k1 = t1 * 2^shift + junk
k2 = (t1 + diff) * 2^shift + junk
k2 - k1 = diff (2^shift)

bf: diff (2^shift)

for bf in range(...):
    if R1 + G * bf * (2^shift) == R2:
        found

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s1 (k1) = h1 - r1 x
s2 (k1 + diff) = h2 - r2 x

(h1 - s1 k1) / r1 = x
(h2 - s2 k1 - s2 diff) / r2 = x

(h1 - s1 k1) r2 = (h2 - s2 k1 - s2 diff) r1
r2 h1 - r2 s1 k1 = r1 h2 - r1 s2 k1 - r1 s2 diff
r1 s2 k1 - r2 s1 k1 = r1 h2 - r1 s2 diff - r2 h1
k1 (r1 s2 - s1 r2) = r1 h2 - r1 s2 diff - r2 h1
k1 = (r1 h2 - r1 s2 diff - r2 h1) / (r1 s2 - s1 r2)