/security/nss/lib/freebl/mpi/utils/README
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1***** BEGIN LICENSE BLOCK ***** 2Version: MPL 1.1/GPL 2.0/LGPL 2.1 3 4The contents of this file are subject to the Mozilla Public License Version 51.1 (the "License"); you may not use this file except in compliance with 6the License. You may obtain a copy of the License at 7http://www.mozilla.org/MPL/ 8 9Software distributed under the License is distributed on an "AS IS" basis, 10WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License 11for the specific language governing rights and limitations under the 12License. 13 14The Original Code is the MPI Arbitrary Precision Integer Arithmetic 15library. 16 17The Initial Developer of the Original Code is 18Michael J. Fromberger <sting@linguist.dartmouth.edu> 19Portions created by the Initial Developer are Copyright (C) 1998, 2000 20the Initial Developer. All Rights Reserved. 21 22Contributor(s): 23 24Alternatively, the contents of this file may be used under the terms of 25either the GNU General Public License Version 2 or later (the "GPL"), or 26the GNU Lesser General Public License Version 2.1 or later (the "LGPL"), 27in which case the provisions of the GPL or the LGPL are applicable instead 28of those above. If you wish to allow use of your version of this file only 29under the terms of either the GPL or the LGPL, and not to allow others to 30use your version of this file under the terms of the MPL, indicate your 31decision by deleting the provisions above and replace them with the notice 32and other provisions required by the GPL or the LGPL. If you do not delete 33the provisions above, a recipient may use your version of this file under 34the terms of any one of the MPL, the GPL or the LGPL. 35 36***** END LICENSE BLOCK ***** 37 38Additional MPI utilities 39------------------------ 40 41The files 'mpprime.h' and 'mpprime.c' define some useful extensions to 42the MPI library for dealing with prime numbers (in particular, testing 43for divisbility, and the Rabin-Miller probabilistic primality test). 44 45The files 'mplogic.h' and 'mplogic.c' define extensions to the MPI 46library for doing bitwise logical operations and shifting. 47 48This document assumes you have read the help file for the MPI library 49and understand its conventions. 50 51Divisibility (mpprime.h) 52------------ 53 54To test a number for divisibility by another number: 55 56mpp_divis(a, b) - test if b|a 57mpp_divis_d(a, d) - test if d|a 58 59Each of these functions returns MP_YES if its initial argument is 60divisible by its second, or MP_NO if it is not. Other errors may be 61returned as appropriate (such as MP_RANGE if you try to test for 62divisibility by zero). 63 64Randomness (mpprime.h) 65---------- 66 67To generate random data: 68 69mpp_random(a) - fill a with random data 70mpp_random_size(a, p) - fill a with p digits of random data 71 72The mpp_random_size() function increases the precision of a to at 73least p, then fills all those digits randomly. The mp_random() 74function fills a to its current precision (as determined by the number 75of significant digits, USED(a)) 76 77Note that these functions simply use the C library's rand() function 78to fill a with random digits up to its precision. This should be 79adequate for primality testing, but should not be used for 80cryptographic applications where truly random values are required for 81security. 82 83You should call srand() in your driver program in order to seed the 84random generator; this function doesn't call it. 85 86Primality Testing (mpprime.h) 87----------------- 88 89mpp_divis_vector(a, v, s, w) - is a divisible by any of the s values 90 in v, and if so, w = which. 91mpp_divis_primes(a, np) - is a divisible by any of the first np primes? 92mpp_fermat(a, w) - is a pseudoprime with respect to witness w? 93mpp_pprime(a, nt) - run nt iterations of Rabin-Miller on a. 94 95The mpp_divis_vector() function tests a for divisibility by each 96member of an array of digits. The array is v, the size of that array 97is s. Returns MP_YES if a is divisible, and stores the index of the 98offending digit in w. Returns MP_NO if a is not divisible by any of 99the digits in the array. 100 101A small table of primes is compiled into the library (typically the 102first 128 primes, although you can change this by editing the file 103'primes.c' before you build). The global variable prime_tab_size 104contains the number of primes in the table, and the values themselves 105are in the array prime_tab[], which is an array of mp_digit. 