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The bedrock principle of public key cryptography is that it is impossible to re-create a private key while only having the public key. This is not "mathematically hard" ; it is currently considered mathematically IMPOSSIBLE. And until such a time as a quantum computer can do it, it remains impossible.
One issue here - It's possible, but computationally expensive. Exponentially more so as key size increases. RSA-768 was successfully factored / private key derived from public key in 2009. The highest successful one before RSA shut down the RSA factoring challenge. It's a matter of time/computer resources, not outright impossible. That was almost 16 years ago. https://eprint.iacr.org/2010/006 & https://arstechnica.com/information-technology/2010/01/768-bit-rsa-cracked-1... Whereas time estimates scale up exponentially as key length increases, with classical computers it is a "solved" problem for this algorithm, but a computationally expensive one. 1024 should be feasible these days in a "reasonable" timeframe - the 2009 RSA-768 took approximately 2 years months of real-time processing across a sizable cluster (80 processors). We can obviously scale much further now. 4096 is still in the realm of geological or universe-scale timeframes for classical computing, however. =========== On Thu, Sep 4, 2025 at 12:16 PM Dan Mahoney <danm@prime.gushi.org> wrote:
On Sep 4, 2025, at 05:21, Tom Beecher <beecher@beecher.cc> wrote:
Dan-
The main concern I have with your post, and the reason I have been so vocal in these messages , centers around the following :
Or you might consider just going back to using inline passwords and consider Cisco’s ssh implementation a failure at launch — at least the “secret” hashing algorithms are salted, but on older kit, it’s also still md5.
It's absolutely fair to criticize their implementation in its current form. I could see it making sense 20 years ago, but they've had time to iterate and improve on it, and should have.
However, Cisco's implementation is not vulnerable to any currently known exploits, and no theoretical attack vectors don't seem to apply either.
The fact that you make a recommendation for readers to *stop using public key SSH auth* because of that is , respectfully, absolutely irresponsible. Someone, somewhere is going to read this, and follow this advice, making their device LESS secure, and for no good reason. We don't tell people that current cryptography might eventually someday be vulnerable to quantum computers , so stop using cryptography completely. You are doing that here, by saying "This might be exploitable some day, so don't use it." Everything MIGHT be exploitable some day, that's how it goes.
Tom,
You see those things on either sides of the words “stop using public key SSH auth” ? Those are called quotation marks, and they mean, in this context, that you are directly citing my words, to the larger group.
Except that those words, in that order, appear nowhere in my article, which hasn’t changed at all, except for one typo which I’ve since corrected.
I make no such recommendation. My usage of the word “you might” is not a recommendation, it’s a statement that people may do their own research and carefully consider how they put an older device online, if at all. Where you’ve cited me bashing md5, I am referring to its crypt() implementation, also used in Cisco type 5 secrets, matching my recommendations with that of the NSA. If anything, I’ll happily suggest that the best answer for an EOL or near-EOL devices is “just use a serial cable”.
But back to your quote.
I believe that you’re seeing words that literally aren’t on the page, and are citing them to a public mailing list, claiming they’re mine.
This is not ok.
-Dan
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