Discussion:
Curves Digest, Vol 235, Issue 1
(too old to reply)
Chang-An Zhao
2016-10-08 02:18:30 UTC
Permalink
Hi,Zooko, 
  Do you have an exact citation for this claim of "BN128 still has at least 96 bits of security"? or any other experts can provide more information for me?

I mean that BN128(take base field p have 256 bits about) was designed for AES-128 bit security. Now due to the progress of NFS algorithm, the security may be not attained. But where can I find this claim of "BN128 still has at least 96 bits of security"?



Best regards

-----------------------------------------
Chang-An Zhao
-----------------------------------------
Department of Mathematics,
Sun Yat-sen University,
P.R. China.
-----------------------------------------



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Today's Topics:

1. Re: Curves for pairings (Zooko Wilcox-OHearn)


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Message: 1
Date: Sun, 25 Sep 2016 23:58:05 +0000
From: Zooko Wilcox-OHearn <***@leastauthority.com>
To: Michael Scott <***@miracl.com>
Cc: "***@moderncrypto.org" <***@moderncrypto.org>
Subject: Re: [curves] Curves for pairings
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Thank you for doing this research and posting, trevp!

I'm glad to hear this opinion from Mike Scott that these attacks on BN
curves are unlikely to get a lot better, because in the Zcash project,
which is the new version of the Zerocash protocol ¹, we decided that
we would just stick with BN128 for now rather than try to hastily find
a new curve. Our deliberations about that are visible here: ².

What it boils down to is:

a) We believed (uncertainly) that BN128 still has at least 96 bits of
security, taking this attack into account. I'm glad of this email
thread to help bring to our attention any reasons to think otherwise!

and

b) Pairing performance is critical for us. A curve like Michael Scott
suggested that took 2.5 times as long for a pairing operation would
almost certainly blow our performance budget and we'd have to do some
serious re-engineering to get it back.

