The NSA designed the SHA-256 and it's variants and some people suspect that they added a backdoor.

Backdoors

Backdoors are hidden ways they can use to break the encryption. However, nobody has since found a security flaw or backdoor.

Today, 20 years after its initial release SHA-256 is considered safe.

Cryptographic Hashes

Cryptographic hashes are needed for blockchain encryption.

Those need to have five properties:

• They need to be deterministic and fast.
• They must only work in one way.
• Reverse engineering must be infeasibly hard.
• Small changes to the input must change the output massively.
  &
• Collisions must be super rare.

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