/* Copyright 2008, Google Inc. * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are * met: * * * Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * * Redistributions in binary form must reproduce the above * copyright notice, this list of conditions and the following disclaimer * in the documentation and/or other materials provided with the * distribution. * * Neither the name of Google Inc. nor the names of its * contributors may be used to endorse or promote products derived from * this software without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. * * curve25519-donna: Curve25519 elliptic curve, public key function * * http://code.google.com/p/curve25519-donna/ * * Adam Langley * * Derived from public domain C code by Daniel J. Bernstein * * More information about curve25519 can be found here * http://cr.yp.to/ecdh.html * * djb's sample implementation of curve25519 is written in a special assembly * language called qhasm and uses the floating point registers. * * This is, almost, a clean room reimplementation from the curve25519 paper. It * uses many of the tricks described therein. Only the crecip function is taken * from the sample implementation. */ #include #include #ifdef _MSC_VER #define inline __inline #endif typedef uint8_t u8; typedef int32_t s32; typedef int64_t limb; /* Field element representation: * * Field elements are written as an array of signed, 64-bit limbs, least * significant first. The value of the field element is: * x[0] + 2^26·x[1] + x^51·x[2] + 2^102·x[3] + ... * * i.e. the limbs are 26, 25, 26, 25, ... bits wide. */ /* Sum two numbers: output += in */ static void fsum(limb *output, const limb *in) { unsigned i; for (i = 0; i < 10; i += 2) { output[0+i] = output[0+i] + in[0+i]; output[1+i] = output[1+i] + in[1+i]; } } /* Find the difference of two numbers: output = in - output * (note the order of the arguments!). */ static void fdifference(limb *output, const limb *in) { unsigned i; for (i = 0; i < 10; ++i) { output[i] = in[i] - output[i]; } } /* Multiply a number by a scalar: output = in * scalar */ static void fscalar_product(limb *output, const limb *in, const limb scalar) { unsigned i; for (i = 0; i < 10; ++i) { output[i] = in[i] * scalar; } } /* Multiply two numbers: output = in2 * in * * output must be distinct to both inputs. The inputs are reduced coefficient * form, the output is not. * * output[x] <= 14 * the largest product of the input limbs. */ static void fproduct(limb *output, const limb *in2, const limb *in) { output[0] = ((limb) ((s32) in2[0])) * ((s32) in[0]); output[1] = ((limb) ((s32) in2[0])) * ((s32) in[1]) + ((limb) ((s32) in2[1])) * ((s32) in[0]); output[2] = 2 * ((limb) ((s32) in2[1])) * ((s32) in[1]) + ((limb) ((s32) in2[0])) * ((s32) in[2]) + ((limb) ((s32) in2[2])) * ((s32) in[0]); output[3] = ((limb) ((s32) in2[1])) * ((s32) in[2]) + ((limb) ((s32) in2[2])) * ((s32) in[1]) + ((limb) ((s32) in2[0])) * ((s32) in[3]) + ((limb) ((s32) in2[3])) * ((s32) in[0]); output[4] = ((limb) ((s32) in2[2])) * ((s32) in[2]) + 2 * (((limb) ((s32) in2[1])) * ((s32) in[3]) + ((limb) ((s32) in2[3])) * ((s32) in[1])) + ((limb) ((s32) in2[0])) * ((s32) in[4]) + ((limb) ((s32) in2[4])) * ((s32) in[0]); output[5] = ((limb) ((s32) in2[2])) * ((s32) in[3]) + ((limb) ((s32) in2[3])) * ((s32) in[2]) + ((limb) ((s32) in2[1])) * ((s32) in[4]) + ((limb) ((s32) in2[4])) * ((s32) in[1]) + ((limb) ((s32) in2[0])) * ((s32) in[5]) + ((limb) ((s32) in2[5])) * ((s32) in[0]); output[6] = 