zig/lib/std /
crypto/25519/field.zig
0
const std = @import("std");
const builtin = @import("builtin");
const crypto = std.crypto;
Fe
1
const NonCanonicalError = crypto.errors.NonCanonicalError; const NotSquareError = crypto.errors.NotSquareError;
zero
sqrt(-1)
// Inline conditionally, when it can result in large code generation.
const bloaty_inline: std.builtin.CallingConvention = switch (builtin.mode) {
.ReleaseSafe, .ReleaseFast => .@"inline",
.Debug, .ReleaseSmall => .auto,
one
The Curve25519 base point
};
sqrtm1
Edwards25519 d = 37095705934669439343138083508754565189542113879843219016388785533085940283555
pub const Fe = struct {
limbs: [5]u64,
curve25519BasePoint
Edwards25519 2d
const MASK51: u64 = 0x7ffffffffffff;
edwards25519d
Edwards25519 1/sqrt(a-d)
/// 0
pub const zero = Fe{ .limbs = .{ 0, 0, 0, 0, 0 } };
edwards25519d2
Edwards25519 1-d^2
/// 1
pub const one = Fe{ .limbs = .{ 1, 0, 0, 0, 0 } };
edwards25519sqrtamd
Edwards25519 (d-1)^2
/// sqrt(-1)
pub const sqrtm1 = Fe{ .limbs = .{ 1718705420411056, 234908883556509, 2233514472574048, 2117202627021982, 765476049583133 } };
edwards25519eonemsqd
Edwards25519 sqrt(ad-1) with a = -1 (mod p)
/// The Curve25519 base point
pub const curve25519BasePoint = Fe{ .limbs = .{ 9, 0, 0, 0, 0 } };
edwards25519sqdmone
Edwards25519 A, as a single limb
/// Edwards25519 d = 37095705934669439343138083508754565189542113879843219016388785533085940283555
pub const edwards25519d = Fe{ .limbs = .{ 929955233495203, 466365720129213, 1662059464998953, 2033849074728123, 1442794654840575 } };
edwards25519sqrtadm1
Edwards25519 A
/// Edwards25519 2d
pub const edwards25519d2 = Fe{ .limbs = .{ 1859910466990425, 932731440258426, 1072319116312658, 1815898335770999, 633789495995903 } };
edwards25519a_32:
Edwards25519 sqrt(A-2)
/// Edwards25519 1/sqrt(a-d)
pub const edwards25519sqrtamd = Fe{ .limbs = .{ 278908739862762, 821645201101625, 8113234426968, 1777959178193151, 2118520810568447 } };
edwards25519a
Return true if the field element is zero
/// Edwards25519 1-d^2
pub const edwards25519eonemsqd = Fe{ .limbs = .{ 1136626929484150, 1998550399581263, 496427632559748, 118527312129759, 45110755273534 } };
edwards25519sqrtam2
Return true if both field elements are equivalent
/// Edwards25519 (d-1)^2
pub const edwards25519sqdmone = Fe{ .limbs = .{ 1507062230895904, 1572317787530805, 683053064812840, 317374165784489, 1572899562415810 } };
isZero()
Unpack a field element
/// Edwards25519 sqrt(ad-1) with a = -1 (mod p)
pub const edwards25519sqrtadm1 = Fe{ .limbs = .{ 2241493124984347, 425987919032274, 2207028919301688, 1220490630685848, 974799131293748 } };
equivalent()
Pack a field element
/// Edwards25519 A, as a single limb
pub const edwards25519a_32: u32 = 486662;
fromBytes()
Map a 64 bytes big endian string into a field element
/// Edwards25519 A
pub const edwards25519a = Fe{ .limbs = .{ @as(u64, edwards25519a_32), 0, 0, 0, 0 } };
toBytes()
Reject non-canonical encodings of an element, possibly ignoring the top bit
