Botan: src/pubkey/workfactor.cpp Source File

Botan 1.10.16

workfactor.cpp
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1 /*
2 * Public Key Work Factor Functions
3 * (C) 1999-2007,2012 Jack Lloyd
4 *
5 * Distributed under the terms of the Botan license
6 */
7
8 #include <botan/internal/workfactor.h>
9 #include <algorithm>
10 #include <cmath>
11
12 namespace Botan {
13
14 size_t dl_work_factor(size_t bits)
15 {
16 /*
17 Based on GNFS work factors. Constant is 1.43 times the asymptotic
18 value; I'm not sure but I believe that came from a paper on 'real
19 world' runtimes, but I don't remember where now.
20
21 Sample return values:
22 |512| -> 64
23 |1024| -> 86
24 |1536| -> 102
25 |2048| -> 116
26 |3072| -> 138
27 |4096| -> 155
28 |8192| -> 206
29
30 For DL algos, we use an exponent of twice the size of the result;
31 the assumption is that an arbitrary discrete log on a group of size
32 bits would take about 2^n effort, and thus using an exponent of
33 size 2^(2*n) implies that all available attacks are about as easy
34 (as e.g Pollard's kangaroo algorithm can compute the DL in sqrt(x)
35 operations) while minimizing the exponent size for performance
36 reasons.
37 */
38
39 const size_t MIN_WORKFACTOR = 64;
40
41 // approximates natural logarithm of p
42 const double log_p = bits / 1.4426;
43
44 const double strength =
45 2.76 * std::pow(log_p, 1.0/3.0) * std::pow(std::log(log_p), 2.0/3.0);
46
47 return std::max(static_cast<size_t>(strength), MIN_WORKFACTOR);
48 }
49
50 }
Botan
Definition: algo_base.h:14
Botan::CT::max
T max(T a, T b)
Definition: ct_utils.h:120
Botan::dl_work_factor
size_t dl_work_factor(size_t bits)
Definition: workfactor.cpp:14