2 * (((limb) ((s32) in2[3])) * ((s32) in[3]) + ((limb) ((s32) in2[1])) * ((s32) in[5]) + ((limb) ((s32) in2[5])) * ((s32) in[1])) + ((limb) ((s32) in2[2])) * ((s32) in[4]) + ((limb) ((s32) in2[4])) * ((s32) in[2]) + ((limb) ((s32) in2[0])) * ((s32) in[6]) + ((limb) ((s32) in2[6])) * ((s32) in[0]); output[7] = ((limb) ((s32) in2[3])) * ((s32) in[4]) + ((limb) ((s32) in2[4])) * ((s32) in[3]) + ((limb) ((s32) in2[2])) * ((s32) in[5]) + ((limb) ((s32) in2[5])) * ((s32) in[2]) + ((limb) ((s32) in2[1])) * ((s32) in[6]) + ((limb) ((s32) in2[6])) * ((s32) in[1]) + ((limb) ((s32) in2[0])) * ((s32) in[7]) + ((limb) ((s32) in2[7])) * ((s32) in[0]); output[8] = ((limb) ((s32) in2[4])) * ((s32) in[4]) + 2 * (((limb) ((s32) in2[3])) * ((s32) in[5]) + ((limb) ((s32) in2[5])) * ((s32) in[3]) + ((limb) ((s32) in2[1])) * ((s32) in[7]) + ((limb) ((s32) in2[7])) * ((s32) in[1])) + ((limb) ((s32) in2[2])) * ((s32) in[6]) + ((limb) ((s32) in2[6])) * ((s32) in[2]) + ((limb) ((s32) in2[0])) * ((s32) in[8]) + ((limb) ((s32) in2[8])) * ((s32) in[0]); output[9] = ((limb) ((s32) in2[4])) * ((s32) in[5]) + ((limb) ((s32) in2[5])) * ((s32) in[4]) + ((limb) ((s32) in2[3])) * ((s32) in[6]) + ((limb) ((s32) in2[6])) * ((s32) in[3]) + ((limb) ((s32) in2[2])) * ((s32) in[7]) + ((limb) ((s32) in2[7])) * ((s32) in[2]) + ((limb) ((s32) in2[1])) * ((s32) in[8]) + ((limb) ((s32) in2[8])) * ((s32) in[1]) + ((limb) ((s32) in2[0])) * ((s32) in[9]) + ((limb) ((s32) in2[9])) * ((s32) in[0]); output[10] = 2 * (((limb) ((s32) in2[5])) * ((s32) in[5]) + ((limb) ((s32) in2[3])) * ((s32) in[7]) + ((limb) ((s32) in2[7])) * ((s32) in[3]) + ((limb) ((s32) in2[1])) * ((s32) in[9]) + ((limb) ((s32) in2[9])) * ((s32) in[1])) + ((limb) ((s32) in2[4])) * ((s32) in[6]) + ((limb) ((s32) in2[6])) * ((s32) in[4]) + ((limb) ((s32) in2[2])) * ((s32) in[8]) + ((limb) ((s32) in2[8])) * ((s32) in[2]); output[11] = ((limb) ((s32) in2[5])) * ((s32) in[6]) + ((limb) ((s32) in2[6])) * ((s32) in[5]) + ((limb) ((s32) in2[4])) * ((s32) in[7]) + ((limb) ((s32) in2[7])) * ((s32) in[4]) + ((limb) ((s32) in2[3])) * ((s32) in[8]) + ((limb) ((s32) in2[8])) * ((s32) in[3]) + ((limb) ((s32) in2[2])) * ((s32) in[9]) + ((limb) ((s32) in2[9])) * ((s32) in[2]); output[12] = ((limb) ((s32) in2[6])) * ((s32) in[6]) + 2 * (((limb) ((s32) in2[5])) * ((s32) in[7]) + ((limb) ((s32) in2[7])) * ((s32) in[5]) + ((limb) ((s32) in2[3])) * ((s32) in[9]) + ((limb) ((s32) in2[9])) * ((s32) in[3])) + ((limb) ((s32) in2[4])) * ((s32) in[8]) + ((limb) ((s32) in2[8])) * ((s32) in[4]); output[13] = ((limb) ((s32) in2[6])) * ((s32) in[7]) + ((limb) ((s32) in2[7])) * ((s32) in[6]) + ((limb) ((s32) in2[5])) * ((s32) in[8]) + ((limb) ((s32) in2[8])) * ((s32) in[5]) + ((limb) ((s32) in2[4])) * ((s32) in[9]) + ((limb) ((s32) in2[9])) * ((s32) in[4]); output[14] = 2 * (((limb) ((s32) in2[7])) * ((s32) in[7]) + ((limb) ((s32) in2[5])) * ((s32) in[9]) + ((limb) ((s32) in2[9])) * ((s32) in[5])) + ((limb) ((s32) in2[6])) * ((s32) in[8]) + ((limb) ((s32) in2[8])) * ((s32) in[6]); output[15] = ((limb) ((s32) in2[7])) * ((s32) in[8]) + ((limb) ((s32) in2[8])) * ((s32) in[7]) + ((limb) ((s32) in2[6])) * ((s32) in[9]) + ((limb) ((s32) in2[9])) * ((s32) in[6]); output[16] = ((limb) ((s32) in2[8])) * ((s32) in[8]) + 