x/crypto/ssh: add padding oracle countermeasures for AES-CBC.
This deprives an attacker of feedback for guesses against the packet
length given by the connection dropping.
Change-Id: I14939a82e5243a86d192bb18be93d45589227147
Reviewed-on: https://go-review.googlesource.com/9908
Reviewed-by: Adam Langley <agl@golang.org>
diff --git a/ssh/cipher_test.go b/ssh/cipher_test.go
index 2fb75d0..d67a93a 100644
--- a/ssh/cipher_test.go
+++ b/ssh/cipher_test.go
@@ -62,3 +62,66 @@
}
}
}
+
+func TestCBCOracleCounterMeasure(t *testing.T) {
+ cipherModes[aes128cbcID] = &streamCipherMode{16, aes.BlockSize, 0, nil}
+ defer delete(cipherModes, aes128cbcID)
+
+ kr := &kexResult{Hash: crypto.SHA1}
+ algs := directionAlgorithms{
+ Cipher: aes128cbcID,
+ MAC: "hmac-sha1",
+ Compression: "none",
+ }
+ client, err := newPacketCipher(clientKeys, algs, kr)
+ if err != nil {
+ t.Fatalf("newPacketCipher(client): %v", err)
+ }
+
+ want := "bla bla"
+ input := []byte(want)
+ buf := &bytes.Buffer{}
+ if err := client.writePacket(0, buf, rand.Reader, input); err != nil {
+ t.Errorf("writePacket: %v", err)
+ }
+
+ packetSize := buf.Len()
+ buf.Write(make([]byte, 2*maxPacket))
+
+ // We corrupt each byte, but this usually will only test the
+ // 'packet too large' or 'MAC failure' cases.
+ lastRead := -1
+ for i := 0; i < packetSize; i++ {
+ server, err := newPacketCipher(clientKeys, algs, kr)
+ if err != nil {
+ t.Fatalf("newPacketCipher(client): %v", err)
+ }
+
+ fresh := &bytes.Buffer{}
+ fresh.Write(buf.Bytes())
+ fresh.Bytes()[i] ^= 0x01
+
+ before := fresh.Len()
+ _, err = server.readPacket(0, fresh)
+ if err == nil {
+ t.Errorf("corrupt byte %d: readPacket succeeded ", i)
+ continue
+ }
+ if _, ok := err.(cbcError); !ok {
+ t.Errorf("corrupt byte %d: got %v (%T), want cbcError", i, err, err)
+ continue
+ }
+
+ after := fresh.Len()
+ bytesRead := before - after
+ if bytesRead < maxPacket {
+ t.Errorf("corrupt byte %d: read %d bytes, want more than %d", i, bytesRead, maxPacket)
+ continue
+ }
+
+ if i > 0 && bytesRead != lastRead {
+ t.Errorf("corrupt byte %d: want %d, got %d bytes read", bytesRead, lastRead)
+ }
+ lastRead = bytesRead
+ }
+}
