PuTTY 使用 ecdsa-sha2-nistp521 的漏洞

看到「PuTTY vulnerability vuln-p521-bias (greenend.org.uk)」這個消息,官網的說明在「PuTTY vulnerability vuln-p521-bias」這邊。

DSA 類的簽名演算法有個得很小心的地方,是 nonce 選擇不當會造成 key recovery,這在原文有提到:

All DSA signature schemes require a random value to be invented during signing, known as the 'nonce' (cryptography jargon for a value used only once), or sometimes by the letter k. It's well known that if an attacker can guess the value of k you used, or find any two signatures you generated with the same k, then they can immediately recover your private key.


With DSA, the entropy, secrecy, and uniqueness of the random signature value {\displaystyle k} are critical. It is so critical that violating any one of those three requirements can reveal the entire private key to an attacker. Using the same value twice (even while keeping {\displaystyle k} secret), using a predictable value, or leaking even a few bits of {\displaystyle k} in each of several signatures, is enough to reveal the private key {\displaystyle x}.

這次爆炸的起因是 PuTTY 用了 SHA-512 產生 nonce,這邊只會有 512 bits 的輸出,而這對 P-521 需要 521 bits 是不夠的 (於是前 9 個 bit 會是 0):

PuTTY's technique worked by making a SHA-512 hash, and then reducing it mod q, where q is the order of the group used in the DSA system. For integer DSA (for which PuTTY's technique was originally developed), q is about 160 bits; for elliptic-curve DSA (which came later) it has about the same number of bits as the curve modulus, so 256 or 384 or 521 bits for the NIST curves.

In all of those cases except P521, the bias introduced by reducing a 512-bit number mod q is negligible. But in the case of P521, where q has 521 bits (i.e. more than 512), reducing a 512-bit number mod q has no effect at all – you get a value of k whose top 9 bits are always zero.

而更糟的是,這不僅僅是將降了 29 的安全性,而是因為 nonce 有 bias,這在 DSA 上已經足以從 60 次的簽出的 signature 中還原出 private key (也就是文章裡提到的 key recovery attack):

This bias is sufficient to allow a key recovery attack. It's less immediate than if an attacker knows all of k, but it turns out that if k has a biased distribution in this way, it's possible to aggregate information from multiple signatures and recover the private key eventually. Apparently the number of signatures required is around 60.

新版會改用 RFC 6979 (Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)) 實作:

To fix this vulnerability, we've completely abandoned PuTTY's old system for generating k, and switched to the RFC 6979 technique, for all DSA and ECDSA key types. (EdDSA keys such as Ed25519 already used a different system, which has not changed.) However, this doesn't affect the fact that information about existing P521 private keys has already been leaked whenever a signature was generated using the old k generator.

所以這次的 fix 得更新 PuTTY 版本,然後重新產生 private key (會假設已經 leak 了),然後看看系統有什麼要處理的...

用 GPT-3 產生 Hacker News 上熱門文章的摘要

看到「Autosummarized HN」這個工具,算是一個組合技的應用:

All summaries have been generated automatically by GPT-3. No responsibility is claimed for their contents nor its accuracy.

透過 GPT-3 解讀並產生出摘要,目前頁面上是沒有 RSS feed,但可以透過一些工具直接拉出來 (像是 PolitePol),然後就可以掛到 Slack 或是 RSS reader 裡面...

Python 新的 HTTP client library:HTTPX

看到「A next-generation HTTP client for Python.」這個專案冒出來,宣稱是下一代 Python 的 HTTP client library:

HTTPX is a fully featured HTTP client for Python 3, which provides sync and async APIs, and support for both HTTP/1.1 and HTTP/2.

目前專案還在 beta,目標是在今年四月出 1.0 版。從說明裡面可以看到,目前的方向是打算相容 requests,讓現有的程式可以不用做太多修改就轉移過來。

再來從開發者的角度來看,HTTPX 比 requests 多了這些功能:

  • Standard synchronous interface, but with async support if you need it.
  • HTTP/1.1 and HTTP/2 support.
  • Ability to make requests directly to WSGI applications or ASGI applications.
  • Strict timeouts everywhere.

但如果就支援這些功能的角度來看,修改 requests 看起來應該會比較快?開新的專案感覺跟 Kenneth Reitz 有關...

Kenneth Reitz 有兩個很有名的作品,一個是這個,另外一個是 Pipenv,也就是在「pipenv 的凋零與替代方案 poetry」這邊提到的事情。

然後看了一下 CHANGELOG.md 內的資訊,裡面最早的記錄是 0.6.0 (2019/06/21),而從 Contributors to encode/httpx 這頁看起來則是 2019/03/31 開始發展的,就這些時間點看起來,原因大概跟 Kenneth Reitz 有關... 雖然沒有找到文章直接提到這件事情。

不過 HTTPX 需要 Python 3.6+ 才能跑,對於版本的要求比較高,如果是 16.04 預設的 python3 (3.5) 就沒辦法跑了,18.04 預設的 python3 (3.6) 也才剛好符合...

之後應該是 HTTPX 與 requests 兩邊都得關注了,看看會有什麼發展...

Tor 釋出,支援下一代的 Hidden Service

Tor 放出了新的 stable 版本:「Tor is released: We have a new stable series!」。

這個版本支援新一代的 hidden service,也就是之前在「下一代的 Tor Hidden Service」這邊提到的東西,將原來 hostname 的部份從 16 chars 變成 56 chars,也就是像 7fa6xlti5joarlmkuhjaifa47ukgcwz6tfndgax45ocyn4rixm632jid.onion 這樣的網址。

這對 hidden service 很重要,因為這代表了淘汰掉舊的演算法,尤其裡面有兩個 (SHA1 與 RSA1024) 都已經確定有問題了:

a) Better crypto (replaced SHA1/DH/RSA1024 with SHA3/ed25519/curve25519)

ROCA:Infineon Technologies 的 RSA 實做問題

最近的另外一個大包,不過這包是 Infineon Technologies 在實做 RSA 算法時的問題,倒不是 RSA 算法本身有問題。之所以會「大」是因為有太多人用了:「ROCA: Vulnerable RSA generation (CVE-2017-15361)」。

起因於 Infineon Technologies 在產生 key 時的組合有限,於是要猜測的 keyspace 小很多。

以研究者的估算,可以看出 CPU year 都被大幅減少了,都是屬於「可行」的範圍:

The time complexity and cost for the selected key lengths (Intel E5-2650 v3@3GHz Q2/2014):

512 bit RSA keys - 2 CPU hours (the cost of $0.06);
1024 bit RSA keys – 97 CPU days (the cost of $40-$80);
2048 bit RSA keys – 140.8 CPU years, (the cost of $20,000 - $40,000).

而且這邊是用 CPU year 估算,如果考慮 FPGA 加速計算,應該會短更多...


2nd of November 2017 - Presentation of all details at the ACM CCS conference (to come)
16th of October 2017 - The initial version of the public disclosure published
May to October 2017 - Cooperation with the manufacturer and other affected parties to help evaluate and mitigate the vulnerability
1st of February - The vulnerability disclosed to Infineon Technologies AG
End of January - The vulnerability found

過一陣子就會去 conference 上報告了...

用 Bitmap 或是用 Tilemap 組成大圖


This program generates bitmaps that are locally similar to the input bitmap.

我覺得這張很有感覺,給幾個 bitmap,然後電路板就出現了:


還有 3D 的: