add SplitBytesToWords circuit to bytes.circom

packages/circuits/utils/bytes.circom

@@ -111,3 +111,39 @@ template DigitBytesToInt(n) {
 
     out <== sums[n];
 }
+
+
+// NOTE: this circuit is unaudited and should not be used in production
+/// @title SplitBytesToWords
+/// @notice split an array of bytes into an array of words
+/// @notice useful for casting a message or modulus before RSA verification
+/// @param l: number of bytes in the input array
+/// @param n: number of bits in a word
+/// @param k: number of words
+/// @input in: array of bytes
+/// @output out: array of words
+template SplitBytesToWords (l,n,k) {
+    signal input in[l];
+    signal output out[k];
+
+    component num2bits[l];
+    for (var i = 0 ; i < l ; i++){
+        num2bits[i] = Num2Bits(8);
+        num2bits[i].in <== in[i];
+    }
+    component bits2num[k];
+    for (var i = 0 ; i < k ; i++){
+        bits2num[i] = Bits2Num(n);
+        for(var j = 0 ; j < n ; j++){
+            if(i*n + j >=  8 * l){
+                bits2num[i].in[j] <==  0;
+            }
+            else{
+                bits2num[i].in[j] <== num2bits[l - (( i * n + j) \ 8) - 1].out[ ((i * n + j) % 8)];
+            }
+        }
+    }
+    for( var i = 0 ; i< k ; i++){
+    out[i] <== bits2num[i].out;
+    }
+}