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Quantum Computers Are Not a Threat to 128-Bit Symmetric Keys

https://words.filippo.io/128-bits/
51•hasheddan•4h ago

Comments

occamofsandwich•2h ago
Disconcerting opening. If you want to put hash algorithms in the same category as symmetric keys in this particular case then say so without referring to them as if they are symmetric keys.
FiloSottile•2h ago
Hashes are symmetric cryptography primitives, and it's even proper to talk about key sizes for e.g. HMAC and HKDF hash-based constructions, to which Grover's algorithm applies analogously to how it applies to cipher keys.
occamofsandwich•50m ago
Assuming a member of the target audience sees the connection between HMAC and symmetric keys AFA usage, would you like them to be making leaps like this in their regular usage of cryptography? (I really couldn't tell you if an algorithm that involves being able to look into the box in the middle might not have characteristics that means part or all the primitives involved are less quantum safe than an algorithm that lacks that possibility yet I'd suspect I have a lot more experience than the average reader drawn in by the title.)
kd913•1h ago
If this is true, I feel teh wifi alliance have a tonne to answer for the ewaste they generate.

WPA3 moved from symmetric AES to ECDH which is vulnerable to Quantum. Gonna be a tonne of IOT inverters waste.

supernetworks_•1h ago
WPA3 moved from PBKDF to ECDH. AES CCMP and GCMP are still the underlying block ciphers in WPA3 with some other extensions for China
evil-olive•1h ago
WPA3 was announced in 2018 [0]. I don't think it's reasonable to blame them for not anticipating the next decade of cryptographic research.

...but even if they had, what realistically could they have done about it? ML-KEM was only standardized in 2024 [1].

also, the addition of ECDH in WPA3 was to address an existing, very real, not-theoretical attack [2]:

> WPA and WPA2 do not provide forward secrecy, meaning that once an adverse person discovers the pre-shared key, they can potentially decrypt all packets encrypted using that PSK transmitted in the future and even past, which could be passively and silently collected by the attacker. This also means an attacker can silently capture and decrypt others' packets if a WPA-protected access point is provided free of charge at a public place, because its password is usually shared to anyone in that place.

0: https://en.wikipedia.org/wiki/Wi-Fi_Protected_Access#WPA3

1: https://en.wikipedia.org/wiki/ML-KEM

2: https://en.wikipedia.org/wiki/Wi-Fi_Protected_Access#Lack_of...

ndriscoll•1h ago
Does it matter if an attacker can decrypt public wifi traffic? You already have to assume the most likely adversary (e.g. the most likely to sell your information) is the entity running the free wifi, and they can already see everything.
bdamm•44m ago
It is precisely because the operator of the wifi is not necessarily the adversary a user may be most concerned about. They may be, but they are not the only one. They are the one you know can be, but they aren't the only one.
tptacek•59m ago
For what it's worth, cryptography engineers were generally not happy with the Dragonfly PAKE, and PQC was a legitimate concern even in 2012.
bob1029•1h ago
I think quantum may be practically mitigated with aggressive key rotation in some cases. I've been prototyping an oauth machine-to-machine integration with a banking vendor that has our ecdsa keys rotate every 5 minutes. The keys are scheduled for deletion after 10 minutes. I see no reason I couldn't reduce this to something like 30s/60s. Our counterparty frequently scans our JWKS endpoint for revocation, so in practice an attacker with a quantum computer would need to be very fast if they wanted to break this particular wire agreement the scary way.
glitchc•1h ago
You're clearly not using these keys in certificates, which would need to be signed by a root or interim CA on every update.
bob1029•44m ago
Correct. The keys are only used for signing JWTs. Trust was established with the vendor out of band from this wire protocol (the URL they scan for public keys).
cortesoft•25m ago
This wouldn’t help symmetric key encryption, which is what this is talking about. The keys you are rotating are asymmetric keys, which are only used to exchange symmetric keys for the actual encryption. In good setups, those symmetric keys are changed every session anyway.

If an attacker can break the symmetric encryption in a reasonable amount of time, they can capture the output and break it later.

In addition, how are you doing the key rotation? You have to have some way of authenticating with the rotation service, and what is to stop them from breaking THAT key, and getting their own new certificate? Or breaking the trusted root authority and giving themselves a key?

bob1029•4m ago
> This wouldn’t help symmetric key encryption, which is what this is talking about.

I agree. The point I am trying to make is that even for asymmetric encryption (which is far more vulnerable), there are still plausible ways to make a quantum break more difficult.

The only thing that could compromise this scheme, aside from breaking the signing keys, would be to have TLS broken to the extent that viewing real-time traffic is possible. Any TLS break delayed by more than 15 minutes would be worthless.

ninjahawk1•1h ago
Very good breakdown, if I’m understanding Grover’s algorithm correctly, are you saying essentially that it would require either too much compute or too much time to be feasible but is still much more realistic than a brute force attack?

If that’s the case, would the time eventually be basically irrelevant with enough compute? For instance, if what’s now a data center is able to fit in the palm of your hand (comparing early computers that took up rooms to phones nowadays). So if compute is (somehow) eventually able to be incredibly well optimized or if we use something new, like how microprocessors were the next big thing, would that then be a quantum threat to 128-bit symmetric keys?

cortesoft•39m ago
I am not an expert, but while you are correct that a fast enough traditional computer (or a parallel enough computer) could brute force a 128 bit key, the amount of improvement required would dwarf what we have already experienced over the last 40 years, and is likely physically impossible without some major fundamental change in how computers work.

Compute has seen in the ballpark of a 5-10 orders of magnitude increase over the last 40 years in terms of instructions per second. We would need an additional 20-30 orders of magnitude increase to make it even close to achievable with brute force in a reasonable time frame. That isn’t happening with how we make computers today.

Strilanc•55m ago
Good post. Entirely correct, and well known amongst quantum researchers, but under appreciated in general.

Grover attacks are very blatantly impractical. When someone describes Grover-type attacks in the same breath as Shor-type attacks, without caveats, that's a red flag.

rolph•52m ago
encryption is not ever to be considered impossible to break.

every encryption scheme has at least one way to be decrypted.

fidelity of information is one use of encryption, if you apply the solution and get garbage, something is wrong, somewhere.

occultation of information is another use, that is commonly abused by extending undue trust. under the proviso that encryption will eventually be broken, you cant trust encryption to keep a secret forever, but you can keep it secret, for long enough that it is no longer applicible to an attack,or slightly askew usecase, thus aggressive rotation of keys becomes desirable

TacticalCoder•51m ago
Tangentially related but regarding RSA and ECC... With RSA can't we just say: "Let's use 16 384 bit keys" and be safe for a long while?

And for ECC, I know many are using the "2 exp 255 - 19" / 25519 for it's unlikely to be backdoored but it's only 256 bits but... Can't we find, say, "2 exp 2047 - 19" (just making that one up) and be safe for a while too?

Basically: for RSA and ECC, is there anything preventing us from using keys 10x bigger?

quinnjh•22m ago
> for RSA and ECC, is there anything preventing us from using keys 10x bigger?

you can run benchmarks yourself: openssl speed rsa1024 rsa2048

also this (slightly dated) java ex writeup covers this well: https://www.javamex.com/tutorials/cryptography/rsa_key_lengt...

tldr trade off is found between better performance and how many years the data needs to be assumed confidential

rugina•32m ago
On one hand I hear that quantum computers will crack factorisation and discrete logarithms, on the other that the max number factorised is 15 and that 21 might not even be feasible.

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Quantum Computers Are Not a Threat to 128-Bit Symmetric Keys

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