Are you aware that Git commits and tags may be signed using OpenSSH? Git signatures may be used to improve integrity and authentication of our software supply-chain. Popular signature algorithms include Ed25519, ECDSA and RSA. Did you consider that these algorithms may not be safe if someone builds a post-quantum computer?
As you may recall, I have earlier blogged about the efficient post-quantum key agreement mechanism called Streamlined NTRU Prime and its use in SSH and I have attempted to promote the conservatively designed Classic McEliece in a similar way, although it remains to be adopted.
What post-quantum signature algorithms are available? There is an effort by NIST to standardize post-quantum algorithms, and they have a category for signature algorithms. According to wikipedia, after round three the selected algorithms are CRYSTALS-Dilithium, FALCON and SPHINCS+. Of these, SPHINCS+ appears to be a conservative choice suitable for long-term digital signatures. Can we get this to work?
Recall that Git uses the ssh-keygen tool from OpenSSH to perform signing and verification. To refresh your memory, let’s study the commands that Git uses under the hood for Ed25519. First generate a Ed25519 private key:
jas@kaka:~$ ssh-keygen -t ed25519 -f my_ed25519_key -P ""
Generating public/private ed25519 key pair.
Your identification has been saved in my_ed25519_key
Your public key has been saved in my_ed25519_key.pub
The key fingerprint is:
SHA256:fDa5+jmC2+/aiLhWeWA3IV8Wj6yMNTSuRzqUZlIGlXQ jas@kaka
The key's randomart image is:
+--[ED25519 256]--+
| .+=.E .. |
| oo=.ooo |
| . =o=+o . |
| =oO+o . |
| .=+S.= |
| oo.o o |
| . o . |
| ...o.+.. |
| .o.o.=**. |
+----[SHA256]-----+
jas@kaka:~$ cat my_ed25519_key
-----BEGIN OPENSSH PRIVATE KEY-----
b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
QyNTUxOQAAACAWP/aZ8hzN0WNRMSpjzbgW1tJXNd2v6/dnbKaQt7iIBQAAAJCeDotOng6L
TgAAAAtzc2gtZWQyNTUxOQAAACAWP/aZ8hzN0WNRMSpjzbgW1tJXNd2v6/dnbKaQt7iIBQ
AAAEBFRvzgcD3YItl9AMmVK4xDKj8NTg4h2Sluj0/x7aSPlhY/9pnyHM3RY1ExKmPNuBbW
0lc13a/r92dsppC3uIgFAAAACGphc0BrYWthAQIDBAU=
-----END OPENSSH PRIVATE KEY-----
jas@kaka:~$ cat my_ed25519_key.pub
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBY/9pnyHM3RY1ExKmPNuBbW0lc13a/r92dsppC3uIgF jas@kaka
jas@kaka:~$ Then let’s sign something with this key:
jas@kaka:~$ echo "Hello world!" > msg
jas@kaka:~$ ssh-keygen -Y sign -f my_ed25519_key -n my-namespace msg
Signing file msg
Write signature to msg.sig
jas@kaka:~$ cat msg.sig
-----BEGIN SSH SIGNATURE-----
U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgFj/2mfIczdFjUTEqY824FtbSVz
Xdr+v3Z2ymkLe4iAUAAAAMbXktbmFtZXNwYWNlAAAAAAAAAAZzaGE1MTIAAABTAAAAC3Nz
aC1lZDI1NTE5AAAAQLmWsq05tqOOZIJqjxy5ZP/YRFoaX30lfIllmfyoeM5lpVnxJ3ZxU8
SF0KodDr8Rtukg2N3Xo80NGvZOzbG/9Aw=
-----END SSH SIGNATURE-----
jas@kaka:~$Now let’s create a list of trusted public-keys and associated identities:
jas@kaka:~$ echo 'my.name@example.org ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBY/9pnyHM3RY1ExKmPNuBbW0lc13a/r92dsppC3uIgF' > allowed-signers
jas@kaka:~$ Then let’s verify the message we just signed:
jas@kaka:~$ cat msg | ssh-keygen -Y verify -f allowed-signers -I my.name@example.org -n my-namespace -s msg.sig
Good "my-namespace" signature for my.name@example.org with ED25519 key SHA256:fDa5+jmC2+/aiLhWeWA3IV8Wj6yMNTSuRzqUZlIGlXQ
jas@kaka:~$
I have implemented support for SPHINCS+ in OpenSSH. This is early work, but I wanted to announce it to get discussion of some of the details going and to make people aware of it.
What would a better way to demonstrate SPHINCS+ support in OpenSSH than by validating the Git commit that implements it using itself?
