1. Quantum-safe authentication.

As Yawning has pointed out,

> I personally don't think that any of the PQ signature schemes are usable
> for us right now, because the smallest key size for an algorithm that
> isn't known to be broken is ~1 KiB (SPHINCS256), and we probably can't
> afford to bloat our descriptors/micro-descriptors that much.

This is also the reason we wanted to roll out QSH first and add the quantum-safe

signature schemes later. We have several good candidate for quantum-safe

encryption algorithms. But for signatures, they are not mature imho.

Another reason that we did not include authentication is due to the attack model.

As I have mentioned (but probably didn't explain very clearly) in previous email, we

are facing two types of attackers. Attacker type I, passive attacker at present, who

cannot break classical authentication nor encryption, record the data anyway, and

decrypt it when quantum computer become available in future. Attack type II, active

attacker who tries to attack the authentication while the communication is taking

place. This attack will be possible in future when quantum computer arrives. But for

now, they will not be successful.

As we believe that there does not exist a general purpose quantum computer at

present (and maybe several years away), we have time to deal with attacker type II.

But the attacker type I is the real threat at the present day. Our proposal is to address

this threat.

2. On NTRU vs NTRU-Prime vs R-LWE and others.

The QSH is modular designed to suite any quantum-safe encryption algorithm. So we

can chose any one we want for trail. And furthermore, we can also hybrid, say ECC,
NTRU and R-LWE, to give a bit more confidence in case one of the quantum-safe encryption

algorithm turns out to be not quantum safe, or broken.

That been said, we chose NTRU for several reasons. NTRU is more mature than R-LWE

from our point o view. NTRU has a full spec, a reference implementation, and is standardized

by several bodies; while for R-LWE, since it enables many interesting cryptographic primitives,

such as FHE, there has been many different parameter proposals, which leads to some kind

of confusion as to which one should reference to. 

> However, if we were to go the route of using NTRU, we'd likely want to instead
> use Dan Bernstein's NTRU Prime parameters, in order to eliminate some of the
> inherent algebraic structure of the ideal lattice which might possibly be
> exploited. [0] [1]

As for NTRU-Prime, I am not aware there is a specific instantiation of this parameter sets, nor

any paper that considers the security of a specific parameter set. Also, this NTRU-Prime would

require some extensive scrutiny before we can use it.

We are happy to roll out any above encryption algorithm as you see fit. But our proposal is mainly

about the QSH approach. I think the best option for now is to buildin a QSH for Tor, with a flexible

API that allows us to switch between algorithms when fit. And for now use any quantum-safe

encryption algorithm that is ready to be used. After all, any QS encryption is better than nothing.

3. License

I am sorry I am not familiar with the license. But my general feeling is that Security Innovation is

willing to let Tor to use NTRU for free. We just need to work out the suitable license to make this

happen.

4. Misc.
> Post-quantum forward-secrecy is what I've been using to describe this
> property.

We will use this terminology. Thanks.

> As I recall, the product form parameter sets are extra encumbered.
> Apart from key/ciphertext size and a minor performance differential, is
> there any reason to not use one of the X9.98 parameter sets (Eg:
> EES613EP1)

Yes we can use non-product form polynomials, if everyone agrees on it.

Non-product form polynomials will make key generation and decryption

a bit slower, but those cost are on the client side. It has no impact on the

load of server side.

> * "For 128 bits quantum security, use NTRU_EESS743EP1." should be
>   "For 256 bits" (Section 2.3).

NTRU_EESS743EP1 provides 256 classical security and 128 bits quantum security. Please see

for arguments of those security levels.

> I'm a little confused about what exactly is meant by "disaster resilience" here.

We will remove "disaster resilience".

Those are most comments I saw. Sorry if I missed some of your comments. Please let me know

if you have questions that I failed to answer.

Happy new year to everyone.

Cheers,

Zhenfei