make I2OSP more constant time

[g-k]

see discussion around https://github.com/mozilla-services/autograph/pull/76#discussion_r184970052

[g-k]

per https://github.com/mozilla-services/go-cose/pull/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.

[g-k]

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.

[franziskuskiefer]

Ugh nasty. Bits looks like a good option.

[hwine]

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