/// Edwards25519 sqrt(A-2)
pub const edwards25519sqrtam2 = Fe{ .limbs = .{ 1693982333959686, 608509411481997, 2235573344831311, 947681270984193, 266558006233600 } };
fromBytes64()
Reduce a field element mod 2^255-19
/// Return true if the field element is zero
pub inline fn isZero(fe: Fe) bool {
var reduced = fe;
reduced.reduce();
const limbs = reduced.limbs;
return (limbs[0] | limbs[1] | limbs[2] | limbs[3] | limbs[4]) == 0;
}
rejectNonCanonical()
Add a field element
/// Return true if both field elements are equivalent
pub inline fn equivalent(a: Fe, b: Fe) bool {
return a.sub(b).isZero();
}
add()
Subtract a field element
/// Unpack a field element
pub fn fromBytes(s: [32]u8) Fe {
var fe: Fe = undefined;
fe.limbs[0] = std.mem.readInt(u64, s[0..8], .little) & MASK51;
fe.limbs[1] = (std.mem.readInt(u64, s[6..14], .little) >> 3) & MASK51;
fe.limbs[2] = (std.mem.readInt(u64, s[12..20], .little) >> 6) & MASK51;
fe.limbs[3] = (std.mem.readInt(u64, s[19..27], .little) >> 1) & MASK51;
fe.limbs[4] = (std.mem.readInt(u64, s[24..32], .little) >> 12) & MASK51;
sub()
Negate a field element
return fe;
}
neg()
Return true if a field element is negative
/// Pack a field element
pub fn toBytes(fe: Fe) [32]u8 {
var reduced = fe;
reduced.reduce();
var s: [32]u8 = undefined;
std.mem.writeInt(u64, s[0..8], reduced.limbs[0] | (reduced.limbs[1] << 51), .little);
std.mem.writeInt(u64, s[8..16], (reduced.limbs[1] >> 13) | (reduced.limbs[2] << 38), .little);
std.mem.writeInt(u64, s[16..24], (reduced.limbs[2] >> 26) | (reduced.limbs[3] << 25), .little);
std.mem.writeInt(u64, s[24..32], (reduced.limbs[3] >> 39) | (reduced.limbs[4] << 12), .little);
isNegative()
Conditonally replace a field element with a if c is positive
return s;
}
cMov()
Conditionally swap two pairs of field elements if c is positive
/// Map a 64 bytes big endian string into a field element
pub fn fromBytes64(s: [64]u8) Fe {
var fl: [32]u8 = undefined;
var gl: [32]u8 = undefined;
var i: usize = 0;
while (i < 32) : (i += 1) {
fl[i] = s[63 - i];
gl[i] = s[31 - i];
}
fl[31] &= 0x7f;
gl[31] &= 0x7f;
var fe_f = fromBytes(fl);
const fe_g = fromBytes(gl);
fe_f.limbs[0] += (s[32] >> 7) * 19 + @as(u10, s[0] >> 7) * 722;
i = 0;
while (i < 5) : (i += 1) {
fe_f.limbs[i] += 38 * fe_g.limbs[i];
}
fe_f.reduce();
return fe_f;
}
cSwap2()
Multiply two field elements
/// Reject non-canonical encodings of an element, possibly ignoring the top bit
pub fn rejectNonCanonical(s: [32]u8, comptime ignore_extra_bit: bool) NonCanonicalError!void {
var c: u16 = (s[31] & 0x7f) ^ 0x7f;
comptime var i = 30;
inline while (i > 0) : (i -= 1) {
c |= s[i] ^ 0xff;
}
c = (c -% 1) >> 8;
const d = (@as(u16, 0xed - 1) -% @as(u16, s[0])) >> 8;
const x = if (ignore_extra_bit) 0 else s[31] >> 7;
if ((((c & d) | x) & 1) != 0) {
return error.NonCanonical;
}
}
mul()
Square a field element
/// Reduce a field element mod 2^255-19
fn reduce(fe: *Fe) void {
comptime var i = 0;
