circuit: add bytes utils

packages/circuits/utils/bytes.circom

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+pragma circom 2.1.5;
+
+include "circomlib/circuits/bitify.circom";
+include "circomlib/circuits/comparators.circom";
+include "circomlib/circuits/poseidon.circom";
+include "./constants.circom";
+
+
+/// @title PackBytesToInts
+/// @notice Converts a byte array into a 31-byte integer array (packing)
+/// @param maxBytes: the maximum number of bytes in the input array
+/// @input in: the input byte array
+/// @output out: the output integer array
+template PackBytesToInts(maxBytes) {
+    var packSize = MAX_BYTES_IN_FIELD();
+
+    // Calculate number of int chunks based in maxBytes
+    var remain = maxBytes % packSize;
+    var maxInts = (maxBytes - remain) / packSize;
+    if (remain > 0) {
+        maxInts += 1;
+    }
+
+    signal input in[maxBytes];
+    signal output out[maxInts];
+
+    signal intSums[maxInts][packSize];
+
+    for (var i = 0; i < maxInts; i++) {
+        for(var j=0; j < packSize; j++) {
+            var idx = packSize * i + j;
+
+            // Copy the previous value if we are out of bounds - we take last item as final result
+            if(idx >= maxBytes) {
+                intSums[i][j] <== intSums[i][j-1];
+            } 
+            // First item of each chunk is the byte itself
+            else if (j == 0){
+                intSums[i][j] <== bytes[idx];
+            }
+            // Every other item is 256^j * byte
+            else {
+                intSums[i][j] <== intSums[i][j-1] + (1 << (8*j)) * bytes[idx];
+            }
+        }
+    }
+
+    // Last item of each chunk is the final sum
+    for (var i = 0; i < maxInts; i++) {
+        ints[i] <== intSums[i][packSize-1];
+    }
+}

packages/circuits/utils/constants.circom

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+pragma circom 2.1.5;
+
+function EMAIL_ADDR_MAX_BYTES() {
+    return 256;
+}
+
+function DOMAIN_MAX_BYTES() {
+    return 255;
+}
+
+// Field support maximum of ~253 bit
+function MAX_BYTES_IN_FIELD() {
+    return 31;
+}