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97 lines
2.5 KiB
Go
97 lines
2.5 KiB
Go
package ecgfp5
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// A custom 161-bit integer type; used for splitting a scalar into a
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// fraction. Negative values use two's complement notation; the value
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// is truncated to 161 bits (upper bits in the top limb are ignored).
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// Elements are mutable containers.
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// WARNING: everything in here is vartime; do not use on secret values.
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type Signed161 [3]uint64
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// Export this value as a 192-bit integer (three 64-bit limbs, in little-endian order).
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func (s Signed161) ToU192() [3]uint64 {
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x := s[2] & 0x00000001FFFFFFFF
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x |= (^(x >> 32) + 1) << 33
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return [3]uint64{s[0], s[1], x}
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}
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// Recode this integer into 33 signed digits for a 5-bit window.
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func (s Signed161) RecodeSigned5() [33]int32 {
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// We first sign-extend the value to 192 bits, then add
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// 2^160 to get a nonnegative value in the 0 to 2^161-1
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// range. We then recode that value; and finally we fix
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// the result by subtracting 1 from the top digit.
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tmp := s.ToU192()
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tmp[2] += 0x0000000100000000
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var ss [33]int32
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RecodeSignedFromLimbs(tmp[:], ss[:], 5)
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ss[32] -= 1
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return ss
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}
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// Add v*2^s to this value.
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func (s *Signed161) AddShifted(v *Signed161, shift int32) {
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if shift == 0 {
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s.Add(v[:])
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} else if shift < 64 {
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s.AddShiftedSmall(v[:], shift)
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} else if shift < 161 {
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s.AddShiftedSmall(v[(shift>>6):], shift&63)
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}
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}
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func (s *Signed161) AddShiftedSmall(v []uint64, shift int32) {
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cc, vbits, j := uint64(0), uint64(0), 3-len(v)
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for i := j; i < 3; i++ {
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vw := v[i-j]
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vws := (vw << (uint32(shift) % 64)) | vbits
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vbits = vw >> ((64 - uint32(shift)) % 64)
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z := U128From64(s[i]).Add64(vws).Add64(cc)
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s[i] = z.Lo
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cc = z.Hi
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}
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}
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func (s *Signed161) Add(v []uint64) {
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cc, j := uint64(0), 3-len(v)
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for i := j; i < 3; i++ {
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z := U128From64(s[i]).Add64(v[i-j]).Add64(cc)
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s[i] = z.Lo
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cc = z.Hi
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}
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}
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// Subtract v*2^s from this value.
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func (s *Signed161) SubShifted(v *Signed161, shift int32) {
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if shift == 0 {
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s.Sub(v[:])
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} else if shift < 64 {
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s.SubShiftedSmall(v[:], shift)
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} else if shift < 161 {
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s.SubShiftedSmall(v[(shift>>6):], shift&63)
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}
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}
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func (s *Signed161) SubShiftedSmall(v []uint64, shift int32) {
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cc, vbits, j := uint64(0), uint64(0), 3-len(v)
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for i := j; i < 3; i++ {
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vw := v[i-j]
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vws := (vw << (uint32(shift) % 64)) | vbits
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vbits = vw >> ((64 - uint32(shift)) % 64)
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z := U128From64(s[i]).Sub64(vws).Sub64(cc)
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s[i] = z.Lo
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cc = z.Hi & 1
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}
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}
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func (s *Signed161) Sub(v []uint64) {
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cc, j := uint64(0), 3-len(v)
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for i := j; i < 3; i++ {
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z := U128From64(s[i]).Sub64(v[i-j]).Sub64(cc)
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s[i] = z.Lo
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cc = z.Hi & 1
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}
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}
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