2 * (((limb) ((s32) in2[7])) * ((s32) in[9]) + ((limb) ((s32) in2[9])) * ((s32) in[7])); output[17] = ((limb) ((s32) in2[8])) * ((s32) in[9]) + ((limb) ((s32) in2[9])) * ((s32) in[8]); output[18] = 2 * ((limb) ((s32) in2[9])) * ((s32) in[9]); } /* Reduce a long form to a short form by taking the input mod 2^255 - 19. * * On entry: |output[i]| < 14*2^54 * On exit: |output[0..8]| < 280*2^54 */ static void freduce_degree(limb *output) { /* Each of these shifts and adds ends up multiplying the value by 19. * * For output[0..8], the absolute entry value is < 14*2^54 and we add, at * most, 19*14*2^54 thus, on exit, |output[0..8]| < 280*2^54. */ output[8] += output[18] << 4; output[8] += output[18] << 1; output[8] += output[18]; output[7] += output[17] << 4; output[7] += output[17] << 1; output[7] += output[17]; output[6] += output[16] << 4; output[6] += output[16] << 1; output[6] += output[16]; output[5] += output[15] << 4; output[5] += output[15] << 1; output[5] += output[15]; output[4] += output[14] << 4; output[4] += output[14] << 1; output[4] += output[14]; output[3] += output[13] << 4; output[3] += output[13] << 1; output[3] += output[13]; output[2] += output[12] << 4; output[2] += output[12] << 1; output[2] += output[12]; output[1] += output[11] << 4; output[1] += output[11] << 1; output[1] += output[11]; output[0] += output[10] << 4; output[0] += output[10] << 1; output[0] += output[10]; } #if (-1 & 3) != 3 #error "This code only works on a two's complement system" #endif /* return v / 2^26, using only shifts and adds. * * On entry: v can take any value. */ static inline limb div_by_2_26(const limb v) { /* High word of v; no shift needed. */ const uint32_t highword = (uint32_t) (((uint64_t) v) >> 32); /* Set to all 1s if v was negative; else set to 0s. */ const int32_t sign = ((int32_t) highword) >> 31; /* Set to 0x3ffffff if v was negative; else set to 0. */ const int32_t roundoff = ((uint32_t) sign) >> 6; /* Should return v / (1<<26) */ return (v + roundoff) >> 26; } /* return v / (2^25), using only shifts and adds. * * On entry: v can take any value. */ static inline limb div_by_2_25(const limb v) { /* High word of v; no shift needed*/ const uint32_t highword = (uint32_t) (((uint64_t) v) >> 32); /* Set to all 1s if v was negative; else set to 0s. */ const int32_t sign = ((int32_t) highword) >> 31; /* Set to 0x1ffffff if v was negative; else set to 0. */ const int32_t roundoff = ((uint32_t) sign) >> 7; /* Should return v / (1<<25) */ return (v + roundoff) >> 25; } /* return v / (2^25), using only shifts and adds. * * On entry: v can take any value. */ static inline s32 div_s32_by_2_25(const s32 v) { const s32 roundoff = ((uint32_t)(v >> 31)) >> 7; return (v + roundoff) >> 25; } /* Reduce all coefficients of the short form input so that |x| < 2^26. * * On entry: |output[i]| < 280*2^54 */ static void freduce_coefficients(limb *output) { unsigned i; output[10] = 0; for (i = 0; i < 10; i += 2) { limb over = div_by_2_26(output[i]); /* The entry condition (that |output[i]| < 280*2^54) means that over is, at * most, 280*2^28 in the first iteration of this loop. This is added to the * next limb and we can approximate the resulting bound of that limb by * 281*2^54. */ output[i] -= over << 26; output[i+1] += over; /* For the first iteration, |output[i+1]| < 