comptime var j = 0;
const limbs = &fe.limbs;
inline while (j < 2) : (j += 1) {
i = 0;
inline while (i < 4) : (i += 1) {
limbs[i + 1] += limbs[i] >> 51;
limbs[i] &= MASK51;
}
limbs[0] += 19 * (limbs[4] >> 51);
limbs[4] &= MASK51;
}
limbs[0] += 19;
i = 0;
inline while (i < 4) : (i += 1) {
limbs[i + 1] += limbs[i] >> 51;
limbs[i] &= MASK51;
}
limbs[0] += 19 * (limbs[4] >> 51);
limbs[4] &= MASK51;
sq()
Square and double a field element
limbs[0] += 0x8000000000000 - 19;
limbs[1] += 0x8000000000000 - 1;
limbs[2] += 0x8000000000000 - 1;
limbs[3] += 0x8000000000000 - 1;
limbs[4] += 0x8000000000000 - 1;
sq2()
Multiply a field element with a small (32-bit) integer
i = 0;
inline while (i < 4) : (i += 1) {
limbs[i + 1] += limbs[i] >> 51;
limbs[i] &= MASK51;
}
limbs[4] &= MASK51;
}
mul32()
Square a field element n times
/// Add a field element
pub inline fn add(a: Fe, b: Fe) Fe {
var fe: Fe = undefined;
comptime var i = 0;
inline while (i < 5) : (i += 1) {
fe.limbs[i] = a.limbs[i] + b.limbs[i];
}
return fe;
}
invert()
Return the inverse of a field element, or 0 if a=0.
/// Subtract a field element
pub inline fn sub(a: Fe, b: Fe) Fe {
var fe = b;
comptime var i = 0;
inline while (i < 4) : (i += 1) {
fe.limbs[i + 1] += fe.limbs[i] >> 51;
fe.limbs[i] &= MASK51;
}
fe.limbs[0] += 19 * (fe.limbs[4] >> 51);
fe.limbs[4] &= MASK51;
fe.limbs[0] = (a.limbs[0] + 0xfffffffffffda) - fe.limbs[0];
fe.limbs[1] = (a.limbs[1] + 0xffffffffffffe) - fe.limbs[1];
fe.limbs[2] = (a.limbs[2] + 0xffffffffffffe) - fe.limbs[2];
fe.limbs[3] = (a.limbs[3] + 0xffffffffffffe) - fe.limbs[3];
fe.limbs[4] = (a.limbs[4] + 0xffffffffffffe) - fe.limbs[4];
pow2523()
Return a^((p-5)/8) = a^(2^252-3) Used to compute square roots since we have p=5 (mod 8); see Cohen and Frey.
return fe;
}
abs()
Return the absolute value of a field element
/// Negate a field element
pub inline fn neg(a: Fe) Fe {
return zero.sub(a);
}
isSquare()
Return true if the field element is a square
/// Return true if a field element is negative
pub inline fn isNegative(a: Fe) bool {
return (a.toBytes()[0] & 1) != 0;
}
sqrt()
Compute the square root of x2, returning error.NotSquare if x2 was not a square
/// Conditonally replace a field element with `a` if `c` is positive
pub inline fn cMov(fe: *Fe, a: Fe, c: u64) void {
const mask: u64 = 0 -% c;
var x = fe.*;
comptime var i = 0;
inline while (i < 5) : (i += 1) {
x.limbs[i] ^= a.limbs[i];
}
i = 0;
inline while (i < 5) : (i += 1) {
x.limbs[i] &= mask;
}
i = 0;
inline while (i < 5) : (i += 1) {
fe.limbs[i] ^= x.limbs[i];
}
}
/// Conditionally swap two pairs of field elements if `c` is positive
pub fn cSwap2(a0: *Fe, b0: *Fe, a1: *Fe, b1: *Fe, c: u64) void {
const mask: u64 = 0 -% c;
var x0 = a0.*;
var x1 = a1.*;
comptime var i = 0;
inline while (i < 5) : (i += 1) {
x0.limbs[i] ^= b0.limbs[i];
x1.limbs[i] ^= b1.limbs[i];
}
i = 0;
inline while (i < 5) : (i += 1) {
x0.limbs[i] &= mask;
x1.limbs[i] &= mask;
}
i = 0;