Sincerely,

Zooko

¹ https://z.cash/blog/fixing-zcash-vulns.html

² https://github.com/zcash/zcash/issues/714# understand the concrete
security level of the BN_128 curve in libsnark
Hello Trevor,
Great that you raise these issues.
In my opinion the new security estimates are reliable. There was always a
suspicion (I recall discussing it over a decade ago) that the DL problem in
these fields might/must be easier due to the special form of the modulus
used in pairing-friendly constructions like BN curves. However for what its
worth my gut tells me that further improvements may be difficult.
Unfortunately these developments will set back efforts at standardization.
Clearly to achieve security levels equivalent to AES-128/192/256 will
require larger fields and/or larger embedding degrees.
I would tentatively suggest that BN curves (which appeared to be such a
perfect fit for AES-128 security), may no longer be so useful. Which is a
bit tragic, as they were so nice! And we will have to live with curves with
a \rho value greater than 1.
I suspect that BLS curves with k=12 must now be a better fit (but maybe not
the best) for AES-128. Here we are experimenting with a 455-bit BLS curve,
derived from the parameter x=0x10000020000080000800. In some experiments our
code is 2.5 times slower to calculate a pairing on this curve compared with
a 256-bit BN curve.
However there is a lot of work to be done now to determine the optimal curve
to use for each desired level of security.
Mike Scott
Hi all,
Last January we discussed elliptic curves for pairing-based crypto
("Curves and code for Identity-based Encryption" [1]).
People recommended 256-bit Barreto-Naehrig ("BN") curves [2] for ~128
- RELIC from Diego Aranha [3,4]
- DCLXVI from Naehrig et al [5]
- Golang's version, from Adam Langley [6]
- MIRACL from Michael Scott [7]
- herumi/ate-pairing from Shigeo and Tadanori [8]
"""
BN curves only really shine at the ~WF128 level, where a 256-bit curve
matches a 256*12-bit extension field discrete log problem (EFDLP). At
higher WFs (or if you’re worried about improvements in EFDLP), you
need a higher embedding degree so that the EFDLP will be harder, or
else a very large base field.
"""
It also seemed like 256-bit BN curves were becoming popular, at least
- Pond [10], using BBS signatures to prevent message spam
- SNARKs [11] as used by Zerocash [12], using pairings for
zero-knowledge proofs of correct program execution
- Apache Milagro [13,14], which proposes ID-based crypto instead of
Certificate Authorities
- "Puncturable encryption" [15,16], which proposes Attribute-Based
Encryption and Hierarchical IBE for message forward secrecy
- (maybe?) Intel SGX, which uses "EPID" group signatures for remote
attestation [17,18]
Unfortunately, "improvements in EFDLP" (as Mike called it) have
seemingly happened, in the last several months.
Kim and Barbulescu's "Extended Tower Number Field Sieve" [19] improves
the efficiency of calculating "finite field" (i.e. NOT elliptic curve)
discrete logs mod p^n, where p is large-ish and n is certain values,
like 6 or 12.
There was a great writeup by Aurore Guillevic on the ellipticnews blog
[20], and in a recent presentation [21]. I'll try to boil it down
The attack is relevant to pairing-friendly curves (like BN) that
support a "pairing" operation. A pairing maps points on the elliptic
curve to values in some "extension field". Security depends on both
the EC-DLP for the elliptic curve, and also DLP on the extension
field.
256-bit BN curves have a field prime p of 256 bits and an "embedding
degree" of 12, so the extension field is mod p^12, where p^12 has 3072
bits. The hope was that 256-bit EC-DLP and 3072-bit DLP would both
have security ~128 bits, so this would be nicely matched.
However, Kim and Barbulescu showed that DLP where the modulus is a
3072-bit (prime^12) is easier than DLP for a 3072-bit prime. How much
easier isn't clear, though by a "crude and naive estimation" they
suggest increasing the size of relevant curves by a third (Section 6).
Mehdi Tibouchi has a similar estimate [22]: "after this attack,
256-bit Barreto-Naehrig curves no longer offer 128 bits of security,
but perhaps closer to 96 or so".
* How reliable are the security estimates?
* Is this attack likely to be improved?
* Can the attacks benefit from precalculation (like Logjam [23]), or
does the work have to be repeated for each discrete log?
* What curves should new systems be based on?
I believe Voltage was using supersingular curves with embedding degree
2 (over a prime field) for Identity-Based Encryption [24]. GCHQ has
been promoting the "SAKKE" IBE scheme for various uses [25,26,27], and
I believe that uses similar curves [28]. However, IIUC, an embedding
degree of 2 means that ~128 bits of security requires a ~1536 bit
elliptic curve (to get a 3072 bit extension field). That seems pretty
slow, e.g. if a BN pairing is an order-of-magnitude slower than a
"regular" elliptic curve scalarmult, this seems like it would be
another order of magnitude slower.
Are there better options, for ~128 bits security?
[1] https://moderncrypto.org/mail-archive/curves/2015/#402
[2] https://eprint.iacr.org/2005/133
[3] https://github.com/relic-toolkit
[4] https://eprint.iacr.org/2013/722
[5] http://polycephaly.org/projects/dclxvi/
[6] https://godoc.org/golang.org/x/crypto/bn256
[7] https://github.com/miracl/MIRACL/
[8] https://github.com/herumi/ate-pairing
[9] https://moderncrypto.org/mail-archive/curves/2015/000405.html
[10] https://github.com/agl/pond
[11] https://eprint.iacr.org/2013/507
[12] http://zerocash-project.org/media/pdf/zerocash-oakland2014.pdf
[13] https://milagro.apache.org/
[14]
http://docs.milagro.io/en/amcl/milagro-crypto-library-white-paper.html
[15] https://github.com/imichaelmiers/libforwardsec/
[16]
https://www.computer.org/csdl/proceedings/sp/2015/6949/00/6949a305.pdf
[17] https://eprint.iacr.org/2009/095
[18]
https://www.blackhat.com/docs/us-16/materials/us-16-Aumasson-SGX-Secure-Enclaves-In-Practice-Security-And-Crypto-Review-wp.pdf
[19] https://eprint.iacr.org/2015/1027
[20]
https://ellipticnews.wordpress.com/2016/05/02/kim-barbulescu-variant-of-the-number-field-sieve-to-compute-discrete-logarithms-in-finite-fields/
[21] https://www.lorentzcenter.nl/lc/web/2016/834/presentations/Guillevic
[22]
https://ellipticnews.wordpress.com/2016/09/02/crypto-and-ches-2016-santa-barbara-ca-usa/
[23] https://weakdh.org/
[24] https://tools.ietf.org/html/rfc5091
[25]
https://www.benthamsgaze.org/wp-content/uploads/2016/01/SA3LI10_099.pdf
[26] https://tools.ietf.org/html/draft-barnes-mikey-sakke-mcptt-00
[27] http://www.securechorus.com/
[28] https://tools.ietf.org/html/rfc6508
Trevor
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Regards,

Zooko Wilcox

Founder and CEO
https://LeastAuthority.com — Freedom matters.


