server: Uniformly random id generator

1 files changed, 14 insertions, 1 deletions

diff --git a/server/serverstore.js b/server/serverstore.js
index e10ae9a..745877f 100755
--- a/server/serverstore.js
+++ b/server/serverstore.js
@@ -30,9 +30,22 @@ renewChallenge();
 
 function gencode(){
 	var alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
+
+	var domainsize = Math.pow(256, 5);
+	var targetsize = Math.pow(62, 6);
+	var limit = domainsize - (domainsize % targetsize);
+
+	var number;
+	do {
+		var bytes = crypto.randomBytes(5);  // 62^6 < 256^5, so large enough
+		number = 0;  // 256^5 < 2^52, so fits in JS's double
+		for (var i = 0; i < 5; i++) number = 256 * number + bytes[i];
+	} while (number >= limit);  // ensure uniformity
+
 	var code = "";
 	for(var i = 0; i < 6; i++) {
-		code += alphabet[~~(Math.random() * alphabet.length)];
+		code += alphabet[number % 62];
+		number = Math.floor(number / 62);
 	}
 	return code;
 }