Here is how to proceed, first get a suitable development environment up and running. I’m using a Debian container launched in a protected environment using podman.
jas@kaka:~$ podman run -it --rm debian:stableThen install the necessary build dependencies for OpenSSH.
# apt-get update
# apt-get install git build-essential autoconf libz-dev libssl-devNow clone my OpenSSH branch with the SPHINCS+ implentation and build it. You may browse the commit on GitHub first if you are curious.
# cd
# git clone https://github.com/jas4711/openssh-portable.git -b sphincsp
# cd openssh-portable
# autoreconf -fvi
# ./configure
# makeConfigure a Git allowed signers list with my SPHINCS+ public key (make sure to keep the public key on one line with the whitespace being one ASCII SPC character):
# mkdir -pv ~/.ssh
# echo 'simon@josefsson.org ssh-sphincsplus@openssh.com AAAAG3NzaC1zcGhpbmNzcGx1c0BvcGVuc3NoLmNvbQAAAECI6eacTxjB36xcPtP0ZyxJNIGCN350GluLD5h0KjKDsZLNmNaPSFH2ynWyKZKOF5eRPIMMKSCIV75y+KP9d6w3' > ~/.ssh/allowed_signers
# git config gpg.ssh.allowedSignersFile ~/.ssh/allowed_signersThen verify the commit using the newly built ssh-keygen binary:
# PATH=$PWD:$PATH
# git log -1 --show-signature
commit ce0b590071e2dc845373734655192241a4ace94b (HEAD -> sphincsp, origin/sphincsp)
Good "git" signature for simon@josefsson.org with SPHINCSPLUS key SHA256:rkAa0fX0lQf/7V7QmuJHSI44L/PAPPsdWpis4nML7EQ
Author: Simon Josefsson <simon@josefsson.org>
Date: Tue Dec 3 18:44:25 2024 +0100
Add SPHINCS+.
# git verify-commit ce0b590071e2dc845373734655192241a4ace94b
Good "git" signature for simon@josefsson.org with SPHINCSPLUS key SHA256:rkAa0fX0lQf/7V7QmuJHSI44L/PAPPsdWpis4nML7EQ
# Yay!
So what are some considerations?
SPHINCS+ comes in many different variants. First it comes with three security levels approximately matching 128/192/256 bit symmetric key strengths. Second choice is between the SHA2-256, SHAKE256 (SHA-3) and Haraka hash algorithms. Final choice is between a “robust” and a “simple” variant with different security and performance characteristics. To get going, I picked the “sphincss256sha256robust” SPHINCS+ implementation from SUPERCOP 20241022. There is a good size comparison table in the sphincsplus implementation, if you want to consider alternative variants.
SPHINCS+ public-keys are really small, as you can see in the allowed signers file. This is really good because they are handled by humans and often by cut’n’paste.
What about private keys? They are slightly longer than Ed25519 private keys but shorter than typical RSA private keys.
# ssh-keygen -t sphincsplus -f my_sphincsplus_key -P ""
Generating public/private sphincsplus key pair.
Your identification has been saved in my_sphincsplus_key
Your public key has been saved in my_sphincsplus_key.pub
The key fingerprint is:
SHA256:4rNfXdmLo/ySQiWYzsBhZIvgLu9sQQz7upG8clKziBg root@ad600ff56253
The key's randomart image is:
+[SPHINCSPLUS 256-+
| . .o |
|o . oo. |
| = .o.. o |
|o o o o . . o |
|.+ = S o o .|
|Eo= . + . . .. .|
|=*.+ o . . oo . |
|B+= o o.o. . |
|o*o ... .oo. |
+----[SHA256]-----+
