crypto/{aes,cipher}: fix panic in CBC on s390x when src length is 0

Adds a test to check that block cipher modes accept a zero-length
input.

Fixes #17435.

Change-Id: Ie093c4cdff756b5c2dcb79342e167b3de5622389
Reviewed-on: https://go-review.googlesource.com/31070
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/aes/cbc_s390x.go

@@ -48,7 +48,9 @@ func (x *cbc) CryptBlocks(dst, src []byte) {
 	if len(dst) < len(src) {
 		panic("crypto/cipher: output smaller than input")
 	}
+	if len(src) > 0 {
 		cryptBlocksChain(x.c, &x.iv[0], &x.b.key[0], &dst[0], &src[0], len(src))
+	}
 }
 
 func (x *cbc) SetIV(iv []byte) {

src/crypto/cipher/cipher_test.go

@@ -5,8 +5,10 @@
 package cipher_test
 
 import (
+	"bytes"
 	"crypto/aes"
 	"crypto/cipher"
+	"crypto/des"
 	"testing"
 )
 
@@ -34,3 +36,55 @@ func mustPanic(t *testing.T, msg string, f func()) {
 	}()
 	f()
 }
+
+func TestEmptyPlaintext(t *testing.T) {
+	var key [16]byte
+	a, err := aes.NewCipher(key[:16])
+	if err != nil {
+		t.Fatal(err)
+	}
+	d, err := des.NewCipher(key[:8])
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	s := 16
+	pt := make([]byte, s)
+	ct := make([]byte, s)
+	for i := 0; i < 16; i++ {
+		pt[i], ct[i] = byte(i), byte(i)
+	}
+
+	assertEqual := func(name string, got, want []byte) {
+		if !bytes.Equal(got, want) {
+			t.Fatalf("%s: got %v, want %v", name, got, want)
+		}
+	}
+
+	for _, b := range []cipher.Block{a, d} {
+		iv := make([]byte, b.BlockSize())
+		cbce := cipher.NewCBCEncrypter(b, iv)
+		cbce.CryptBlocks(ct, pt[:0])
+		assertEqual("CBC encrypt", ct, pt)
+
+		cbcd := cipher.NewCBCDecrypter(b, iv)
+		cbcd.CryptBlocks(ct, pt[:0])
+		assertEqual("CBC decrypt", ct, pt)
+
+		cfbe := cipher.NewCFBEncrypter(b, iv)
+		cfbe.XORKeyStream(ct, pt[:0])
+		assertEqual("CFB encrypt", ct, pt)
+
+		cfbd := cipher.NewCFBDecrypter(b, iv)
+		cfbd.XORKeyStream(ct, pt[:0])
+		assertEqual("CFB decrypt", ct, pt)
+
+		ctr := cipher.NewCTR(b, iv)
+		ctr.XORKeyStream(ct, pt[:0])
+		assertEqual("CTR", ct, pt)
+
+		ofb := cipher.NewOFB(b, iv)
+		ofb.XORKeyStream(ct, pt[:0])
+		assertEqual("OFB", ct, pt)
+	}
+}