mirror of
https://github.com/henrydcase/nobs.git
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53 lines
1.4 KiB
Go
53 lines
1.4 KiB
Go
// +build !amd64 noasm
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package sidh
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var three238m1 = []uint8{
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0xf8, 0x84, 0x83, 0x82, 0x8a, 0x71, 0xcd, 0xed,
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0x14, 0x7a, 0x42, 0xd4, 0xbf, 0x35, 0x3b, 0x73,
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0x38, 0xcf, 0xd7, 0x94, 0xcf, 0x29, 0x82, 0xf8,
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0xd6, 0x2a, 0x7c, 0x0c, 0x99, 0x6c, 0xc5, 0x63,
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0xc7, 0x22, 0x42, 0x8f, 0x7e, 0xa8, 0x58, 0xb8,
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0xf5, 0xea, 0x25, 0xb5, 0xc6, 0xc9, 0x54, 0x02}
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func addc8(cin, a, b uint8) (ret, cout uint8) {
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t := a + cin
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ret = b + t
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cout = ((a & b) | ((a | b) & (^ret))) >> 7
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return
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}
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func subc8(bIn, a, b uint8) (ret, bOut uint8) {
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var tmp1 = a - b
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ret = tmp1 - bIn
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// Set bOut if bIn!=0 and tmp1==0 in constant time
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bOut = bIn & (1 ^ ((tmp1 | uint8(0-tmp1)) >> 7))
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// Constant time check if a<b
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bOut |= (a ^ ((a ^ b) | (uint8(a-b) ^ b))) >> 7
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return
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}
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// Set result to zero if the input scalar is <= 3^238, otherwise result is 1.
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// Scalar must be array of 48 bytes. This function is specific to P751.
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func checkLessThanThree238(scalar []byte) uint8 {
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var borrow uint8
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for i := 0; i < len(three238m1); i++ {
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_, borrow = subc8(borrow, three238m1[i], scalar[i])
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}
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return borrow
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}
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// Multiply 48-byte scalar by 3 to get a scalar in 3*[0,3^238). This
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// function is specific to P751.
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func multiplyByThree(scalar []byte) {
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var carry uint8
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var dbl [48]uint8
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for i := 0; i < len(scalar); i++ {
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dbl[i], carry = addc8(carry, scalar[i], scalar[i])
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}
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for i := 0; i < len(scalar); i++ {
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scalar[i], carry = addc8(carry, dbl[i], scalar[i])
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}
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}
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