Botan 2.13.0 Crypto and TLS for C++11

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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
12 namespace Botan {
13
14 size_t ecp_work_factor(size_t bits)
15 {
16 return bits / 2;
17 }
18
19 namespace {
20
21 size_t nfs_workfactor(size_t bits, double k)
22 {
23 // approximates natural logarithm of integer of given bitsize
24 const double log2_e = std::log2(std::exp(1));
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>(std::log2(k) + log2_e * est);
34 }
35
36 }
37
38 size_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 return nfs_workfactor(bits, .02);
43 }
44
45 size_t dl_work_factor(size_t bits)
46 {
47 // Lacking better estimates...
48 return if_work_factor(bits);
49 }
50
51 size_t dl_exponent_size(size_t bits)
52 {
53 /*
54 This uses a slightly tweaked version of the standard work factor
55 function above. It assumes k is 1 (thus overestimating the strength
56 of the prime group by 5-6 bits), and always returns at least 128 bits
57 (this only matters for very small primes).
58 */
59 const size_t MIN_WORKFACTOR = 64;
60
61 return 2 * std::max<size_t>(MIN_WORKFACTOR, nfs_workfactor(bits, 1));
62 }
63
64 }
Botan::if_work_factor
size_t if_work_factor(size_t bits)
Definition: workfactor.cpp:38
Botan::ecp_work_factor
size_t ecp_work_factor(size_t bits)
Definition: workfactor.cpp:14
Botan
Definition: alg_id.cpp:13
Botan::dl_exponent_size
size_t dl_exponent_size(size_t bits)
Definition: workfactor.cpp:51
Botan::dl_work_factor
size_t dl_work_factor(size_t bits)
Definition: workfactor.cpp:45