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Trevor Perrin
2016-10-08 02:34:37 UTC
Permalink
On Fri, Oct 7, 2016 at 7:18 PM, Chang-An Zhao
Post by Chang-An Zhao
  Do you have an exact citation for this claim of "BN128 still has at least 96 bits of security"? or any other experts can provide more information for me?
Hi Chang,

See the discussion in my original post:

https://moderncrypto.org/mail-archive/curves/2016/000740.html

The security situation isn't entirely clear yet, though that post
mentions some estimates.

Trevor
Michael Scott
2016-10-10 17:33:35 UTC
Permalink
This standard C program might help...

/*
L Function calculation - complexity of Integer factorisation/discrete
logarithm
gcc -O2 complexity.c -o complexity.exe
*/

#include <stdio.h>
#include <math.h>

#define FIDDLE_FACTOR 12 // To answer your next question - I have no idea!

/* Enter number of bits in modulus (or extension field) and assumed
complexity - usually 64, 48 or 32 (over 9) depending on the actual
calculation */
/* Its assumed to be (64/9) for factorisation, but maybe as low as (32/9)
for extension field discrete log */
/* Returns approximate amount of work required for optimal Index Calculus
method */

/* Ideally for pairing friendly curve NB*2*k*rho bits should require work
2^NB */
/* The number of bits in the curve modulus is NB*2*rho */
/* 2^NB is work required to break using Pollard-rho, and k is embedding
degree */
/* So for example a BN curve is ideal at the NB level if inputting
NB*2*12*1 bits
were to produce an output of 2^NB, for the assumed complexity (64, 48 or
32)
If (64/9) is appropriate, then 256-bit BN curves are ideal for the
128-bit level
But it would appear that if (32/9) applies, 256-bit BN curves provide
only 99-bits of security.
*/

void L(int bts,int cpx)
{
double w=bts*log(2.0);
double c= exp(pow(((double)cpx/9.0)*w,1.0/3.0)*pow(log(w),2.0/3.0));
printf("bits= %d Complexity (%d/9)
work=2^%d\n",bts,cpx,1+(int)log2(c)-FIDDLE_FACTOR);
return;
}

int main()
{
L(80*2*12*1,32); // 160-bit BN curve ideal for 80-bit security
L(3072,64); // factoring a 3072 bit number
L(128*2*12*1,32); // 256-bit BN curve
L(128*2*12*1,16); // hope this never happens...
L(224*2*12*1,32); // restoring faith with 448-bit BN curve - but group
size too big!
L(128*2*8*2,48); // 512-bit Cocks-Pinch curve, embedding degree 8

L(112*2*12*3/2,32); // BLS k=12 curve ideal at 112-bit security
L(128*2*16*5/4,32); // KSS k=16 curve ideal at 128-bit security
L(128*2*18*4/3,32); // KSS k=18

L(192*2*24*5/4,32); // BLS k=24
L(192*2*32*9/8,32); // KSS k=32 curve ideal at 192-bit level
L(256*2*36*7/6,32); // KSS k=36
L(256*2*48*9/8,32); // BLS k=48 curve ideal at 256-bit level

return 0;
}


Mike
Post by Trevor Perrin
On Fri, Oct 7, 2016 at 7:18 PM, Chang-An Zhao
Post by Chang-An Zhao
Do you have an exact citation for this claim of "BN128 still has at
least 96 bits of security"? or any other experts can provide more
information for me?
Hi Chang,
https://moderncrypto.org/mail-archive/curves/2016/000740.html
The security situation isn't entirely clear yet, though that post
mentions some estimates.
Trevor
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