// StretchedKey implements BES' key schedule. It returns the 11 round keys derived from the master key.
func (constr *Construction) StretchedKey() [11]gfmatrix.Row {
var (
i int = 0
stretched [4 * 11]gfmatrix.Row
split [11]gfmatrix.Row
)
for ; i < 4; i++ {
stretched[i] = constr.Key[32*i : 32*(i+1)]
}
for ; i < (4 * 11); i++ {
temp := stretched[i-1]
if (i % 4) == 0 {
temp = append(temp[8:], temp[:8]...)
temp = constr.subBytes(temp)
temp = wordSubBytes.Mul(temp).Add(wordSubBytesConst)
temp = temp.Add(Expand([]byte{powx[i/4-1], 0, 0, 0}))
}
stretched[i] = stretched[i-4].Add(temp)
}
for j := 0; j < 11; j++ {
split[j] = gfmatrix.NewRow(128)
for k := 0; k < 4; k++ {
copy(split[j][32*k:32*(k+1)], stretched[4*j+k])
}
}
return split
}
// subBytes rewrites each byte of the state with the S-Box. unSubBytes and subBytes are the same.
func (constr *Construction) subBytes(in gfmatrix.Row) gfmatrix.Row {
out := gfmatrix.NewRow(in.Size())
for pos := 0; pos < in.Size(); pos++ {
out[pos] = in[pos].Invert()
}
return out
}
// Decrypt decrypts the first block in src into dst. Dst and src may point at the same memory.
func (constr Construction) Decrypt(dst, src []byte) {
state := gfmatrix.NewRow(128)
for pos := 0; pos < 128; pos++ {
state[pos] = number.ByteFieldElem(src[pos])
}
state = constr.decrypt(state)
for pos := 0; pos < 128; pos++ {
dst[pos] = byte(state[pos])
}
}
// Expand takes a value from AES's space and embeds it in BES's space.
func Expand(in []byte) gfmatrix.Row {
out := gfmatrix.NewRow(8 * len(in))
for k, v := range in {
num := number.ByteFieldElem(v)
for pos := 0; pos < 8; pos++ {
out[8*k+pos] = num
num = num.Mul(num)
}
}
return out
}
