chris 08ac545567 Harden ALTCHA: raise cost and add expiration
cost 100 → 50000: was solvable in milliseconds by a bot, now takes
~1-2s in a real browser, making mass automation impractical.

expiresAt 10min: embeds expiry in the HMAC-signed challenge so the
server rejects replayed tokens without needing to store seen challenges.

ALTCHA_HMAC_KEY must be set to a strong secret in production.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
2026-06-13 19:11:19 -04:00
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