//split takes a document and splits it into shares using a
//k out of n shamir secret sharing scheme. It returns the shares
//in a 2D array, or nil and a non-nil error
func split(document []byte, n int, k int) ([][]byte, error) {
fmt.Printf("document: %x.\n", document)
length := len(document)
fmt.Printf("Document has length %d.\n", length)
shares := make([][]byte, n)
for i := 0; i < n; i++ {
//the length of each share is a 2 byte x value plus
//the length of the document
shares[i] = make([]byte, len(document)+2)
//populate shares with x values
x := shares[i][:2]
if _, err := io.ReadFull(rand.Reader, x); err != nil {
checkError(err)
return nil, err
}
}
//choose k elements uniformly at random
as := make([][]byte, k)
for i := 0; i < k; i++ {
as[i] = make([]byte, 2)
if _, err := io.ReadFull(rand.Reader, as[i]); err != nil {
checkError(err)
return nil, err
}
}
//operate on 2 bytes at a time
for i := 0; i < length-1; i += 2 {
//working on document[i:i+2].
for j := 0; j < n; j++ {
x := C.int(bytes2int(shares[j][:2]))
y := C.int(0)
for l := 0; l < k; l++ {
a := C.int(bytes2int(as[l]))
tmp := C.int(1)
for m := 0; m <= l; m++ {
tmp = C.galois_single_multiply(x, tmp, 16)
}
y = y ^ C.galois_single_multiply(a, tmp, 16)
}
ybytes := int2bytes(int(y))
out := shares[j][i+2 : i+4]
out[0] = document[i] ^ ybytes[0]
out[1] = document[i+1] ^ ybytes[1]
}
}
return shares, nil
}
// Create and print a matrix in GF(2^w)
func CreateAndPrint(r, c, w int) {
matrix := make([]int, r*c)
n := 1
for i := 0; i < r*c; i++ {
matrix[i] = n
n = int(C.galois_single_multiply(C.int(n), 2, C.int(w)))
}
C.jerasure_print_matrix((*C.int)(unsafe.Pointer(&matrix[0])), C.int(r), C.int(c), C.int(w))
}
func mend(shares [][]byte) []byte {
length := len(shares[0])
fmt.Printf("Shares have length = %d.\n", length)
num := len(shares)
document := make([]byte, length-2)
for i := 2; i < length-1; i += 2 {
//interpolate shares[][i:i+2]
out := document[i-2 : i]
out[0] = 0
out[1] = 0
cout := C.int(0)
for j := 0; j < num; j++ {
b := C.int(1)
tmp := C.int(0)
for k := 0; k < num; k++ {
if k != j {
xk := C.int(bytes2int(shares[k][:2]))
xj := C.int(bytes2int(shares[j][:2]))
b = C.galois_single_multiply(b, xk, 16)
tmp = xk ^ xj
b = C.galois_single_divide(b, tmp, 16)
}
}
ttmp := make([]byte, 2)
ctmp := C.CString(string(ttmp))
C.galois_w16_region_multiply(C.CString(string(shares[j][i:i+2])), b, 2, ctmp, 0)
wootmp := bytes2int([]byte(C.GoBytes(unsafe.Pointer(ctmp), C.int(2))))
tmpint1 := C.int(wootmp)
cout = tmpint1 ^ cout
}
out = int2bytes(int(cout))
copy(document[i-2:i], out)
}
fmt.Printf("document: %x.\n", document)
fmt.Printf("Document has length %d.\n", len(document))
return document
}