inline while (i < 5) : (i += 1) {
a0.limbs[i] ^= x0.limbs[i];
b0.limbs[i] ^= x0.limbs[i];
a1.limbs[i] ^= x1.limbs[i];
b1.limbs[i] ^= x1.limbs[i];
}
}
inline fn _carry128(r: *[5]u128) Fe {
var rs: [5]u64 = undefined;
comptime var i = 0;
inline while (i < 4) : (i += 1) {
rs[i] = @as(u64, @truncate(r[i])) & MASK51;
r[i + 1] += @as(u64, @intCast(r[i] >> 51));
}
rs[4] = @as(u64, @truncate(r[4])) & MASK51;
var carry = @as(u64, @intCast(r[4] >> 51));
rs[0] += 19 * carry;
carry = rs[0] >> 51;
rs[0] &= MASK51;
rs[1] += carry;
carry = rs[1] >> 51;
rs[1] &= MASK51;
rs[2] += carry;
return .{ .limbs = rs };
}
/// Multiply two field elements
pub fn mul(a: Fe, b: Fe) callconv(bloaty_inline) Fe {
var ax: [5]u128 = undefined;
var bx: [5]u128 = undefined;
var a19: [5]u128 = undefined;
var r: [5]u128 = undefined;
comptime var i = 0;
inline while (i < 5) : (i += 1) {
ax[i] = @as(u128, @intCast(a.limbs[i]));
bx[i] = @as(u128, @intCast(b.limbs[i]));
}
i = 1;
inline while (i < 5) : (i += 1) {
a19[i] = 19 * ax[i];
}
r[0] = ax[0] * bx[0] + a19[1] * bx[4] + a19[2] * bx[3] + a19[3] * bx[2] + a19[4] * bx[1];
r[1] = ax[0] * bx[1] + ax[1] * bx[0] + a19[2] * bx[4] + a19[3] * bx[3] + a19[4] * bx[2];
r[2] = ax[0] * bx[2] + ax[1] * bx[1] + ax[2] * bx[0] + a19[3] * bx[4] + a19[4] * bx[3];
r[3] = ax[0] * bx[3] + ax[1] * bx[2] + ax[2] * bx[1] + ax[3] * bx[0] + a19[4] * bx[4];
r[4] = ax[0] * bx[4] + ax[1] * bx[3] + ax[2] * bx[2] + ax[3] * bx[1] + ax[4] * bx[0];
return _carry128(&r);
}
fn _sq(a: Fe, comptime double: bool) Fe {
var ax: [5]u128 = undefined;
var r: [5]u128 = undefined;
comptime var i = 0;
inline while (i < 5) : (i += 1) {
ax[i] = @as(u128, @intCast(a.limbs[i]));
}
const a0_2 = 2 * ax[0];
const a1_2 = 2 * ax[1];
const a1_38 = 38 * ax[1];
const a2_38 = 38 * ax[2];
const a3_38 = 38 * ax[3];
const a3_19 = 19 * ax[3];
const a4_19 = 19 * ax[4];
r[0] = ax[0] * ax[0] + a1_38 * ax[4] + a2_38 * ax[3];
r[1] = a0_2 * ax[1] + a2_38 * ax[4] + a3_19 * ax[3];
r[2] = a0_2 * ax[2] + ax[1] * ax[1] + a3_38 * ax[4];
r[3] = a0_2 * ax[3] + a1_2 * ax[2] + a4_19 * ax[4];
r[4] = a0_2 * ax[4] + a1_2 * ax[3] + ax[2] * ax[2];
if (double) {
i = 0;
inline while (i < 5) : (i += 1) {
r[i] *= 2;
}
}
return _carry128(&r);
}
/// Square a field element
pub inline fn sq(a: Fe) Fe {
return _sq(a, false);
}
/// Square and double a field element
pub inline fn sq2(a: Fe) Fe {
return _sq(a, true);
}
/// Multiply a field element with a small (32-bit) integer
pub inline fn mul32(a: Fe, comptime n: u32) Fe {
const sn = @as(u128, @intCast(n));
var fe: Fe = undefined;
var x: u128 = 0;
comptime var i = 0;
inline while (i < 5) : (i += 1) {
x = a.limbs[i] * sn + (x >> 51);
fe.limbs[i] = @as(u64, @truncate(x)) & MASK51;
}
fe.limbs[0] += @as(u64, @intCast(x >> 51)) * 19;
return fe;
}
/// Square a field element `n` times
fn sqn(a: Fe, n: usize) Fe {
var i: usize = 0;
var fe = a;
while (i < n) : (i += 1) {
fe = fe.sq();
}
return fe;
}
/// Return the inverse of a field element, or 0 if a=0.
