Cryptographic Hash
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Cryptographic hash is the output (digest or hash) of the hash function that generates the single binary string of the fixed length from the message i.e. from the input linear sequence (string) of ones and zeros of arbitrary length that may be as long as you want. Both nxt and btc use SHA-256 cryptographic function (SHA stands for Secure Hash Algorithm) that produces digest with following attributes:

    1. 256 bits in length.
    2. always the same for the given message.
    3. collision resistant (which means that it is almost impossible to find two different messages with the same hash. To be quite frankly collisions do exist but they are not findable by regular people using regular computers. Just to have feeling of that it is turned out that over 2130 randomly chosen messages at least two of them are colliding with the chance of 0.998. However to find collisions with such high probability computer needs to hash 2130 messages. It requires time on astronomical scale. Even if every computer ever made by humanity was hashing since the beginning of Universe up to now the chance they would find out collision is infinitesimally small .)
    4. hides all info relevant to input message even if the last one is very simple and consists of a few characters (It means that there is no feasible way to find message knowing its hash).
    5. looks like really random in nature.

SHA-256 itself is efficiently computable algorithm that allows to find on regular computer in the reasonable length of time what the output is for the given message.