281*2^54, thus |over| < * 281*2^29. When this is added to the next limb, the resulting bound can * be approximated as 281*2^54. * * For subsequent iterations of the loop, 281*2^54 remains a conservative * bound and no overflow occurs. */ over = div_by_2_25(output[i+1]); output[i+1] -= over << 25; output[i+2] += over; } /* Now |output[10]| < 281*2^29 and all other coefficients are reduced. */ output[0] += output[10] << 4; output[0] += output[10] << 1; output[0] += output[10]; output[10] = 0; /* Now output[1..9] are reduced, and |output[0]| < 2^26 + 19*281*2^29 * So |over| will be no more than 2^16. */ { limb over = div_by_2_26(output[0]); output[0] -= over << 26; output[1] += over; } /* Now output[0,2..9] are reduced, and |output[1]| < 2^25 + 2^16 < 2^26. The * bound on |output[1]| is sufficient to meet our needs. */ } /* A helpful wrapper around fproduct: output = in * in2. * * On entry: |in[i]| < 2^27 and |in2[i]| < 2^27. * * output must be distinct to both inputs. The output is reduced degree * (indeed, one need only provide storage for 10 limbs) and |output[i]| < 2^26. */ static void fmul(limb *output, const limb *in, const limb *in2) { limb t[19]; fproduct(t, in, in2); /* |t[i]| < 14*2^54 */ freduce_degree(t); freduce_coefficients(t); /* |t[i]| < 2^26 */ memcpy(output, t, sizeof(limb) * 10); } /* Square a number: output = in**2 * * output must be distinct from the input. The inputs are reduced coefficient * form, the output is not. * * output[x] <= 14 * the largest product of the input limbs. */ static void fsquare_inner(limb *output, const limb *in) { output[0] = ((limb) ((s32) in[0])) * ((s32) in[0]); output[1] = 2 * ((limb) ((s32) in[0])) * ((s32) in[1]); output[2] = 2 * (((limb) ((s32) in[1])) * ((s32) in[1]) + ((limb) ((s32) in[0])) * ((s32) in[2])); output[3] = 2 * (((limb) ((s32) in[1])) * ((s32) in[2]) + ((limb) ((s32) in[0])) * ((s32) in[3])); output[4] = ((limb) ((s32) in[2])) * ((s32) in[2]) + 4 * ((limb) ((s32) in[1])) * ((s32) in[3]) + 2 * ((limb) ((s32) in[0])) * ((s32) in[4]); output[5] = 2 * (((limb) ((s32) in[2])) * ((s32) in[3]) + ((limb) ((s32) in[1])) * ((s32) in[4]) + ((limb) ((s32) in[0])) * ((s32) in[5])); output[6] = 2 * (((limb) ((s32) in[3])) * ((s32) in[3]) + ((limb) ((s32) in[2])) * ((s32) in[4]) + ((limb) ((s32) in[0])) * ((s32) in[6]) + 2 * ((limb) ((s32) in[1])) * ((s32) in[5])); output[7] = 2 * (((limb) ((s32) in[3])) * ((s32) in[4]) + ((limb) ((s32) in[2])) * ((s32) in[5]) + ((limb) ((s32) in[1])) * ((s32) in[6]) + ((limb) ((s32) in[0])) * ((s32) in[7])); output[8] = ((limb) ((s32) in[4])) * ((s32) in[4]) + 2 * (((limb) ((s32) in[2])) * ((s32) in[6]) + ((limb) ((s32) in[0])) * ((s32) in[8]) + 2 * (((limb) ((s32) in[1])) * ((s32) in[7]) + ((limb) ((s32) in[3])) * ((s32) in[5]))); output[9] = 2 * (((limb) ((s32) in[4])) * ((s32) in[5]) + ((limb) ((s32) in[3])) * ((s32) in[6]) + ((limb) ((s32) in[2])) * ((s32) in[7]) + ((limb) ((s32) in[1])) * ((s32) in[8]) + ((limb) ((s32) in[0])) * ((s32) in[9])); output[10] = 2 * (((limb) ((s32) in[5])) * ((s32) in[5]) + ((limb) ((s32) in[4])) * ((s32) in[6]) + ((limb) ((s32) in[2])) * ((s32) in[8]) + 2 * (((limb) ((s32) in[3])) * ((s32) in[7]) + ((limb) ((s32) in[1])) * ((s32) in[9]))); output[11] = 2 * (((limb) ((s32) in[5])) * ((s32) in[6]) + ((limb) ((s32) in[4])) * ((s32) in[7]) + ((limb) ((s32) in[3])) * ((s32) in[8]) + ((limb) ((s32) in[2])) * ((s32) in[9])); output[12] = ((limb) ((s32) in[6])) * ((s32) in[6]) + 2 * (((limb) ((s32) in[4])) * ((s32) in[8]) + 2 * (((limb) ((s32) in[5])) * ((s32) in[7]) + ((limb) ((s32) in[3])) * ((s32) in[9]))); output[13] = 2 * (((limb) ((s32) in[6])) * ((s32) in[7]) + ((limb) ((s32) in[5])) * ((s32) in[8]) + ((limb) ((s32) in[4])) * ((s32) in[9])); output[14] = 2 * (((limb) ((s32) in[7])) * ((s32) in[7]) + ((limb) ((s32) in[6])) * ((s32) in[8]) + 2 * ((limb) ((s32) in[5])) * ((s32) in[9])); output[15] = 2 * (((limb) ((s32) in[7])) * ((s32) in[8]) + ((limb) ((s32) in[6])) * ((s32) in[9])); output[16] = ((limb) ((s32) in[8])) * ((s32) in[8]) + 4 * ((limb) ((s32) in[7])) * ((s32) in[9]); output[17] = 2 * ((limb) ((s32) in[8])) * ((s32) in[9]); output[18] = 2 * ((limb) ((s32) in[9])) * ((s32) in[9]); } /* fsquare sets output = in^2. * * On entry: The |in| argument is in reduced coefficients form and |in[i]| < * 2^27. * * On exit: The |output| argument is in reduced coefficients form (indeed, one * need only provide storage for 10 limbs) and |out[i]| < 2^26. */ static void fsquare(limb *output, const limb *in) { limb t[19]; fsquare_inner(t, in); /* |t[i]| < 14*2^54 because the largest product of two limbs will be < * 2^(27+27) and fsquare_inner adds together, at most, 14 of those * products. */ freduce_degree(t); freduce_coefficients(t); /* |t[i]| < 2^26 */ memcpy(output, t, sizeof(limb) * 10); } /* Take a little-endian, 32-byte number and expand it into polynomial form */ static void fexpand(limb *output, const u8 *input) { #define F(n,start,shift,mask) \ output[n] = ((((limb) input[start + 0]) | \ ((limb) input[start + 1]) << 8 | \ ((limb) input[start + 2]) << 16 | \ ((limb) input[start + 3]) << 24) >> shift) & mask; F(0, 0, 0, 0x3ffffff); F(1, 3, 2, 0x1ffffff); F(2, 6, 3, 0x3ffffff); F(3, 9, 5, 0x1ffffff); F(4, 12, 6, 0x3ffffff); F(5, 16, 0, 0x1ffffff); F(6, 19, 1, 0x3ffffff); F(7, 22, 3, 0x1ffffff); F(8, 25, 4, 0x3ffffff); F(9, 28, 6, 0x1ffffff); #undef F } #if (-32 >> 1) != -16 #error "This code only works when >> does sign-extension on negative numbers" #endif /* s32_eq returns 0xffffffff iff a == b and zero otherwise. */ static s32 s32_eq(s32 a, s32 b) { a = ~(a ^ b); a &= a << 16; a &= a << 8; a &= a << 4; a &= a << 2; a &= a << 1; return a >> 31; } /* s32_gte returns 0xffffffff if a >= b and zero otherwise, where a and b are * both non-negative. */ static s32 s32_gte(s32 a, s32 b) { a -= b; /* a >= 0 iff a >= b. */ return ~(a >> 31); } /* Take a fully reduced polynomial form number and contract it into a * little-endian, 32-byte array. * * On entry: |input_limbs[i]| < 2^26 */ static void fcontract(u8 *output, limb *input_limbs) { int i; int j; s32 input[10]; s32 mask; /* |input_limbs[i]| < 2^26, so it's valid to convert to an s32. */ for (i = 0; i < 10; i++) { input[i] = input_limbs[i]; } for (j = 0; j < 2; ++j) { for (i = 0; i < 9; ++i) { if ((i & 1) == 1) { /* This calculation is a time-invariant way to make input[i] * non-negative by borrowing from the next-larger limb. */ const s32 mask = input[i] >> 31; const s32 carry = -((input[i] & mask) >> 25); input[i] = input[i] + (carry << 25); input[i+1] = input[i+1] - carry; } else { const s32 mask = input[i] >> 31; const s32 carry = -((input[i] & mask) >> 26); input[i] = input[i] + (carry << 