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If I wanted to use hmac for proving that a value hadn't been tampered with, how many times can I use a key before it needs to be rotated?

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There is no known inherent weakness in HMAC-SHA256 that requires key rotation for a suitably secret key.

However, other aspects of the system in which you are using HMAC-SHA256 may make key rotation necessary. For instance if you don't want the same message to have the same hmac over time for some reason a key rotation would achieve that (but so would a salt). Or if you know your key comes from an unusually low entropy pool making it easier to brute force than one would expect.

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  • $\begingroup$ Thanks. So basically with a good key from something quality like /dev/urandom I should be good to go. $\endgroup$ – mr blobby Dec 23 '14 at 23:49
  • $\begingroup$ Yes. as long as your key isn't leaked via some other mechanism then 32 bytes from /dev/urandom would be a fine key. $\endgroup$ – John Meacham Dec 23 '14 at 23:51
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With HMAC, can an attacker recover the key, given many known plaintext/tag pairs?

I guess this constitutes an answer. I can keep the key as long as I think I can keep it secure. Rotating the key would give me forward secrecy at least.

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