In the diagram below, "T" designates a transaction, "H" a hash. Note that the image is highly simplified; a block can contain hundreds or thousands of transactions.

For more than six years, the SHA1 cryptographic hash function underpinning Internet security has been at death's door.Now it's officially dead, thanks to the submission of the first known instance ...

With a strong hash function it is almost impossible to reverse the hash value to its original content. The National Security Agency (NSA) designed the SHA-1 cryptographic hash function 10 years ago.