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Let's suppose I want to use a (keyed) hash function as a stream cipher, I provide a key and a message to be hashed as a counter (like a nonce), and so, XOR each hashed output block on the plaintext.

Is it possible that one or more different keys produce the same hashed block and so can revel part of plaintext when XORed? If equivalent keys exist, what is the percentage of key space should be brute-forced to find them?

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