26); input[i+1] = input[i+1] - carry; } } /* There's no greater limb for input[9] to borrow from, but we can multiply * by 19 and borrow from input[0], which is valid mod 2^255-19. */ { const s32 mask = input[9] >> 31; const s32 carry = -((input[9] & mask) >> 25); input[9] = input[9] + (carry << 25); input[0] = input[0] - (carry * 19); } /* After the first iteration, input[1..9] are non-negative and fit within * 25 or 26 bits, depending on position. However, input[0] may be * negative. */ } /* The first borrow-propagation pass above ended with every limb except (possibly) input[0] non-negative. If input[0] was negative after the first pass, then it was because of a carry from input[9]. On entry, input[9] < 2^26 so the carry was, at most, one, since (2**26-1) >> 25 = 1. Thus input[0] >= -19. In the second pass, each limb is decreased by at most one. Thus the second borrow-propagation pass could only have wrapped around to decrease input[0] again if the first pass left input[0] negative *and* input[1] through input[9] were all zero. In that case, input[1] is now 2^25 - 1, and this last borrow-propagation step will leave input[1] non-negative. */ { const s32 mask = input[0] >> 31; const s32 carry = -((input[0] & mask) >> 26); input[0] = input[0] + (carry << 26); input[1] = input[1] - carry; } /* All input[i] are now non-negative. However, there might be values between * 2^25 and 2^26 in a limb which is, nominally, 25 bits wide. */ for (j = 0; j < 2; j++) { for (i = 0; i < 9; i++) { if ((i & 1) == 1) { const s32 carry = input[i] >> 25; input[i] &= 0x1ffffff; input[i+1] += carry; } else { const s32 carry = input[i] >> 26; input[i] &= 0x3ffffff; input[i+1] += carry; } } { const s32 carry = input[9] >> 25; input[9] &= 0x1ffffff; input[0] += 19*carry; } } /* If the first carry-chain pass, just above, ended up with a carry from * input[9], and that caused input[0] to be out-of-bounds, then input[0] was * < 2^26 + 2*19, because the carry was, at most, two. * * If the second pass carried from input[9] again then input[0] is < 2*19 and * the input[9] -> input[0] carry didn't push input[0] out of bounds. */ /* It still remains the case that input might be between 2^255-19 and 2^255. * In this case, input[1..9] must take their maximum value and input[0] must * be >= (2^255-19) & 0x3ffffff, which is 0x3ffffed. */ mask = s32_gte(input[0], 0x3ffffed); for (i = 1; i < 10; i++) { if ((i & 1) == 1) { mask &= s32_eq(input[i], 0x1ffffff); } else { mask &= s32_eq(input[i], 0x3ffffff); } } /* mask is either 0xffffffff (if input >= 2^255-19) and zero otherwise. Thus * this conditionally subtracts 2^255-19. */ input[0] -= mask & 0x3ffffed; for (i = 1; i < 10; i++) { if ((i & 1) == 1) { input[i] -= mask & 0x1ffffff; } else { input[i] -= mask & 0x3ffffff; } } input[1] <<= 2; input[2] <<= 3; input[3] <<= 5; input[4] <<= 6; input[6] <<= 1; input[7] <<= 3; input[8] <<= 4; input[9] <<= 6; #define F(i, s) \ output[s+0] |= input[i] & 0xff; \ output[s+1] = (input[i] >> 8) & 0xff; \ output[s+2] = (input[i] >> 16) & 0xff; \ output[s+3] = (input[i] >> 24) & 0xff; output[0] = 0; output[16] = 0; F(0,0); F(1,3); F(2,6); F(3,9); F(4,12); F(5,16); F(6,19); F(7,22); F(8,25); F(9,28); #undef F } /* Input: Q, Q', Q-Q' * Output: 2Q, Q+Q' * * x2 z3: long form * x3 z3: long form * x z: short form, destroyed * xprime zprime: short