PPK v3 Adoption Decision: Keep v2 Compatibility or Upgrade for Stronger Protection

Direct recommendation

Keep generating and distributing PPK v2 keys until every SSH client, automation script, and CI node in your environment is confirmed to read v3 reliably. If all passphrases are already strong (≥20 random characters), the security gain from Argon2id is marginal; the interoperability risk outweighs it. Upgrade only after a version‑matrix audit and a dry‑run validation pass.

Practical risks of silent failures

• Automation that ignores exit codes – scripts calling puttygen, plink, or pageant may treat a non‑zero exit as success, leaving the key unused.
• Configuration‑management tools (Ansible, Chef, Puppet) that capture only stdout will miss “unrecognised key format” messages on stderr.
• CI pipelines that validate only public‑key extraction (ssh-keygen -y) will pass even when the private part cannot be loaded by the target agent.
• Pageant ≤0.70 cannot load v3 keys; SSH sessions fall back to “no supported authentication methods” without a clear error.

Enforcing a consistent key version across heterogeneous toolchains

1. Create a TOOL_VERSIONS.md matrix listing the minimum verified version for each consumer (PuTTY, WinSCP, FileZilla, Paramiko, libssh2, OpenSSH for Windows, etc.).
2. Generate all new keys explicitly as v2:

   puttygen -t ed25519 -o key.ppk -O private-v2 -P ""

3. Add a CI gate that runs puttygen candidate.ppk -L >/dev/null || exit 1 for every committed private key; this fails fast on unreadable formats.
4. Block merges that introduce a v3 key unless the matrix shows every consumer ≥ the required version (PuTTY ≥0.71, WinSCP ≥5.15, Paramiko ≥2.7, libssh2 ≥1.10).

Recommended validation step before bulk conversion

Run a dry‑run script over the key store:

for f in *.ppk; do
  puttygen "$f" -O private -o /dev/null -P "$PASS" || { echo "FAIL: $f"; exit 1; }
  puttygen "$f" -L >/dev/null || { echo "NO PUBLIC: $f"; exit 1; }
done

Only proceed with conversion (puttygen -O private -o new.ppk) after every key exits 0. Keep the original v2 files until the new v3 keys have been exercised in a staging environment.

One diagnostic detail that changes the recommendation

What is the oldest PuTTY (or WinSCP/Paramiko/libssh2) version still in active use? If any component is ≤0.70 / ≤5.14 / ≤2.6, stay on v2.