ssh: obscure keystroke interval timing #24
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djmdjm
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This adds a pair of transport messages SSH2_MSG_PING/PONG to implement a protocol-level ping capability.
Quantises packet output times at the client for interactive connections when there is only a keystroke or two in the output buffer. Send chaff packets on intervals were no keystroke was observed. Controlled by a new ssh_config:ObscureKeystrokeTiming keyword
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This implements some obscurity for keystroke timings in the
sshclient. It is intended to frustrate basic inter-keystroke measurements by an adversary who can passively observe ciphertext packets.The basic idea is twofold:
Chaff packets are sent using a new transport-level
SSH2_MSG_PINGextension, as nothing else lets us send a message that will elicit a reply, with a size that exactly matches a single keystroke. Currently this message uses an implementation-local message number, but I wrote an ID to see if it can be standardised.