Re: [viff-devel] Optimizing preprocessing

2009-10-21 Thread Marcel Keller

Martin Geisler wrote:

Janus Dam Nielsen janus.niel...@alexandra.dk writes:


Hi Marcel,

I am not opposed to your suggestion. However I would like to point out
that in VIFF you compute on shares and not field elements!


Well, we've actually made the outer runtime interfaces in such a way
that add, mul, xor, etc... accept both integers, FieldElements and
Shares. The methods then wrap their input as needed -- or they *dont*
wrap it if that leads to a short cut (e.g., constant multiplication)


I agree (see also my answer).


Computing directly on the field elements is hacking the abstractions
of VIFF. Computation on field elements or rather the representation of
a Share can be useful as an optimization, however this optimization
should be confined within applications or runtimes, and should not
progress over interface boundaries as I fear you are suggesting.


I think we are in agreement: public methods on the runtimes will keep
returning Shares. Methods used internally in runtimes can return other
things as needed. To me it sounds like a better API to require
preprocessing functions to return a list of Deferreds:

  [D(?), D(?), ...],

instead of a Deferred list of tuples containing Deferreds :-)

I think it will simplify the interface nicely, at least for consumers.
Using simpler types also leads to less memory usage which has a positive
effect on performance, as Marcel notes. So let's go for it.


So this makes 2 votes in favour of it and 1 against it. Maybe we should 
have a meeting to discuss it. What do you think?


Best regards,
Marcel
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Re: [viff-devel] Optimizing preprocessing

2009-10-09 Thread Janus Dam Nielsen

Hi Marcel,

I am not opposed to your suggestion. However I would like to point out  
that in VIFF you compute on shares and not field elements!. Computing  
directly on the field elements is hacking the abstractions of VIFF.
Computation on field elements or rather the representation of a Share  
can be useful as an optimization, however this optimization should be  
confined within applications or runtimes, and should not progress over  
interface boundaries as I fear you are suggesting.



On 08/10/2009, at 20.11, Marcel Keller wrote:


Dear friends of VIFF,

I have a proprosal to optimize preprocessing in VIFF, which I would  
like to put up for discussion.


Notation:
- D(x): a Deferred object whose callback function will be called  
with x

- S(x): same for a Share object
- F: a FieldElement object
- [x, ...]: a Python list
- (x, ...): a Python tuple

Current situation:
The two generate_triples() functions in the active runtimes return
n, D([(S(F), S(F), S(F)), ...]),
where n denotes the length of the list. Runtime.preprocess() puts  
one D(S(F), S(F), S(F)) per program counter in the preprocessing  
pool and uses util.deep_wait() to return a Deferred whose callback  
is called when all field elements are available.


The get_triple() functions return
D(S(F), S(F), S(F)),
either taken from the preprocessing pool, or by calling  
generate_triples() and adding an extra callback to extract the first  
triple.


My proposal:
The generate_triples() functions return
[D([F, F, F]), ...],
and Runtime.preprocess() puts one [F, F, F] per program counter in  
the preprocessing pool. There is no need for deep_wait, we can just  
use gatherResults on the Deferreds return by the generator  
functions. The generator functions can use gatherResults to get  
D([F, F, F]) from [S(F), S(F), S(F)].


The get_triple() functions return
[F, F, F], True  if there is a triple in the pool, and
[S(F), S(F), S(F)], Falseotherwise.
For the multiplication in BasicActiveRuntime it doesn't make a  
difference whether FieldElements or Shares are returned. Future  
applications which require Shares can wrap the FieldElements in  
Shares if the Boolean is True.


Benchmarks:
- 1 multiplications in parallel with active security using PRSS
 - 4% faster preprocessing
 - 5% faster online operation
 - 50 MB less memory used
- 10 AES blocks in parallel using masked exponentiation with active  
security using PRSS

 - 10% faster preprocessing
 - 10% faster online operation
 - 140 MB less memory used

Further considerations:
- I don't see any problem for replacing FieldElement with anything  
that does not contain Deferreds in any form.
- Probably, it would also be possible to leave the generator  
functions as they are and use something similar to deep_wait().  
However, I consider it as an overhead.
- One could also always wrap the FieldElements in Shares, but this  
would again be an overhead.


Best regards,
Marcel
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Janus Dam Nielsen

Research and Innovationspecialist, PhD.
CENTRE FOR IT-SECURITY

THE ALEXANDRA INSTITUTE LTD.

T +45 42 22 93 56
E janus.niel...@alexandra.dk
W alexandra.dk


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