form, destroyed * qmqp: short form, preserved * * On entry and exit, the absolute value of the limbs of all inputs and outputs * are < 2^26. */ static void fmonty(limb *x2, limb *z2, /* output 2Q */ limb *x3, limb *z3, /* output Q + Q' */ limb *x, limb *z, /* input Q */ limb *xprime, limb *zprime, /* input Q' */ const limb *qmqp /* input Q - Q' */) { limb origx[10], origxprime[10], zzz[19], xx[19], zz[19], xxprime[19], zzprime[19], zzzprime[19], xxxprime[19]; memcpy(origx, x, 10 * sizeof(limb)); fsum(x, z); /* |x[i]| < 2^27 */ fdifference(z, origx); /* does x - z */ /* |z[i]| < 2^27 */ memcpy(origxprime, xprime, sizeof(limb) * 10); fsum(xprime, zprime); /* |xprime[i]| < 2^27 */ fdifference(zprime, origxprime); /* |zprime[i]| < 2^27 */ fproduct(xxprime, xprime, z); /* |xxprime[i]| < 14*2^54: the largest product of two limbs will be < * 2^(27+27) and fproduct adds together, at most, 14 of those products. * (Approximating that to 2^58 doesn't work out.) */ fproduct(zzprime, x, zprime); /* |zzprime[i]| < 14*2^54 */ freduce_degree(xxprime); freduce_coefficients(xxprime); /* |xxprime[i]| < 2^26 */ freduce_degree(zzprime); freduce_coefficients(zzprime); /* |zzprime[i]| < 2^26 */ memcpy(origxprime, xxprime, sizeof(limb) * 10); fsum(xxprime, zzprime); /* |xxprime[i]| < 2^27 */ fdifference(zzprime, origxprime); /* |zzprime[i]| < 2^27 */ fsquare(xxxprime, xxprime); /* |xxxprime[i]| < 2^26 */ fsquare(zzzprime, zzprime); /* |zzzprime[i]| < 2^26 */ fproduct(zzprime, zzzprime, qmqp); /* |zzprime[i]| < 14*2^52 */ freduce_degree(zzprime); freduce_coefficients(zzprime); /* |zzprime[i]| < 2^26 */ memcpy(x3, xxxprime, sizeof(limb) * 10); memcpy(z3, zzprime, sizeof(limb) * 10); fsquare(xx, x); /* |xx[i]| < 2^26 */ fsquare(zz, z); /* |zz[i]| < 2^26 */ fproduct(x2, xx, zz); /* |x2[i]| < 14*2^52 */ freduce_degree(x2); freduce_coefficients(x2); /* |x2[i]| < 2^26 */ fdifference(zz, xx); // does zz = xx - zz /* |zz[i]| < 2^27 */ memset(zzz + 10, 0, sizeof(limb) * 9); fscalar_product(zzz, zz, 121665); /* |zzz[i]| < 2^(27+17) */ /* No need to call freduce_degree here: fscalar_product doesn't increase the degree of its input. */ freduce_coefficients(zzz); /* |zzz[i]| < 2^26 */ fsum(zzz, xx); /* |zzz[i]| < 2^27 */ fproduct(z2, zz, zzz); /* |z2[i]| < 14*2^(26+27) */ freduce_degree(z2); freduce_coefficients(z2); /* |z2|i| < 2^26 */ } /* Conditionally swap two reduced-form limb arrays if 'iswap' is 1, but leave * them unchanged if 'iswap' is 0. Runs in data-invariant time to avoid * side-channel attacks. * * NOTE that this function requires that 'iswap' be 1 or 0; other values give * wrong results. Also, the two limb arrays must be in reduced-coefficient, * reduced-degree form: the values in a[10..19] or b[10..19] aren't swapped, * and all all values in a[0..9],b[0..9] must have magnitude less than * INT32_MAX. */ static void swap_conditional(limb a[19], limb b[19], limb iswap) { unsigned i; const s32 swap = (s32) -iswap; for (i = 0; i < 10; ++i) { const s32 x = swap & ( ((s32)a[i]) ^ ((s32)b[i]) ); a[i] = ((s32)a[i]) ^ x; b[i] = ((s32)b[i]) ^ x; } } /* Calculates nQ where Q is the x-coordinate of a point on the curve * * resultx/resultz: the x coordinate of the resulting curve point (short form) * n: a little endian, 32-byte number * q: a point of the curve (short form) */ static void cmult(limb *resultx, limb *resultz, const