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Comparison of hash functions
I have a question regarding the use of hash functions.
I have read that hash functions are used to achieve compression. I understand that if an attacker knows the hash function, they can try to calculate the hash for messages. In this case, how is this prevented?
I know the hash function is secret, but can the attacker calculate the hash of messages in an attempt to guess the function?
I am also aware of the use of hash functions in hash tables. In this case, are all hash functions equal or are there differences in each hash function?
A:
If an attacker can intercept (i.e. hijack) your messages, then no hash function prevents him from computing a hash value for each message. However, if the attacker does not have access to your messages, but to the hashes calculated over them, he still cannot recover the original messages.
If you use a cryptographic hash function, then attackers cannot perform a preimage attack against it: they cannot calculate any $h(M)$ for a message $M$ such that $h(M)=h(N)$ for some $N
e M$. Hence, even if you were not careful with the implementation of your hash function, it would be secure against a preimage attack.
Hash functions are used for a different purpose, namely to identify the message; however, the secret key to such a hash function is usually much shorter than the message itself. Hash functions are only used to reduce the message size and thus the memory footprint required to store the message.
Hash functions can be used as hash tables. However, some applications require that the message is not allowed to be modified in-place: for instance, you might want to store the message in a file system or in a database, but you do not want the attacker to be able to modify it. In this case, you need to use a cryptographic hash function.
Walter Baxter Brookes
Walter Baxter Brookes (April 12 be359ba680
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