Home » Posts tagged "padding"

這次 PKCS #1 1.5 的 ROBOT 攻擊,Cisco 沒打算修...

1998 年就發現的 security issue 因為 workaround 也很複雜,所以不是每一家都修對方法,於是 19 年後又被爆破了。這次叫做 ROBOT:「1998 attack that messes with sites’ secret crypto keys is back in a big way」。

可以看到中獎的表:

這次的攻擊在 client 端無法修正,只能在 server 端修正:

Do I need to update my browser?
No. This is an implementation bug in servers, there is nothing clients can do to prevent it.

如果 server 端無法盡快修正的話,想辦法避開 RSA encryption 可以躲開這個問題,而且因為現代瀏覽器都有非 RSA 的替代方案,這樣做應該都還有退路,可以維持連線的可能性:

Disable RSA encryption!
ROBOT only affects TLS cipher modes that use RSA encryption. Most modern TLS connections use an Elliptic Curve Diffie Hellman key exchange and need RSA only for signatures. We believe RSA encryption modes are so risky that the only safe course of action is to disable them. Apart from being risky these modes also lack forward secrecy.

但使用 Cisco ACE 就哭了,因為 Cisco ACE 只支援 RSA encryption,而 Cisco 官方以產品線已經關閉,不再提供維護而沒有提供更新的計畫,所以就進入一個死胡同...

不過 Cisco 自己也還在用 Cisco ACE 就是了,不在意就不會痛的感覺 XD

I have a Cisco ACE device.
Cisco informed us that the ACE product line was discontinued several years ago and that they won't provide an update. Still, we found plenty of vulnerable hosts that use these devices.

These devices don't support any other cipher suites, therefore disabling RSA is not an option. To our knowledge it is not possible to use these devices for TLS connections in a secure way.

However, if you use these products you're in good company: As far as we can tell Cisco is using them to serve the cisco.com domain.

IEEE P1735 漏洞,又是 Padding Oracle Attack...

在「IEEE P1735 Encryption Is Broken—Flaws Allow Intellectual Property Theft」這邊看到 US-CERT 發表的「IEEE P1735 implementations may have weak cryptographic protections」,裡面提到的主要漏洞:

The methods are flawed and, in the most egregious cases, enable attack vectors that allow recovery of the entire underlying plaintext IP.

主要應該是第一包:

CVE-2017-13091: improperly specified padding in CBC mode allows use of an EDA tool as a decryption oracle.

又是 CBCpadding oracle attack 啊... 看起來是標準沒有強制定義好造成的?

The main vulnerability (CVE-2017-13091) resides in the IEEE P1735 standard's use of AES-CBC mode.

Since the standard makes no recommendation for any specific padding scheme, the developers often choose the wrong scheme, making it possible for attackers to use a well-known classic padding-oracle attack (POA) technique to decrypt the system-on-chip blueprints without knowledge of the key.

去年 Cloudflare 寫的「Padding oracles and the decline of CBC-mode cipher suites」這邊有提到 padding oracle attack 的方式,比較一般性的解法是避開要自己決定 Encrypt-then-MAC (IPsec;也是數學上證明安全性) 或 Encrypt-and-MAC (SSH) 或是 MAC-then-Encrypt (SSL),而是用 AEAD 類的加密元件直接躲開 padding oracle attack 的某些必要條件 (像是 AES-GCM 或是 ChaCha20-Poly1305)。

不過這也是這幾年大家才了解這樣做的重要性,當年在訂規格的時候都比較沒在在意這些...

SSL/TLS 的問題...

這篇與「對稱式加密系統的爆炸歷史 (Authenticated encryption 的問題)」這篇相關,建議可以一起看一看。

TLS (Transport Layer Security),前身是 SSL (Secure Sockets Layer),是目前 HTTPS 所使用的加密協議。發展的順序上是 SSLv2、SSLv3、TLSv1、TLSv1.1、TLSv1.2。

然後有兩篇文章可以看:

第一篇文章講 Padding oracle attack,第二篇文章是酸 SSL/TLS 的修正愈修愈歪... XD

AES 這類的 block cipher 在加密或解密時會要求切齊 block size,以 AES 的要求就是 128bits (16 bytes)。

而對於不齊的資料要怎麼加密呢?其中一個方法是 PKCS#7:(圖片取自第二篇文章)

Padding

要想辦法補齊 128bits (16bytes),如果像上圖需要補 7bytes 進去,就都補上 \x07 (剛好就是補上長度),另外在最後面會補上 padding 的長度,而問題出就出在這個設計先天就有缺陷:在 SSL/TLS 所使用的 MAC-then-Encrypt 中,MAC 只計算原文的值,沒有保護到 padding 的部份,於是就可以針對 padding 的部份想辦法找到洞鑽。

pseudo code 可能是這樣:

// Decrypt to plaintext + mac + padding
$plaintext_mac_padding = decrypt($ciphertext);
if (NULL != $plaintext_mac_padding) {

    // Now decode padding part
    $plaintext_mac = decode_padding($plaintext_mac_padding, $padding_length);
    if (NULL != $plaintext_mac) {

        // Now check MAC part
        $plaintext = check_mac(plaintext_mac);
        if (NULL != $plaintext) {

            // Now it's okay
        }
    }
}

攻擊者亂改 $ciphertext 會導致解出來的 padding 也亂掉,但早期的 SSL 會回傳「padding error」這種對攻擊者有利的資訊,而導致攻擊者可以利用這個資訊想辦法得知更多內容。

而 TLS 並沒有從根本改善,而是試著加上機制補西牆:當遇到錯誤時就跳過,不要傳回錯誤資訊。

但因為攻擊者亂改封包造成 decode_padding() 會失敗,而沒有呼叫到 check_mac()。這導致了大量的計算時間差與能量差,而使得攻擊者可以藉由這些資訊而得知是否成功。而官方在 TLSv1.2 的建議是再補上機制來補洞:

In general, the best way to do this is to compute the MAC even if the padding is incorrect, and only then reject the packet. For instance, if the pad appears to be incorrect, the implementation might assume a zero-length pad and then compute the MAC.

而官方認為雖然這樣還是有 timing channel,但已經小到會被雜訊覆蓋,所以「應該」可以解決問題:

This leaves a small timing channel, since MAC performance depends to some extent on the size of the data fragment, but it is not believed to be large enough to be exploitable, due to the large block size of existing MACs and the small size of the timing signal.

於是,只要覺得「應該安全吧」,就會「應該會被破」:「Lucky Thirteen: Breaking the TLS and DTLS Record Protocols」:

The attacks apply to all TLS and DTLS implementations that are compliant with TLS 1.1 or 1.2, or with DTLS 1.0 or 1.2. They also apply to implementations of SSL 3.0 and TLS 1.0 that incorporate countermeasures to previous padding oracle attacks. Variant attacks may also apply to non-compliant implementations.

這 SSL/TLS 的設計讓人補到快起笑了... XD

資安的東西通常是愈複雜就愈容易被抓問題出來,在 SSL/TLS 的歷史包袱下,不知道什麼時候才想換 Encrypt-then-MAC 來改善底層問題...

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