For example I want to send text document with content:

---start of hello.txt---
Hello world!

SHA-256 hash of this document is: e5cefc21ce1131cc0e8759f5e3b46c016319883c6f9b0d714502f3cf7226864f
---end of hello.txt---
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    So you are asking if you could create a document with known text where hash_as_hex(text + hexdigest) = hexdigest ? What is the use case for this or is it just curiosity? – Steffen Ullrich Mar 1 '17 at 10:38
  • Are you trying to hash the document and then putting that same hash in the same document? It's like 'hashception'? – Lester T. Mar 1 '17 at 11:00
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    There is no standard way to do this for SHAs. Also see stackoverflow.com/questions/4432669/… – Jedi Mar 1 '17 at 11:01

No. It is a problem akin to that of the self-enumerating pangram, but since the hash function is not reversible, the only way of doing this is by brute force. Given the size of the key space, this is unfeasible.

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I guess you could potentially include a hash of the other content in the file in the file - e.g.

some content
Hash of CONTENT: sha256:aaaaf

It isn't clear what that achieves, though, because anyone able to intercept the message can replace both the content and hash - something using e.g. HMAC would prevent.

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