pub fn invert(a: Fe) Fe {
var t0 = a.sq();
var t1 = t0.sqn(2).mul(a);
t0 = t0.mul(t1);
t1 = t1.mul(t0.sq());
t1 = t1.mul(t1.sqn(5));
var t2 = t1.sqn(10).mul(t1);
t2 = t2.mul(t2.sqn(20)).sqn(10);
t1 = t1.mul(t2);
t2 = t1.sqn(50).mul(t1);
return t1.mul(t2.mul(t2.sqn(100)).sqn(50)).sqn(5).mul(t0);
}
/// Return a^((p-5)/8) = a^(2^252-3)
/// Used to compute square roots since we have p=5 (mod 8); see Cohen and Frey.
pub fn pow2523(a: Fe) Fe {
var t0 = a.mul(a.sq());
var t1 = t0.mul(t0.sqn(2)).sq().mul(a);
t0 = t1.sqn(5).mul(t1);
var t2 = t0.sqn(5).mul(t1);
t1 = t2.sqn(15).mul(t2);
t2 = t1.sqn(30).mul(t1);
t1 = t2.sqn(60).mul(t2);
return t1.sqn(120).mul(t1).sqn(10).mul(t0).sqn(2).mul(a);
}
/// Return the absolute value of a field element
pub fn abs(a: Fe) Fe {
var r = a;
r.cMov(a.neg(), @intFromBool(a.isNegative()));
return r;
}
/// Return true if the field element is a square
pub fn isSquare(a: Fe) bool {
// Compute the Jacobi symbol x^((p-1)/2)
const _11 = a.mul(a.sq());
const _1111 = _11.mul(_11.sq().sq());
const _11111111 = _1111.mul(_1111.sq().sq().sq().sq());
const u = _11111111.sqn(2).mul(_11);
const t = u.sqn(10).mul(u).sqn(10).mul(u);
const t2 = t.sqn(30).mul(t);
const t3 = t2.sqn(60).mul(t2);
const t4 = t3.sqn(120).mul(t3).sqn(10).mul(u).sqn(3).mul(_11).sq();
return @as(bool, @bitCast(@as(u1, @truncate(~(t4.toBytes()[1] & 1)))));
}
fn uncheckedSqrt(x2: Fe) Fe {
var e = x2.pow2523();
const p_root = e.mul(x2); // positive root
const m_root = p_root.mul(Fe.sqrtm1); // negative root
const m_root2 = m_root.sq();
e = x2.sub(m_root2);
var x = p_root;
x.cMov(m_root, @intFromBool(e.isZero()));
return x;
}
/// Compute the square root of `x2`, returning `error.NotSquare` if `x2` was not a square
pub fn sqrt(x2: Fe) NotSquareError!Fe {
const x2_copy = x2;
const x = x2.uncheckedSqrt();
const check = x.sq().sub(x2_copy);
if (check.isZero()) {
return x;
}
return error.NotSquare;
}
};