106 107The mpp_divis_primes() function is basically just a wrapper around 108mpp_divis_vector() that uses prime_tab[] as the test vector. The np 109parameter is a pointer to an mp_digit -- on input, it should specify 110the number of primes to be tested against. If a is divisible by any 111of the primes, MP_YES is returned and np is given the prime value that 112divided a (you can use this if you're factoring, for example). 113Otherwise, MP_NO is returned and np is untouched. 114 115The function mpp_fermat() performs Fermat's test, using w as a 116witness. This test basically relies on the fact that if a is prime, 117and w is relatively prime to a, then: 118 119 w^a = w (mod a) 120 121That is, 122 123 w^(a - 1) = 1 (mod a) 124 125The function returns MP_YES if the test passes, MP_NO if it fails. If 126w is relatively prime to a, and the test fails, a is definitely 127composite. If w is relatively prime to a and the test passes, then a 128is either prime, or w is a false witness (the probability of this 129happening depends on the choice of w and of a ... consult a number 130theory textbook for more information about this). 131 132Note: If (w, a) != 1, the output of this test is meaningless. 133---- 134 135The function mpp_pprime() performs the Rabin-Miller probabilistic 136primality test for nt rounds. If all the tests pass, MP_YES is 137returned, and a is probably prime. The probability that an answer of 138MP_YES is incorrect is no greater than 1 in 4^nt, and in fact is 139usually much less than that (this is a pessimistic estimate). If any 140test fails, MP_NO is returned, and a is definitely composite. 141 142Bruce Schneier recommends at least 5 iterations of this test for most 143cryptographic applications; Knuth suggests that 25 are reasonable. 144Run it as many times as you feel are necessary. 145 146See the programs 'makeprime.c' and 'isprime.c' for reasonable examples 147of how to use these functions for primality testing. 148 149 150Bitwise Logic (mplogic.c) 151------------- 152 153The four commonest logical operations are implemented as: 154 155mpl_not(a, b) - Compute bitwise (one's) complement, b = ~a 156 157mpl_and(a, b, c) - Compute bitwise AND, c = a & b 158 159mpl_or(a, b, c) - Compute bitwise OR, c = a | b 160 161mpl_xor(a, b, c) - Compute bitwise XOR, c = a ^ b 162 163Left and right shifts are available as well. These take a number to 164shift, a destination, and a shift amount. The shift amount must be a 165digit value between 0 and DIGIT_BIT inclusive; if it is not, MP_RANGE 166will be returned and the shift will not happen. 167 168mpl_rsh(a, b, d) - Compute logical right shift, b = a >> d 169 170mpl_lsh(a, b, d) - Compute logical left shift, b = a << d 171 172Since these are logical shifts, they fill with zeroes (the library 173uses a signed magnitude representation, so there are no sign bits to 174extend anyway). 175 176 177Command-line Utilities 178---------------------- 179 180A handful of interesting command-line utilities are provided. These 181are: 182 183lap.c - Find the order of a mod m. Usage is 'lap <a> <m>'. 184 This uses a dumb algorithm, so don't use it for 185 a really big modulus. 186 187invmod.c - Find the inverse of a mod m, if it exists. Usage 188 is 'invmod <a> <m>' 189 190sieve.c - A simple bitmap-based implementation of the Sieve 191 of Eratosthenes. Used to generate the table of 192 primes in primes.c. Usage is 'sieve <nbits>' 193 194prng.c - Uses the routines in bbs_rand.{h,c} to generate 195 one or more 32-bit pseudo-random integers. This 196 is mainly an example, not intended for use in a 197 cryptographic application (the system time is 198 the only source of entropy used) 199 200dec2hex.c - Convert decimal to hexadecimal 201 202hex2dec.c - Convert hexadecimal to decimal 203 204basecvt.c - General radix conversion tool (supports 2-64) 205 206isprime.c - Probabilistically test an integer for primality 207 using the Rabin-Miller pseudoprime test combined 208 with division by small primes. 209 210primegen.c - Generate primes at random. 211 212exptmod.c - Perform modular exponentiation 213 214ptab.pl - A Perl script to munge the output of the sieve 215 program into a compilable C structure. 216 217 218Other Files 219----------- 220 221PRIMES - Some randomly generated numbers which are prime with 222 extremely high probability. 223 224README - You're reading me already. 225 226 227About the Author 228---------------- 229 230This software was written by Michael J. Fromberger. You can contact 231the author as follows: 232 233E-mail: <sting@linguist.dartmouth.edu> 234 235Postal: 8000 Cummings Hall, Thayer School of Engineering 236 Dartmouth College, Hanover, New Hampshire, USA 237 238PGP key: http://linguist.dartmouth.edu/~sting/keys/mjf.html 239 9736 188B 5AFA 23D6 D6AA BE0D 5856 4525 289D 9907 240 241Last updated: $Id: README,v 1.3 2005/02/02 22:28:23 gerv%gerv.net Exp $