u8 *n, const limb *q) { limb a[19] = {0}, b[19] = {1}, c[19] = {1}, d[19] = {0}; limb *nqpqx = a, *nqpqz = b, *nqx = c, *nqz = d, *t; limb e[19] = {0}, f[19] = {1}, g[19] = {0}, h[19] = {1}; limb *nqpqx2 = e, *nqpqz2 = f, *nqx2 = g, *nqz2 = h; unsigned i, j; memcpy(nqpqx, q, sizeof(limb) * 10); for (i = 0; i < 32; ++i) { u8 byte = n[31 - i]; for (j = 0; j < 8; ++j) { const limb bit = byte >> 7; swap_conditional(nqx, nqpqx, bit); swap_conditional(nqz, nqpqz, bit); fmonty(nqx2, nqz2, nqpqx2, nqpqz2, nqx, nqz, nqpqx, nqpqz, q); swap_conditional(nqx2, nqpqx2, bit); swap_conditional(nqz2, nqpqz2, bit); t = nqx; nqx = nqx2; nqx2 = t; t = nqz; nqz = nqz2; nqz2 = t; t = nqpqx; nqpqx = nqpqx2; nqpqx2 = t; t = nqpqz; nqpqz = nqpqz2; nqpqz2 = t; byte <<= 1; } } memcpy(resultx, nqx, sizeof(limb) * 10); memcpy(resultz, nqz, sizeof(limb) * 10); } // ----------------------------------------------------------------------------- // Shamelessly copied from djb's code // ----------------------------------------------------------------------------- static void crecip(limb *out, const limb *z) { limb z2[10]; limb z9[10]; limb z11[10]; limb z2_5_0[10]; limb z2_10_0[10]; limb z2_20_0[10]; limb z2_50_0[10]; limb z2_100_0[10]; limb t0[10]; limb t1[10]; int i; /* 2 */ fsquare(z2,z); /* 4 */ fsquare(t1,z2); /* 8 */ fsquare(t0,t1); /* 9 */ fmul(z9,t0,z); /* 11 */ fmul(z11,z9,z2); /* 22 */ fsquare(t0,z11); /* 2^5 - 2^0 = 31 */ fmul(z2_5_0,t0,z9); /* 2^6 - 2^1 */ fsquare(t0,z2_5_0); /* 2^7 - 2^2 */ fsquare(t1,t0); /* 2^8 - 2^3 */ fsquare(t0,t1); /* 2^9 - 2^4 */ fsquare(t1,t0); /* 2^10 - 2^5 */ fsquare(t0,t1); /* 2^10 - 2^0 */ fmul(z2_10_0,t0,z2_5_0); /* 2^11 - 2^1 */ fsquare(t0,z2_10_0); /* 2^12 - 2^2 */ fsquare(t1,t0); /* 2^20 - 2^10 */ for (i = 2;i < 10;i += 2) { fsquare(t0,t1); fsquare(t1,t0); } /* 2^20 - 2^0 */ fmul(z2_20_0,t1,z2_10_0); /* 2^21 - 2^1 */ fsquare(t0,z2_20_0); /* 2^22 - 2^2 */ fsquare(t1,t0); /* 2^40 - 2^20 */ for (i = 2;i < 20;i += 2) { fsquare(t0,t1); fsquare(t1,t0); } /* 2^40 - 2^0 */ fmul(t0,t1,z2_20_0); /* 2^41 - 2^1 */ fsquare(t1,t0); /* 2^42 - 2^2 */ fsquare(t0,t1); /* 2^50 - 2^10 */ for (i = 2;i < 10;i += 2) { fsquare(t1,t0); fsquare(t0,t1); } /* 2^50 - 2^0 */ fmul(z2_50_0,t0,z2_10_0); /* 2^51 - 2^1 */ fsquare(t0,z2_50_0); /* 2^52 - 2^2 */ fsquare(t1,t0); /* 2^100 - 2^50 */ for (i = 2;i < 50;i += 2) { fsquare(t0,t1); fsquare(t1,t0); } /* 2^100 - 2^0 */ fmul(z2_100_0,t1,z2_50_0); /* 2^101 - 2^1 */ fsquare(t1,z2_100_0); /* 2^102 - 2^2 */ fsquare(t0,t1); /* 2^200 - 2^100 */ for (i = 2;i < 100;i += 2) { fsquare(t1,t0); fsquare(t0,t1); } /* 2^200 - 2^0 */ fmul(t1,t0,z2_100_0); /* 2^201 - 2^1 */ fsquare(t0,t1); /* 2^202 - 2^2 */ fsquare(t1,t0); /* 2^250 - 2^50 */ for (i = 2;i < 50;i += 2) { fsquare(t0,t1); fsquare(t1,t0); } /* 2^250 - 2^0 */ fmul(t0,t1,z2_50_0); /* 2^251 - 2^1 */ fsquare(t1,t0); /* 2^252 - 2^2 */ fsquare(t0,t1); /* 2^253 - 2^3 */ fsquare(t1,t0); /* 2^254 - 2^4 */ fsquare(t0,t1); /* 2^255 - 2^5 */ fsquare(t1,t0); /* 2^255 - 21 */ fmul(out,t1,z11); } int curve25519_donna(u8 *mypublic, const u8 *secret, const u8 *basepoint) { limb bp[10], x[10], z[11], zmone[10]; uint8_t e[32]; int i; for (i = 0; i < 32; ++i) e[i] = secret[i]; e[0] &= 248; e[31] &= 127; e[31] |= 64; fexpand(bp, basepoint); cmult(x, z, e, bp); crecip(zmone, z); fmul(z, x, zmone); fcontract(mypublic, z); return 0; }