refactor: remove math/big, fix RNG bias, optimize builds, and bump Go version

main.go

@@ -5,7 +5,6 @@ package main
 import (
 	"crypto/rand"
 	"fmt"
-	"math/big"
 	"os"
 	"runtime"
 	"strconv"
@@ -15,7 +14,7 @@ import (
 var allowedCharacters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890`~!@#$%^&*()_+[]\\{}|;':,./<>?")
 
 const (
-	Version   = "1.3.4"
+	Version   = "1.3.5"
 	symbols   = "`~!@#$%^&*()_+[]\\{}|;':,./<>?"
 	chunkSize = 16 // The size of each chunk of the password to be generated by the worker goroutines
 )
@@ -40,7 +39,7 @@ func matchArguments(args []string) string {
 			"\nFor help (this output): gopass -h\n" +
 			"For version: gopass -v\n"
 	default:
-		err := error(nil)
+		var err error
 		size, err = strconv.Atoi(args[0])
 		if err != nil {
 			return "Invalid first argument (\"" + args[0] + "\") supplied! (Type gopass -h for help)"
@@ -95,21 +94,52 @@ func generatePassword(size int) string {
 	numWorkers := runtime.NumCPU()
 
 	// Launch the worker goroutines
-	for i := 0; i < numWorkers; i++ {
+	for range numWorkers {
 		go func() {
 			allowedLen := len(allowedCharacters)
+
+			// Calculate the rejection limit to avoid modulo bias.
+			// Any random byte value > maxByte must be discarded.
+			maxByte := 255 - (256 % allowedLen)
+
+			// Create a buffer for random bytes.
+			// We read enough bytes for the whole chunk at once (plus some extra
+			// in case of rejections) to reduce system calls.
+			randBuf := make([]byte, chunkSize*2)
+
 			for {
-				// Generate a chunk of the password
+				// Fill the buffer initially
+				if _, err := rand.Read(randBuf); err != nil {
+					println("Error securely generating random character chunk!")
+					return
+				}
+				bufIdx := 0
+
 				chunk := make([]rune, chunkSize)
 				for i := range chunk {
-					index, err := rand.Int(rand.Reader, big.NewInt(int64(allowedLen)))
-					if err != nil {
-						println("Error securely generating random character chunk!")
-						return
-					}
-					chunk[i] = allowedCharacters[index.Int64()]
-				}
+					for {
+						// If we exhausted the buffer due to rejections, refill it
+						if bufIdx >= len(randBuf) {
+							if _, err := rand.Read(randBuf); err != nil {
+								println("Error securely generating random character chunk!")
+								return
+							}
+							bufIdx = 0
+						}
 
+						b := randBuf[bufIdx]
+						bufIdx++
+
+						// If the byte causes bias, discard and retry
+						if int(b) > maxByte {
+							continue
+						}
+
+						// Safe to map to character set
+						chunk[i] = allowedCharacters[int(b)%allowedLen]
+						break
+					}
+				}
 				// Send the chunk of the password to the main goroutine
 				passChan <- chunk
 			}