Secret Key Recovery of NIST P-521 Private Keys through biased ECDSA nonces in PuTTY client

The PuTTY client and all related components generate heavily biased ECDSA nonces in the case of NIST P-521. This allows for full secret key recovery in roughly 60 signatures by using state-of-the-art techniques. These signatures can either be harvested by a malicious server (machine-in-the-middle attacks are not possible given that clients do not transmit their signature in the clear) or from any other source, e.g. signed git commits through forwarded agents.