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I keep most of my business documents in Microsoft Words (.docx) file format. Problem is these documents can be edited and tampered with. For my signatures, I just copy and paste a signature image onto the file.

Question, will these documents hold in the court of law? If yes, what prevents someone from altering the contents of the document?

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    PDFs are almost as "editable" as Word documents. – chrylis -cautiouslyoptimistic- Jan 2 at 8:42
  • Files are just bytes. You can trivially modify any byte you want. If you want to have proof that a file was not tampered with you should look into digitally signing them. There are services that not only provide digital signature but also timestamping, so that you can proof that the document existed at a certain time in the past and you didn't create it the day before. – Giacomo Alzetta Jan 8 at 13:29
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Whether they are admissible as evidence is up to the trier of law

The “trier of law” (judge) decides what evidence is admissible according to the rules of evidence.

Documents of most kinds are not admissible on their own (exceptions include “business” documents like invoices and receipts). To be admissible, someone (presumably you) has to testify as to what they are, how they were created and maintained and why they are relevant before the judge will decide if they will be accepted as evidence.

Whether they prove anything is up to the trier of fact

Evidence is not proof

The “trier of fact” (jury in a jury trial, judge in a bench trial) decides what weight to give to any and all evidence presented. They may accept the testimony about the documents or they may reject it. If presented with conflicting testimony they have to decide which they prefer. Judges have to give reasons for their decisions which can be appealed. Juries don’t.

Files that are readily changeable are likely to be given less weight than files which have a verifiable audit trail. For example, if you kept your .docx files in a GitHub repository they will be stronger evidence than those kept on a local hard drive.

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Evidence is evidence.

The word document can be evidence of something.

Yes it is true that the document can be forged, modified etc.

But this only affects how much weight is attached to the evidence (i.e how convincing it is)

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If in doubt, why not make PDFs?

PDF software generally has a signature feature. I use foxit, which is free and easy to use, and it has a handy signing tool. You just upload a scanned signature image in.

When I needed to send official communications to a hearing officer, with copy to the other side, I emailed for simple things like scheduling discussions. For motions or a response to a motion, I attached an electronically signed pdf.

That is also the pattern I have seen from others.

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will these documents hold in the court of law?

They could be. It is up to a party to dispute the adversary's evidence regardless of how easy it is to alter that evidence. Absent an objection, the '.docx' file would be considered admitted.

what prevents someone from altering the contents of the document?

Nothing. But in the event of forgery, the victim of that forgery should point to further evidence (perhaps circumstantial evidence and/or inconsistencies in the parties' conduct) with which to discredit the altered document.

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You may consider attaching a digital signature as the last line of each document.

Make the last line read "sha3-256: 0feba.....", where that line is the signature of everything in that file before the signature line. This would make them very difficult to tamper with. Especially if you also keep a catalog of the signatures and sizes for all the files you have.

In fact, a similar technique is used when distributing binary packages. A digital "signature" (which is a hash) can be downloaded from the package distributor separately from the package. This way the person who gets the package can produce a hash of that package on their end and verify that the hashes match.

The added benefit of doing this is that hashes can be communicated via unsecured channels. So if there is any need to preserve confidentiality of the documents, the documents with hashes can be sent by secure channels while hashes can be queried from the sender at a later time by any other channel.

Consider the following scenario: you need to send over 10 .docx documents. You generate the hashes for all of them and append a hash to each document. You can then print out the list of document names and their sizes and hashes on 1 piece of paper. You can then store that piece of paper securely. You can generate copies of that piece of paper without worrying about its confidentiality. You can send that list of digital signatures around without worrying about confidentiality. It can effectively act as a receipt for the digital communication which you sent.

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    A hash, such as one produced with one of the secure hashing algorithms (SHA), is not a digital signature. Since the slightest variation in how the hashing is performed will give a different result, a reproducible procedure is needed. Such procedures are built in to most software, such as Microsoft Word or Adobe Acrobat; to be effective the procedures need a digital certificate, and preferably, a secure time-stamping service. – Gerard Ashton Jan 2 at 9:17
  • @GerardAshton it's my understanding that sha-3 is standardized (within variants) by NIST. You can use a signing service, but a local catalog of size+sha3 will do. If the documents are sensitive in nature, it may mean that all of them have to kept in house though. This isn't really a place to discuss this. If you have a link to security.SE question about this or cryptography.SE, I would be curious to see what those communities have to say about it. Otherwise, it would still be a better subject matter for a question in one of those communities rather than here. – grovkin Jan 2 at 21:18
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    This is appallingly bad advice. It just gives a false sense of confidence. What is to stop the forger editing the document and the hash line? – Martin Bonner supports Monica Jan 4 at 11:12
  • Note that the OP is concerned that they will be accused of being the forger, so access to any catalog is not a problem. – Martin Bonner supports Monica Jan 4 at 11:14
  • @MartinBonnersupportsMonica how do you figure that they are concerned about being accused of a forgery? The very last line of the question seems to express a concern that someone else would alter the document. I read that to be a concern that the document would be altered while in transit. If the hash is altered, or if it doesn't match, the sender and the recipient have a way of verifying that (by checking that the hashes match or don't match). – grovkin Jan 5 at 6:00
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Even a paper signature can be forged.

What Dale said is the best, it's up to the courts to decide whether the evidence is strong enough. You need to convince the courts that the document could not be tempered.

A document sealer like SignRequest would provide impeccable evidence

A document sealing service can "seal" the document by adding a digital signature together with the words or pdf file. Only SignRequest has the authority to seal the document. A public key is used to verify the legitimacy of the seal.

https://signrequest.com/

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