Re: [Cfrg] [irsg] IRSG review of draft-irtf-cfrg-xmss-hash-based-signatures-08

["A. Huelsing" <[e-mail]>]

Dear Stephen,

we tried to make the required changes. Please have a look if you are fine
with the changes (especially regarding section 5 and the reference
implementation).

You find our answers to your review below and the new draft version attached.

Thanks again for your time,

Andreas, Aziz, Denis, Joost & Stefan


################# Answers ####################
possible errors:
----------------

- 3.1.2: Algorithm 2: "if ( (i + s) > w - 1 )..." seems to be
missing parenthesis around the "(w-1)" to me.  Without those
brackets I could interpret that test to always result in false.

#Done

- 4.1.9: should the call to setIdx in alg 12 be after treeSig?
  as-is you seem to have incremented the index too soon so
that when alg 11 does getIdx it'd presumably get the
incremented index and cause verification failure. I think
the same is true of alg 16 as well, in section 4.2.4.

#Done

significant comments, but likely fixable:
-----------------------------------------

- section 5: there are waaaay too many options defined here.
  As-is, this will damage potential deployment of xmss. I
would strongly suggest deleting all of the options except the
minimum, that being one (and only one) set of parameters for
XMSS and one for XMSS^MT. If others are needed later, those
can be defined later. (Note that the damage done here includes
the hours of developer time that would be wasted debating
which of these choices to implement/use. Consider the case of
pre-hash variants of eddsa for an ongoing example.)

####
#   We significantly changed section 5, please check if this
#   satisfies your remarks
####

- section 5 (or an appendix) should contain some test vectors
  (including intermediate values). Without those, implementers
have a much harder time of getting their code right.

# Added section on reference implementation


nits, near-nits and other ignorable things:
-------------------------------------------

- abstract: I'd suggest s/can withstand attacks/ can withstand
  so-far known attacks/
 
# Done

- 1.1: You say if used >1 time "no cryptographic security
  guarantees remain." It might be clearer to give some
examples of consequences, e.g. that the attacker can forge new
signatures or whatever.

# Done

- 1.1: I think you might mention that XMSS and other OTS ideas
  require some new crypto APIs. I'm not aware if anyone has
developed proposals for such, but would be interested if
someone has.

# Done

- 2.3, 2nd last para: you might want to say what happens with
  e.g.  B<<2 where B=0xf0. I assume the result is 0xc0 but
someone might think it's 0x3c0 or even 0xc3.

# Done: Removed the left shift as it is never used for single bytes

- 2.5: having the "type word" as octet 15 of a 32 byte address
  seems odd. Is there a reason why? (Just wondering.)

# Yes: We got the space and think that it simlifies implementation if
one always has to manipulate whole words (and can treat the address as
uint32_t[8])
 
- 2.6: It seems odd to given an example where the input and
  output of base_w() are the same. A different example may be
more useful. (More examples generally would be great.)

# Done

- 3.1.3: maybe note that "/" means nothing? Which I assume it
  does? Better might be to just say that.
 
# Done

- 3.1.5: "a maximum value of len_1 * (w - 1) * 2^8" is missing
  units
 
# Done: value -> Integer value

- 3.1.5: "the variable" - which one?

# Done: Added explanation

- 3.1.5: "For the parameter sets given in Section 5 a 32-bit
  unsigned integer is sufficient." Sufficient for what?

# Done: Added " to hold the checksum"
 
- 3.1.5: The ascii art at the end of p16 doesn't help much.

# while the art does not contain new information, we think it's
a handy reminder. Would prefer to keep it like this.

- 3.1.7: The "MUST match" statement doesn't seem enforceable
  nor testable so I'm not sure it's a good idea to include.
OTOH, I do get the idea of using 2119 terms for emphasis.

# Ok, so what should we do? I agree that key handling cannot be
verified. Still, it seems necessary to emphasize this.
If it is just about the use of the 2119 term,
how about:
"but it is crucial for security that the cryptographic strength
matches that of the used WOTS+ parameters."?

- 3.1.7: I think it might be useful to point out any specific
  problems associated with using a low entropy human memorable
secret (password) for the value S. No matter what you say,
people will do that, so better if you can say you told them
specifically about downsides of doing that.

# We adressed this in a new sentence at the end of the
# paragraph.

- 4.1.12: I'm not sure if the MAY there is correct or not.  If
  it means "you MAY use a different algorithm to get the same
output as alg 12" then that'd be fine. If something else is
meant I'm not sure what you're saying, and it'd probably be
better to not even mention it.
# That is exactly what it means. There are far more efficient
# algorithms to compute that output (but also far more complex)

- section 5 should also spell out the signature and
public key sizes in bytes and ideally, if you keep multiple
options, (but please don't:-) describe relative or measured
timings.

# Done