From: Colin P. Mccabe <colin@cmccabe.xyz>
Date: Sat, 9 May 2020 05:33:34 +0000 (-0700)
Subject: pickrand.go: use cryptographic randomness
X-Git-Url: http://www.club.cc.cmu.edu/~cmccabe/cgi-bin/gitweb.cgi?p=cmccabe-bin;a=commitdiff_plain;h=235c8f2e5692d8c89145085057609de827dc0aee

pickrand.go: use cryptographic randomness

Use cryptographic randomness to avoid getting the same random file
multiple times in a row due to coarse clock granularity.
---

diff --git a/pickrand.go b/pickrand.go
index a21efda..1bf4844 100644
--- a/pickrand.go
+++ b/pickrand.go
@@ -1,11 +1,12 @@
 package main
 
 import (
+	"bytes"
+	"crypto/rand"
 	"fmt"
+	"math/big"
 	"os"
 	"path/filepath"
-	"math/rand"
-	"time"
 )
 
 func main() {
@@ -27,7 +28,19 @@ func main() {
 		fmt.Fprintf(os.Stderr, "** Error: %s\n", err.Error())
 		os.Exit(1)
 	}
-	rand.Seed(time.Now().UTC().UnixNano())
-	i := rand.Int31n(int32(len(files)))
-	fmt.Printf("%s\n", root + "/" + files[i])
+	var b [8]byte
+	_, err = rand.Read(b[:])
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to access cryptographic randomness.  " +
+			"Error: %s\n\n", err.Error())
+		os.Exit(1)
+	}
+	i, err := rand.Int(bytes.NewReader(b[:]), big.NewInt(int64(len(files))))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to get a random int.  Error: %s\n", err.Error())
+		os.Exit(1)
+	}
+	j := int(i.Uint64())
+
+	fmt.Printf("%s\n", root + "/" + files[j])
 }