Build a tiny CA for your homelab with a Raspberry Pi
16 comments
·January 19, 2025ostensible
myself248
How does the disconnected audio input of any random PC or thinclient compare?
I continue to find it a bit silly to see "with a raspberry pi" when people just mean "with any random linux box that doesn't need to be very powerful".
It's like listening to NPR, where every smartphone is an iPhone even if it's an Android, you know?
tashian
I love this idea!
rmoriz
I'm running smallstep CA in my homelab. While it's nicely done and clearly focuses to the containerized enterprise market, its defaults are very harsh.
Take for example the maximum certificate duration. While from a production/security perspective short-lived certificates are great, you don't want to renew certs in your homelab every 24-48hrs. Also, many things just don't support ACME but still benefit from a valid certificate, e.g. router/firewall/appliance web interfaces. Out of the box, the limit for traditionally issued certificates using the CLI is very low, too.
The default prevents expired certificates to be renewed. If your homelab does not offer a couple of nines behind the comma, you'll pretty much have to intervene on a regular basis UNLESS you adjust the defaults. You can't set the max duration to years, months or days but only hours:
"claims": {
"minTLSCertDuration": "24h",
"maxTLSCertDuration": "26400h",
"defaultTLSCertDuration": "9000h"
},
CAVE: macOS enforces duration limits even for trusted enterprise CAs, e.g. Safari won't accept your 1000 days certificate anymore.
tashian
It's true, the defaults are quite strict.
As for the "hours" max interval, this is the result of a design decision in Go's time duration library, dealing with the quirks of our calendaring system.
bpye
Is there any way to replace the dedicated RNG with the YubiKey RNG? The OpenPGP applet allows you to query its internal RNG [0].
[0] - https://developers.yubico.com/PGP/YubiKey_5.2.3_Enhancements...
nimbius
For those interested in an hsm pi, theres picohsm https://github.com/polhenarejos/pico-hsm
zikduruqe
That isn't so tiny.
This is tiny. https://github.com/FiloSottile/mkcert
pkaye
Does that provide an ACME server?
pandemic_region
Not appreciating the hijacking of the back button until the cookie form is dismissed.
likeabatterycar
This is littered with so many missteps I don't know where to start.
-Complete overkill requiring the use of a YubiKey for key storage and external RNG source - what problems does this solve? For a Yubikey to act as a poor man's HSM you have to store the PIN in plaintext on the disk. So if the device is compromised, they can just issue their own certs. If it's to protect against physical theft of the keys, they'll just put the entire Raspberry Pi in their pocket. You could choose to enter the PIN manually but this precludes any automation including CRL generation. It's also a waste of a good YubiKey.
-Creates a two-tier PKI... on the same device. This completely defeats the purpose so you can't revoke anything in case of key compromise. You could make it a 100-tier PKI and it would make no difference if they're on the same device. Though they would need a whole lot of YubiKeys and USB hubs for that.
-They're generating the private key on disk then importing into the YubiKey. Which defeats having an external key storage device because you have left traces of the key on disk.
-All this digital duct taping the windows and doors yet the article instructs you to download and run random binaries off GitHub with no verification whatsoever.
-Why do you need ACME in a homelab and can't just hand issue long lived certificates?
-OpenSC and the crypto libraries are notoriously difficult to set up and working properly. A tiny CA this is not.
An instance of openssl or xca covers 99.9% of "homelab" use cases. This is like using a battery operated drill to open a can of soup.
cwalv
> Complete overkill requiring the use of a YubiKey for key storage and external RNG source - what problems does solve? For a Yubikey to act as a poor man's HSM you have to store the PIN in plaintext on the disk
You still can't exfiltrate the key material.
> If it's to protect against physical theft of the keys, they'll just put the entire Raspberry Pi in their pocket.
Just because someone has compromised your device doesn't mean they have physical access. That's the point.
> They're generating the private key on disk then importing into the YubiKey. Which defeats having an external key storage device because you have left traces of the key on disk.
The traces don't have to be left behind. Is this excessive 'overkill', or is the 'digital duct taping the windows and doors' insufficient?
> An instance of openssl or xca covers 99.9% of "homelab" use cases
The interesting thing about this article is that it adds a few 9's that are covered, and it's both easy and cheap.
likeabatterycar
> You still can't exfiltrate the key material
And? What actual problem does this solve or realistic threat does this prevent? They are not decryption keys they are used to digitally sign certificates.
