Digital signatures with Ed25519: sign, verify, and publish a public key

What a signature proves

A valid signature shows that the holder of a specific private key signed exactly these bytes. It provides integrity and origin, and, if the private key is well protected, non-repudiation. It says nothing about whether the content is good, and it is only meaningful if you trust that the public key belongs to the claimed signer.

Ed25519 is a modern signature scheme with small keys and signatures (32 and 64 bytes), deterministic signing that avoids random-number failures, and fast verification.

In Python

Serialize the public key to share it, keep the private key in a protected file or KMS, and verify by catching InvalidSignature.

from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
from cryptography.hazmat.primitives import serialization
from cryptography.exceptions import InvalidSignature

priv = Ed25519PrivateKey.generate()
pub_pem = priv.public_key().public_bytes(serialization.Encoding.PEM, serialization.PublicFormat.SubjectPublicKeyInfo)
message = open('release.tar.gz', 'rb').read()
sig = priv.sign(message)

pub = serialization.load_pem_public_key(pub_pem)
try:
    pub.verify(sig, message)
    print('valid')
    pub.verify(sig, message + b'x')
except InvalidSignature:
    print('invalid after change')

Lab: sign files with SSH keys

OpenSSH can sign arbitrary files with ssh-keygen -Y. Create an allowed_signers file mapping an identity to your public key, sign a file, verify it, then modify the file and verify again. Git can use the same mechanism to sign commits with gpg.format ssh.

ssh-keygen -t ed25519 -f lab_sign -N '' -C 'lab signer'
echo "lab@example.org $(cat lab_sign.pub)" > allowed_signers
ssh-keygen -Y sign -f lab_sign -n file release.tar.gz          # writes release.tar.gz.sig
ssh-keygen -Y verify -f allowed_signers -I lab@example.org -n file -s release.tar.gz.sig < release.tar.gz

git config gpg.format ssh && git config user.signingkey ~/lab_sign.pub && git commit -S -m 'signed lab commit'

Distributing the public key

Publish the public key or its fingerprint in several independent places: your website, repository README, and a profile on a code hosting site. Users who compare at least two sources make it much harder for an attacker to substitute a key.