// generateSelfEquivalence returns a random self-equivalence of the S-box layer, so that \zeta(x) = bInv(\zeta(a(x))).
func generateSelfEquivalence(r io.Reader) (a, bInv encoding.Block) {
// Sample a byte-wise permutation to apply to the input.
p := &permutation{encoding.GenerateShuffle(r)}
// Sample one non-zero scalar for each byte. Each byte of the input is multiplied by this scalar.
buff := make([]byte, 1)
scalars := encoding.ConcatenatedBlock{}
for pos := 0; pos < 16; {
r.Read(buff)
if buff[0] != 0x00 {
scalars[pos] = encoding.NewByteMultiplication(number.ByteFieldElem(buff[0]))
pos++
}
}
// Sample a random value in [0, 8) for each byte. This is the number of times to apply the Frobenius.
frobs := encoding.ConcatenatedBlock{}
for pos := 0; pos < 16; pos++ {
r.Read(buff)
frobs[pos] = frobenius(buff[0] & 0x7)
}
return encoding.ComposedBlocks{
frobs, scalars, p,
}, encoding.ComposedBlocks{
encoding.InverseBlock{p}, scalars, encoding.InverseBlock{frobs},
}
}
// stripScalars gets rid of unknown scalars in each block of the affine layer. It leaves it exactly equal to MixColumns,
// but there is an unknown scalar in each block that will move into the S-box layers.
func (al *affineLayer) stripScalars() (encoding.ConcatenatedBlock, encoding.ConcatenatedBlock) {
input, output := [16]encoding.ByteLinear{}, [16]encoding.ByteLinear{}
for pos := 0; pos < 16; pos += 4 {
found := false
for guess := 1; guess < 256 && !found; guess++ { // Take a guess for the input scalar on the first column.
input[pos], _ = encoding.DecomposeByteLinear(encoding.NewByteMultiplication(number.ByteFieldElem(guess)))
// Given input scalar on first column, calculate output scalars on all rows.
for i := pos; i < pos+4; i++ {
mc, _ := mixColumns[i%4][0].Invert()
output[i] = encoding.NewByteLinear(
al.getBlock(i, pos).Compose(input[pos].Backwards).Compose(mc),
)
}
// Given output scalar on each row, calculate input scalars on all columns.
for i := pos + 1; i < pos+4; i++ {
mc, _ := mixColumns[0][i%4].Invert()
input[i] = encoding.NewByteLinear(
al.getBlock(pos, i).Compose(output[pos].Backwards).Compose(mc),
)
}
// Verify that guess is consistent.
found = true
for i := pos; i < pos+4 && found; i++ {
for j := pos; j < pos+4 && found; j++ {
cand := al.getBlock(i, j).Compose(output[i].Backwards).Compose(input[j].Backwards)
real := mixColumns[i%4][j%4]
if !cand.Equals(real) {
found = false
}
}
}
}
if !found {
panic("Failed to disambiguate block affine layer!")
}
}
in, out := encoding.ConcatenatedBlock{}, encoding.ConcatenatedBlock{}
for pos := 0; pos < 16; pos++ {
in[pos], out[pos] = input[pos], output[pos]
}
return in, out
}