What the DigiNotar hack taught us years ago is if your CA is compromised you are already 0wned doesn't matter if the key is stored in an HSM or not.
All they can do with a stolen key is issue more certificates. Which they can do anyway if they have root access to the CA.
You can put 12 locks on your door but if they're all keyed to the same key you've stored under the plant on the porch, it doesn't really matter.
> The interesting thing about this article is that it adds a few 9's that are covered, and it's both easy and cheap.
Hard to say if those extra 9's need an external RNG for extra entropy.
cwalv
> Which they can do anyway if they have root access to the CA.
Until you turn it off. If they exfiltrate the keys, it's more complicated.
This goes back to your comment:
> Creates a two-tier PKI... on the same device. This completely defeats the purpose so you can't revoke anything in case of key compromise
But the root key is just created; it doesn't stay on the device and can't be used to sign anything.
> What actual problem does this solve or realistic threat does this prevent?
The problem is exfiltrating the key without physical access. Whether or not that's "realistic" enough to matter isn't a question that can be answered generally.
> Hard to say if those extra 9's need an external RNG for extra entropy.
IMO it's not. In the author's words: Optional, but fire
tashian
Hi, I'm the author of the post. Thanks for your questions here.
> -Complete overkill requiring the use of a YubiKey for key storage and external RNG source - what problems does this solve? For a Yubikey to act as a poor man's HSM you have to store the PIN in plaintext on the disk. So if the device is compromised, they can just issue their own certs. If it's to protect against physical theft of the keys, they'll just put the entire Raspberry Pi in their pocket.
Yep, it's overkill. Homelabs are learning environments. People want tutorials when trying new things. It's a poor man's HSM because not many people will buy an HSM for their homelab, but almost everyone already has a YubiKey they can play with.
The project solves the problem of people wanting to learn and play with new technology.
And it's a way to kickstart a decently solid local PKI, if that's something you're interested in.
The RNG is completely unnecessary flair that just adds to the fun.
> -Creates a two-tier PKI... on the same device. This completely defeats the purpose so you can't revoke anything in case of key compromise. > -They're generating the private key on disk then importing into the YubiKey. Which defeats having an external key storage device because you have left traces of the key on disk.
The tutorial shows how to generate and store the private key offline on a USB stick, not on the device or the YubiKey. The key material never touches the disk of the Raspberry Pi.
Why store a copy of the CA keys offline? Because YubiKeys don't have the key-wrapped backup and restore feature of HSMs. So, if the YubiKey ever fails, you need a way to restore your CA. Storing the root on a USB stick is the backup. Put the USB stick in a safe.
If you want active revocation, you can set it up so that the intermediate is revocable—in case physical theft of the key is important to you. (We have instructions to do that in our docs.)
> -All this digital duct taping the windows and doors yet the article instructs you to download and run random binaries off GitHub with no verification whatsoever.
It's open source software downloaded from GitHub. The only non-smallstep code is the RNG driver (GitHub is the distribution point for that project). Was there a kind of verification that you expected to see?
> -Why do you need ACME in a homelab and can't just hand issue long lived certificates? -OpenSC and the crypto libraries are notoriously difficult to set up and working properly. A tiny CA this is not.
Most people don't need ACME in their homelab, they just want to learn stuff. That said, we have homelabbers in our community issuing certs to dozens of endpoints in their homelab.
Whether you issue long-lived or short-lived certs is a philosophical issue. If a short-lived cert is compromised, it's simply less valuable to the attacker. Short-lived certs encourage automation. Long-lived certs can be easier to manage and you can just manually renew them. But unplanned expiry of long-lived certs has caused a lot of multi-million dollar outages.
I hope this helps clarify things.
mschuster91
> Why do you need ACME in a homelab and can't just hand issue long lived certificates?
If there is one thing I hate it is hand issuing certificates. Even for a homelab.
SSL just plain sucks and OpenSSLs incantation and especially config files make an already bad problem even worse.
This being raspberry pi absolves you from needing to buy a separate hardware noise generator: it has plenty of GPIO. For example, one can obtain entropy by sampling random noise generated by reverse-biasing a junction in a cheap pn transistor. Here is an example: http://holdenc.altervista.org/avalanche/. Bonus — maybe it will get you hooked on electrical engineering!
Btw, some versions of raspberry pi already have hardware random number number generator accessible at /dev/hwrng.