Reorganize crypto code

Move all crypto functions to package "crypto", move random generators
for tests into helper package.

crypto/buffer_pool.go

@@ -0,0 +1,19 @@
+package crypto
+
+import "sync"
+
+const defaultBufSize = 2048
+
+var bufPool = sync.Pool{
+	New: func() interface{} {
+		return make([]byte, defaultBufSize)
+	},
+}
+
+func getBuffer() []byte {
+	return bufPool.Get().([]byte)
+}
+
+func freeBuffer(buf []byte) {
+	bufPool.Put(buf)
+}