems with PQC, hybrid crypto, and quantum-ready policies, reach out. Contact us today, and our PKI experts will guide you through adopting post-quantum cryptography across your organisation.
Janki Mehta
Janki Mehta is a passionate Cyber-Security Enthusiast who keenly monitors the latest developments in the Web/Cyber Security industry. She puts her knowledge into practice and helps web users by arming them with the necessary security measures to stay safe in the digital world.
*** This is a Security Bloggers Network syndicated blog from EncryptedFence by Certera – Web & Cyber Security Blog authored by Janki Mehta. Read the original post at: https://certera.com/blog/post-quantum-cryptography-for-dkim-pgp-and-s-mime/
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