Prove Message Ownership

G5 Cyber Security

1 week ago

TL;DR

You can prove you wrote an anonymous message using a digital signature based on cryptography. This involves creating a unique ‘fingerprint’ of the message and your private key, which others can verify with your public key.

How to Prove You Wrote an Anonymous Message

Understand Digital Signatures: A digital signature isn’t like writing your name. It uses cryptography (complex maths) to create a unique code linked to both the message *and* your secret key. If someone changes even one letter of the message, the signature won’t work anymore.
Generate a Key Pair: You need two keys:
- Private Key: Keep this absolutely secret! This is what creates the signature.
- Public Key: Share this with anyone who needs to verify your messages.
You can use tools like OpenSSL or GPG for this. Here’s an example using OpenSSL:
```
openssl genrsa -out private.pem 2048
```
```
openssl rsa -in private.pem -pubout -out public.pem
```
Sign the Message: Use your private key to create a signature for the message.
```
openssl dgst -sha256 -sign private.pem -out signature.sig "Your Anonymous Message"
```
This creates a file called signature.sig containing the digital signature. The SHA256 algorithm is commonly used for hashing (creating the fingerprint).
Share Your Public Key and Signature: Give the recipient your public key (public.pem) and the signature file (signature.sig). Also, share the original message text.
Verification: The recipient uses your public key to verify that the signature matches the message. They’ll use a tool like OpenSSL:
```
openssl dgst -sha256 -verify public.pem -signature signature.sig "Your Anonymous Message"
```
If the verification is successful, it will output something like Verified OK. If not, the message has been altered or wasn’t signed with your private key.

Important Considerations

Key Security: Protect your private key! If someone gets hold of it, they can forge signatures in your name. Store it securely (e.g., password-protected file, hardware security module).
Hashing Algorithm: SHA256 is a good choice for the hashing algorithm, but newer algorithms like SHA3 are also available.
Message Integrity: Ensure the recipient receives the *exact* original message text along with the signature and public key. Any changes will invalidate the signature.
Complexity: This process can be complex for non-technical users. Consider using tools that simplify it, or seek help from someone experienced in cryptography.