Block chain , a very interesting topic to go on , but before that lets learn about hashing .
What ever amount input you give (It can be up to infinite) ,those are processed by hashing algorithm and output comes in a fixed length (let us say it as 256bits) even bitcoin uses 256 bit algorithm called as SHA-256 [Secure Hashing Algorithm].
No matter how long a input but the output size in always fixed.
Each Block is given a hash code.
Each blocks in the block chain has different and unique hash code which indicates the block in which a particular transaction is made.
Every block has its previous block's hash code
See this pic
So if any modifications are done , it is clearly visible , that's why block chains are considered as safe.
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