Botan 2.19.1
Crypto and TLS for C&
workfactor.cpp
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1/*
2* Public Key Work Factor Functions
3* (C) 1999-2007,2012 Jack Lloyd
4*
5* Botan is released under the Simplified BSD License (see license.txt)
6*/
7
8#include <botan/workfactor.h>
9#include <algorithm>
10#include <cmath>
11
12namespace Botan {
13
14size_t ecp_work_factor(size_t bits)
15 {
16 return bits / 2;
17 }
18
19namespace {
20
21size_t nfs_workfactor(size_t bits, double log2_k)
22 {
23 // approximates natural logarithm of an integer of given bitsize
24 const double log2_e = 1.44269504088896340736;
25 const double log_p = bits / log2_e;
26
27 const double log_log_p = std::log(log_p);
28
29 // RFC 3766: k * e^((1.92 + o(1)) * cubrt(ln(n) * (ln(ln(n)))^2))
30 const double est = 1.92 * std::pow(log_p * log_log_p * log_log_p, 1.0/3.0);
31
32 // return log2 of the workfactor
33 return static_cast<size_t>(log2_k + log2_e * est);
34 }
35
36}
37
38size_t if_work_factor(size_t bits)
39 {
40 // RFC 3766 estimates k at .02 and o(1) to be effectively zero for sizes of interest
41
42 const double log2_k = -5.6438; // log2(.02)
43 return nfs_workfactor(bits, log2_k);
44 }
45
46size_t dl_work_factor(size_t bits)
47 {
48 // Lacking better estimates...
49 return if_work_factor(bits);
50 }
51
52size_t dl_exponent_size(size_t bits)
53 {
54 /*
55 This uses a slightly tweaked version of the standard work factor
56 function above. It assumes k is 1 (thus overestimating the strength
57 of the prime group by 5-6 bits), and always returns at least 128 bits
58 (this only matters for very small primes).
59 */
60 const size_t min_workfactor = 64;
61 const double log2_k = 0;
62
63 return 2 * std::max<size_t>(min_workfactor, nfs_workfactor(bits, log2_k));
64 }
65
66}
Definition: alg_id.cpp:13
size_t ecp_work_factor(size_t bits)
Definition: workfactor.cpp:14
size_t dl_work_factor(size_t bits)
Definition: workfactor.cpp:46
size_t dl_exponent_size(size_t bits)
Definition: workfactor.cpp:52
size_t if_work_factor(size_t bits)
Definition: workfactor.cpp:38