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Using $$token = SHA256(APIKey \mathbin\| SecretKey \mathbin\| expireTimestamp) \mathbin\| expireTimestamp$$
is not secure since it can cause a length extension attack as;
$$token2 = SHA256(APIKey \mathbin\| SecretKey \mathbin\| expireTimestamp \| \color{red}{extension}) \mathbin\| (expireTimestamp \| \color{red}{extension}) $$
This is a valid token for the ...
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