from_pem error for an OpenSSH-format key mentions only PKCS8 — wrong remediation #335

Problem

ssh-keygen's default output since OpenSSH 7.8 is -----BEGIN OPENSSH PRIVATE KEY-----, not PKCS8. cryptography.serialization.load_pem_private_key does not parse that format — it requires load_ssh_private_key. A user who generated their Kalshi key with ssh-keygen -t rsa gets KalshiAuthError: Invalid PEM private key: Could not deserialize key data.... The current hint tells them to verify PKCS8 format — but does not tell them their key is actually OpenSSH or how to convert it.

First-run friction; the v2.x passphrase work added password handling but kept the PKCS8-only error string.

Evidence

• kalshi/auth.py:137-143:

except (ValueError, UnsupportedAlgorithm) as e:
    # Wrong password and malformed PEM both surface here as ValueError.
    raise KalshiAuthError(
        f"Invalid PEM private key: {e}. Ensure the key is an RSA private key "
        "in PKCS8 PEM format (-----BEGIN PRIVATE KEY-----) and that "
        "the supplied `password=` (if any) is correct."
    ) from e

Suggested fix

Before calling load_pem_private_key, peek at the first line of pem_bytes. If it starts with -----BEGIN OPENSSH PRIVATE KEY-----, raise a targeted error:

raise KalshiAuthError(
    "OpenSSH private-key format detected. Kalshi requires PKCS8 PEM. "
    "Convert in place with: ssh-keygen -p -m PEM -f <path>  "
    "or: openssl rsa -in <path> -out <path>.pem"
)

Leave the generic PKCS8 message for genuinely malformed bytes. Add a unit test that feeds an OpenSSH-format key and asserts the targeted message.

Source

Round-3 independent audit (reviewer: security_auth).