kubeadm: more random tokens

The strategy of hex encoding a random byte array only uses the
following characters:

	0123456789abcdef

Instead of the entire bootstrapping token character set:

	0123456789abcdefghijklmnopqrstuvwxyz

Update the token generation to use the entire character set. This
increases the token secret from 48 bits of entropy to ~82 bits.

256^8 (1.8e+19) vs. 36^16 (7.9e+24).

cmd/kubeadm/app/util/token/tokens.go

@@ -17,8 +17,8 @@ limitations under the License.
 package token
 
 import (
+	"bufio"
 	"crypto/rand"
-	"encoding/hex"
 	"fmt"
 	"regexp"
 
@@ -27,9 +27,9 @@ import (
 
 const (
 	// TokenIDBytes defines a number of bytes used for a token id
-	TokenIDBytes = 3
+	TokenIDBytes = 6
 	// TokenSecretBytes defines a number of bytes used for a secret
-	TokenSecretBytes = 8
+	TokenSecretBytes = 16
 )
 
 var (
@@ -43,13 +43,35 @@ var (
 	TokenRegexp = regexp.MustCompile(TokenRegexpString)
 )
 
+const validBootstrapTokenChars = "0123456789abcdefghijklmnopqrstuvwxyz"
+
 func randBytes(length int) (string, error) {
-	b := make([]byte, length)
-	_, err := rand.Read(b)
-	if err != nil {
-		return "", err
+	// len("0123456789abcdefghijklmnopqrstuvwxyz") = 36 which doesn't evenly divide
+	// the possible values of a byte: 256 mod 36 = 4. Discard any random bytes we
+	// read that are >= 252 so the bytes we evenly divide the character set.
+	const maxByteValue = 252
+
+	var (
+		b     byte
+		err   error
+		token = make([]byte, length)
+	)
+
+	reader := bufio.NewReaderSize(rand.Reader, length*2)
+	for i := range token {
+		for {
+			if b, err = reader.ReadByte(); err != nil {
+				return "", err
+			}
+			if b < maxByteValue {
+				break
+			}
+		}
+
+		token[i] = validBootstrapTokenChars[int(b)%len(validBootstrapTokenChars)]
 	}
-	return hex.EncodeToString(b), nil
+
+	return string(token), nil
 }
 
 // GenerateToken generates a new token with a token ID that is valid as a

cmd/kubeadm/app/util/token/tokens_test.go

@@ -147,8 +147,8 @@ func TestRandBytes(t *testing.T) {
 		if err != nil {
 			t.Errorf("failed randBytes: %v", err)
 		}
-		if len(actual) != rt*2 {
-			t.Errorf("failed randBytes:\n\texpected: %d\n\t  actual: %d\n", rt*2, len(actual))
+		if len(actual) != rt {
+			t.Errorf("failed randBytes:\n\texpected: %d\n\t  actual: %d\n", rt, len(actual))
 		}
 	}
 }