# cat my_sphincsplus_key.pub
ssh-sphincsplus@openssh.com AAAAG3NzaC1zcGhpbmNzcGx1c0BvcGVuc3NoLmNvbQAAAEAltAX1VhZ8pdW9FgC+NdM6QfLxVXVaf1v2yW4v+tk2Oj5lxmVgZftfT37GOMOlK9iBm9SQHZZVYZddkEJ9F1D7 root@ad600ff56253
# cat my_sphincsplus_key
-----BEGIN OPENSSH PRIVATE KEY-----
b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAYwAAABtzc2gtc3
BoaW5jc3BsdXNAb3BlbnNzaC5jb20AAABAJbQF9VYWfKXVvRYAvjXTOkHy8VV1Wn9b9slu
L/rZNjo+ZcZlYGX7X09+xjjDpSvYgZvUkB2WVWGXXZBCfRdQ+wAAAQidiIwanYiMGgAAAB
tzc2gtc3BoaW5jc3BsdXNAb3BlbnNzaC5jb20AAABAJbQF9VYWfKXVvRYAvjXTOkHy8VV1
Wn9b9sluL/rZNjo+ZcZlYGX7X09+xjjDpSvYgZvUkB2WVWGXXZBCfRdQ+wAAAIAbwBxEhA
NYzITN6VeCMqUyvw/59JM+WOLXBlRbu3R8qS7ljc4qFVWUtmhy8B3t9e4jrhdO6w0n5I4l
mnLnBi2hJbQF9VYWfKXVvRYAvjXTOkHy8VV1Wn9b9sluL/rZNjo+ZcZlYGX7X09+xjjDpS
vYgZvUkB2WVWGXXZBCfRdQ+wAAABFyb290QGFkNjAwZmY1NjI1MwECAwQ=
-----END OPENSSH PRIVATE KEY-----
# Signature size? Now here is the challenge, for this variant the size is around 29kb or close to 600 lines of base64 data:
# git cat-file -p ce0b590071e2dc845373734655192241a4ace94b | head -10
tree ede42093e7d5acd37fde02065a4a19ac1f418703
parent 826483d51a9fee60703298bbf839d9ce37943474
author Simon Josefsson <simon@josefsson.org> 1733247865 +0100
committer Simon Josefsson <simon@josefsson.org> 1734907869 +0100
gpgsig -----BEGIN SSH SIGNATURE-----
U1NIU0lHAAAAAQAAAGMAAAAbc3NoLXNwaGluY3NwbHVzQG9wZW5zc2guY29tAAAAQIjp5p
xPGMHfrFw+0/RnLEk0gYI3fnQaW4sPmHQqMoOxks2Y1o9IUfbKdbIpko4Xl5E8gwwpIIhX
vnL4o/13rDcAAAADZ2l0AAAAAAAAAAZzaGE1MTIAAHSDAAAAG3NzaC1zcGhpbmNzcGx1c0
BvcGVuc3NoLmNvbQAAdGDHlobgfgkKKQBo3UHmnEnNXczCMNdzJmeYJau67QM6xZcAU+d+
2mvhbksm5D34m75DWEngzBb3usJTqWJeeDdplHHRe3BKVCQ05LHqRYzcSdN6eoeZqoOBvR
# git cat-file -p ce0b590071e2dc845373734655192241a4ace94b | tail -5
ChvXUk4jfiNp85RDZ1kljVecfdB2/6CHFRtxrKHJRDiIavYjucgHF1bjz0fqaOSGa90UYL
RZjZ0OhdHOQjNP5QErlIOcZeqcnwi0+RtCJ1D1wH2psuXIQEyr1mCA==
-----END SSH SIGNATURE-----
Add SPHINCS+.
# git cat-file -p ce0b590071e2dc845373734655192241a4ace94b | wc -l
579
# What about performance? Verification is really fast:
# time git verify-commit ce0b590071e2dc845373734655192241a4ace94b
Good "git" signature for simon@josefsson.org with SPHINCSPLUS key SHA256:rkAa0fX0lQf/7V7QmuJHSI44L/PAPPsdWpis4nML7EQ
real 0m0.010s
user 0m0.005s
sys 0m0.005s
# On this machine, verifying an Ed25519 signature is a couple of times slower, and needs around 0.07 seconds.
Signing is slower, it takes a bit over 2 seconds on my laptop.
# echo "Hello world!" > msg
# time ssh-keygen -Y sign -f my_sphincsplus_key -n my-namespace msg
Signing file msg
Write signature to msg.sig
real 0m2.226s
user 0m2.226s
sys 0m0.000s
# echo 'my.name@example.org ssh-sphincsplus@openssh.com AAAAG3NzaC1zcGhpbmNzcGx1c0BvcGVuc3NoLmNvbQAAAEAltAX1VhZ8pdW9FgC+NdM6QfLxVXVaf1v2yW4v+tk2Oj5lxmVgZftfT37GOMOlK9iBm9SQHZZVYZddkEJ9F1D7' > allowed-signers
# cat msg | ssh-keygen -Y verify -f allowed-signers -I my.name@example.org -n my-namespace -s msg.sig
Good "my-namespace" signature for my.name@example.org with SPHINCSPLUS key SHA256:4rNfXdmLo/ySQiWYzsBhZIvgLu9sQQz7upG8clKziBg
# Welcome to our new world of Post-Quantum safe digital signatures of Git commits, and Happy Hacking!