| Exponentiation |
| |
| For exponentiation, the MPI library uses a simple and fairly standard |
| square-and-multiply method. The algorithm is this: |
| |
| Input: a, b |
| Output: a ** b |
| |
| s = 1 |
| |
| while(b != 0) |
| if(b is odd) |
| s = s * a |
| endif |
| |
| b = b / 2 |
| |
| x = x * x |
| endwhile |
| |
| return s |
| |
| The modular exponentiation is done the same way, except replacing: |
| |
| s = s * a |
| |
| with |
| s = (s * a) mod m |
| |
| and replacing |
| |
| x = x * x |
| |
| with |
| |
| x = (x * x) mod m |
| |
| Here is a sample exponentiation using the MPI library, as compared to |
| the same problem solved by the Unix 'bc' program on my system: |
| |
| Computation of 2,381,283 ** 235 |
| |
| 'bc' says: |
| |
| 4385CA4A804D199FBEAD95FAD0796FAD0D0B51FC9C16743C45568C789666985DB719\ |
| 4D90E393522F74C9601262C0514145A49F3B53D00983F95FDFCEA3D0043ECEF6227E\ |
| 6FB59C924C3EE74447B359B5BF12A555D46CB819809EF423F004B55C587D6F0E8A55\ |
| 4988036A42ACEF9F71459F97CEF6E574BD7373657111648626B1FF8EE15F663B2C0E\ |
| 6BBE5082D4CDE8E14F263635AE8F35DB2C280819517BE388B5573B84C5A19C871685\ |
| FD408A6471F9D6AFAF5129A7548EAE926B40874B340285F44765BF5468CE20A13267\ |
| CD88CE6BC786ACED36EC7EA50F67FF27622575319068A332C3C0CB23E26FB55E26F4\ |
| 5F732753A52B8E2FB4D4F42D894242613CA912A25486C3DEC9C66E5DB6182F6C1761\ |
| CF8CD0D255BE64B93836B27D452AE38F950EB98B517D4CF50D48F0165EF0CCCE1F5C\ |
| 49BF18219FDBA0EEDD1A7E8B187B70C2BAED5EC5C6821EF27FAFB1CFF70111C52235\ |
| 5E948B93A015AA1AE152B110BB5658CB14D3E45A48BFE7F082C1182672A455A695CD\ |
| A1855E8781E625F25B41B516E77F589FA420C3B058861EA138CF7A2C58DB3C7504FD\ |
| D29554D78237834CC5AE710D403CC4F6973D5012B7E117A8976B14A0B5AFA889BD47\ |
| 92C461F0F96116F00A97AE9E83DC5203680CAF9A18A062566C145650AB86BE4F907F\ |
| A9F7AB4A700B29E1E5BACCD6DCBFA513E10832815F710807EED2E279081FEC61D619\ |
| AB270BEB3D3A1787B35A9DD41A8766CF21F3B5C693B3BAB1C2FA14A4ED202BC35743\ |
| E5CBE2391624D4F8C9BFBBC78D69764E7C6C5B11BF005677BFAD17D9278FFC1F158F\ |
| 1B3683FF7960FA0608103792C4163DC0AF3E06287BB8624F8FE3A0FFBDF82ACECA2F\ |
| CFFF2E1AC93F3CA264A1B |
| |
| MPI says: |
| |
| 4385CA4A804D199FBEAD95FAD0796FAD0D0B51FC9C16743C45568C789666985DB719\ |
| 4D90E393522F74C9601262C0514145A49F3B53D00983F95FDFCEA3D0043ECEF6227E\ |
| 6FB59C924C3EE74447B359B5BF12A555D46CB819809EF423F004B55C587D6F0E8A55\ |
| 4988036A42ACEF9F71459F97CEF6E574BD7373657111648626B1FF8EE15F663B2C0E\ |
| 6BBE5082D4CDE8E14F263635AE8F35DB2C280819517BE388B5573B84C5A19C871685\ |
| FD408A6471F9D6AFAF5129A7548EAE926B40874B340285F44765BF5468CE20A13267\ |
| CD88CE6BC786ACED36EC7EA50F67FF27622575319068A332C3C0CB23E26FB55E26F4\ |
| 5F732753A52B8E2FB4D4F42D894242613CA912A25486C3DEC9C66E5DB6182F6C1761\ |
| CF8CD0D255BE64B93836B27D452AE38F950EB98B517D4CF50D48F0165EF0CCCE1F5C\ |
| 49BF18219FDBA0EEDD1A7E8B187B70C2BAED5EC5C6821EF27FAFB1CFF70111C52235\ |
| 5E948B93A015AA1AE152B110BB5658CB14D3E45A48BFE7F082C1182672A455A695CD\ |
| A1855E8781E625F25B41B516E77F589FA420C3B058861EA138CF7A2C58DB3C7504FD\ |
| D29554D78237834CC5AE710D403CC4F6973D5012B7E117A8976B14A0B5AFA889BD47\ |
| 92C461F0F96116F00A97AE9E83DC5203680CAF9A18A062566C145650AB86BE4F907F\ |
| A9F7AB4A700B29E1E5BACCD6DCBFA513E10832815F710807EED2E279081FEC61D619\ |
| AB270BEB3D3A1787B35A9DD41A8766CF21F3B5C693B3BAB1C2FA14A4ED202BC35743\ |
| E5CBE2391624D4F8C9BFBBC78D69764E7C6C5B11BF005677BFAD17D9278FFC1F158F\ |
| 1B3683FF7960FA0608103792C4163DC0AF3E06287BB8624F8FE3A0FFBDF82ACECA2F\ |
| CFFF2E1AC93F3CA264A1B |
| |
| Diff says: |
| % diff bc.txt mp.txt |
| % |
| |
| ------------------------------------------------------------------ |
| This Source Code Form is subject to the terms of the Mozilla Public |
| # License, v. 2.0. If a copy of the MPL was not distributed with this |
| # file, You can obtain one at http://mozilla.org/MPL/2.0/. |