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authorsiddharth <s@ricketyspace.net>2021-09-04 16:57:14 -0400
committersiddharth <s@ricketyspace.net>2021-09-04 16:57:14 -0400
commit823471e97d03d0f2c020c344e941668ea7e7769d (patch)
treea7579f7a85e8bca4044ac6142162360d86674028 /lib
parentfbf74bd2d84db61a68b181b8923335b1f2d501fa (diff)
lib: add MTXORStream
Diffstat (limited to 'lib')
-rw-r--r--lib/rng.go49
1 files changed, 49 insertions, 0 deletions
diff --git a/lib/rng.go b/lib/rng.go
index 4c566e0..c1038f4 100644
--- a/lib/rng.go
+++ b/lib/rng.go
@@ -93,3 +93,52 @@ func (r *MTRand) UnTemper(y uint32) uint32 {
y = 0xFFFFFFFF & y
return y
}
+
+// Returns a stream function. The stream function returns a random
+// byte when invoked.
+//
+// The `seed` argument must be exactly bytes.
+func keystream(seed []byte) func() byte {
+ if len(seed) != 2 {
+ return nil
+ }
+
+ // Pack seed into a uint32
+ s := (uint32(seed[0]) << 8) ^ uint32(seed[1])
+
+ // Init MT19937.
+ mtR := new(MTRand)
+ mtR.Seed(s)
+
+ // Stream func.
+ n := uint32(0)
+ return func() byte {
+ if n == uint32(0) {
+ n = mtR.Extract()
+ }
+ r := byte(n & 0xFF) // Extract last 8 bits.
+ n = n >> 8 // Get rid of the last 8 bits.
+
+ return r
+ }
+}
+
+// XORs `stream` with the MT19937 keystream seeded with `seed`.
+func MTXORStream(stream, seed []byte) []byte {
+ if len(stream) == 0 {
+ return []byte{}
+ }
+ if len(seed) != 2 {
+ return nil
+ }
+
+ ks := keystream(seed)
+ if ks == nil {
+ return nil
+ }
+ s := make([]byte, 0)
+ for i := 0; i < len(stream); i++ {
+ s = append(s, stream[i]^ks())
+ }
+ return s
+}