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In HMAC, the key $K$ [after it has been replaced by $H(K)$ if $K$ was wider than the hash's internal block size] is padded with zeros to the hash's internal block size. The question asks why this padding. In a nutshell: the security argument of HMAC would not hold without that. HMAC's original and improved security arguments make heavy use of the ...


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Does allowing users to test VALUEs increase the likelihood that SECRET will be broken or illegal hashed values generated, relative to the CONTROL scenario? By definition a cryptographic Message Authentication Code such as HMAC is secure only if resists existential forgery under chosen-plaintext attacks. i.e. if allowing users to test VALUEs increases ...


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Would you use HMAC-SHA1 or HMAC-SHA256 for message authentication? Yes. That is a semi-serious answer; both are very good choices, assuming, of course, that a Message Authentication Code is the appropriate solution (that is, both sides share a secret key), and you don't need extreme speed. How much HMAC-SHA256 is slower than HMAC-SHA1? Those ...


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If you truly can't be dissuaded from 'using' an RSA key for HMAC, be sure to derive a strong symmetric key using HKDF with a salt and some associated data. I have a suggestion for you based on your comment to Stephen's answer. If all you need to do is store the symmetric key in the key/cert store, why not encode some generated symmetric key in the format ...


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The reason I am asking about the RSA private key is the HMAC key needs to be stored so that the HMAC can be validated by the server on future requests. An RSA private key is an easy to manage, persistent value. You seem to be under the misguided and mistaken belief that an RSA key is somehow easier to manage and persist than a symmetric key. I am ...


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Password hashes need first pre-image resistance and should not cause many collisions among typical passwords (preserve the entropy). This collision "attack" violates neither requirement and causes no practical security issues. While this issue can find trivial collisions, they're not between commonly chosen passwords. A SHA-1 hash (and thus the shorter of ...


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No, it is not broken. This is NOT A PROBLEM for PBKDF2-HMAC-SHA1. The PBKDF2-HMAC-SHA1 function is a key derivation function (password-based key derivation). It is fairly good function, for instance it is recommended by NIST (NIST SP 800-132). It is (relatively) rare for this function to have a collision, but collisions generally are not a problem for key ...



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