This is a text-only version of the following page on https://raymii.org:

This is a text-only version of the following page on https://raymii.org:
---
Title : Sign and verify text/files to public keys via the OpenSSL Command Line
Author : Remy van Elst
Date : 09-11-2015
URL : https://raymii.org/s/tutorials/Sign_and_verify_text_files_to_public_keys_via_the_OpenSSL_Command_Line.html
Format : Markdown/HTML
---

This small guide will shows you how to use the OpenSSL Command Line to sign a
file, and how to verify the signing of this file. You can do this to prove
ownership of a key, or to prove that a file hasn't been modified since you
signed it. This works both with small text files as well as huge photo's,
documents or PDF files.

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### Generate a keypair

We'll generate a new keypair for this. You can also use an exisiting one. Change
the subject in the following command and execute it to generate a self signed
keypair:

openssl req -nodes -x509 -sha256 -newkey rsa:4096 -keyout "$(whoami)s Sign Key.key" -out "$(whoami)s Sign Key.crt" -days 365 -subj "/C=NL/ST=Zuid Holland/L=Rotterdam/O=Sparkling Network/OU=IT Dept/CN=$(whoami)s Sign Key"

Also create a small text file to test the signing process on:

echo "Hello, World!" > sign.txt

### Sign the file

Use the following command to sign the file. We actually take the sha256 hash of
the file and sign that, all in one `openssl` command:

openssl dgst -sha256 -sign "$(whoami)s Sign Key.key" -out sign.txt.sha256 sign.txt

This will result in a file `sign.txt` with the contents, and the file
`sign.txt.sha256` with the signed hash of this file.

You can place the file and the public key (`$(whoami)s Sign Key.crt`) on the
internet or anywhere you like. Keep the private key (`$(whoami)s Sign Key.key`)
very safe and private.

### Verify the signature

To verify the signature, you need the specific certificate's public key. We can
get that from the certificate using the following command:

openssl x509 -in "$(whoami)s Sign Key.crt"

But that is quite a burden and we have a shell that can automate this away for
us. The below command validates the file using the hashed signature:

openssl dgst -sha256 -verify <(openssl x509 -in "$(whoami)s Sign Key.crt" -pubkey -noout) -signature sign.txt.sha256 sign.txt

If the contents have not changed since the signing was done, the output is like
below:

Verified OK

If the validation failed, that means the file hash doesn't correspond to the
signed hash. The file has very likely been modified or tampered. The result of a
failed validation looks like below:

Verification Failure

### Signature

To get a text version of the signature (the file contains binary content) you
can use the `base64` command. The textual version is easier to public online
with the file:

base64 sign.txt.sha256 > sign.txt.sha256.txt

To get this back into `openssl` parsable output, use the `base64 -d` command:

base64 -d sign.txt.sha256.txt > sign.txt.sha256

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---

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