Propability of collissions using checksums.

hi there,

currently i am using a checksum from a file (sha-algorithm) combined with the size of that file as a key in a database.
my question is:
how save is this?
how can i calculate the propability that two files (of the same size) will have the same checksum?

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Jaime OlivaresSoftware ArchitectCommented:
You can have 2 ^  n (2 to the nth power) where n is number of bits of your hashing/cypher algorithm.

In this case n=160 for SHA algorithm, so probability is near to impossible.

2^160 is aproximately 15000000000000000000000000000000000000000000000000 posible combinations.

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