make I2OSP more constant time
g-k opened this issue · comments
see discussion around mozilla-services/autograph#76 (comment)
per #41 further investigation might help:
Need to profile this, but we could:
iterate through by byte instead try to make the error checking constant time find out if the big.Int.Bytes() call is accounting for the discrepancy
Also, the timing test is flaky.
Looks like the implementations of both Bytes and Sign methods are O(n) with int size (assuming the make([]byte, len(x.abs)*_S) and len(x.abs) are):
// Bytes returns the absolute value of x as a big-endian byte slice.
func (x *Int) Bytes() []byte {
buf := make([]byte, len(x.abs)*_S)
return buf[x.abs.bytes(buf):]
}https://golang.org/src/math/big/int.go?s=10730:10758#L423
// Sign returns:
//
// -1 if x < 0
// 0 if x == 0
// +1 if x > 0
//
func (x *Int) Sign() int {
if len(x.abs) == 0 {
return 0
}
if x.neg {
return -1
}
return 1
}https://golang.org/src/math/big/int.go?s=575:599#L21
The a big int's Bits() that returns the little-endian Word slice method should be constant time.
Ugh nasty. Bits looks like a good option.
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