Signing an Image with a Digital Signature and Protecting It

Leon Chase

• 2 min read

Signing an image with a digital signature and protecting it involves a combination of cryptographic techniques and practical steps to ensure authenticity, integrity, and security. Here's a straightforward guide:

Signing an Image with a Digital Signature

A digital signature verifies that the image comes from you and hasn’t been tampered with. Here’s how you can do it:

  1. Choose a Digital Signature Tool or Software

    • Use tools like Adobe Acrobat (for PDFs with embedded images), GIMP with a cryptography plugin, or dedicated software like OpenSSL for advanced users.
    • Alternatively, online platforms like DocuSign or SignNow can sign files, though they’re more common for documents.

  2. Create a Digital Certificate

    • Obtain a digital certificate from a trusted Certificate Authority (CA) like DigiCert, GlobalSign, or Let’s Encrypt. This acts as your digital identity.
    • If it’s just for personal use, you can generate a self-signed certificate using tools like OpenSSL:
      openssl req -x509 -newkey rsa:2048 -keyout privatekey.pem -out certificate.pem -days 365
      This creates a private key and a certificate valid for a year.

  3. Hash the Image

    • Generate a unique fingerprint (hash) of the image using a cryptographic hash function like SHA-256. This ensures any change to the image alters the hash.
      Example with OpenSSL:
      openssl dgst -sha256 image.jpg > hash.txt

  4. Sign the Hash

    • Use your private key to encrypt the hash, creating the digital signature.
      Example with OpenSSL:
      openssl rsautl -sign -in hash.txt -inkey privatekey.pem -out signature.bin
    • Attach the signature.bin file or embed the signature metadata in the image (e.g., via EXIF data if the format supports it).

  5. Distribute the Signed Image

    • Share the image, the signature, and your public certificate. Anyone can verify the signature by decrypting it with your public key and comparing it to a freshly computed hash of the image.

Protecting the Image

Beyond signing, you can take steps to safeguard the image from unauthorized use or alteration:

  1. Encrypt the Image

    • Use encryption tools like GPG or AES to lock the image file, ensuring only authorized people with the decryption key can view it.
      Example with OpenSSL:
      openssl enc -aes-256-cbc -salt -in image.jpg -out image.jpg.enc -k YOURPASSWORD
    • Share the password or key securely with intended recipients.

  2. Watermarking

    • Add a visible or invisible watermark to deter theft or prove ownership. Tools like Photoshop, GIMP, or online services (e.g., Watermark.ws) can do this. Invisible watermarks can be embedded using steganography tools like Steghide.

  3. Restrict Access

    • Store the image in a secure location (e.g., password-protected cloud storage like Google Drive or Dropbox) and share access only with trusted parties.
    • Use file permissions if hosting on a server to limit who can download or view it.

  4. Monitor Usage

    • Use reverse image search tools like Google Images or TinEye to check if your image is being used elsewhere without permission.

Verification Process for Others

To let someone verify your signed image:

  • They compute the hash of the image they received.
  • They decrypt your signature with your public key to get the original hash.
  • If the two hashes match, the image is authentic and unaltered.

Notes

  • If you’re not tech-savvy, software like Adobe Acrobat or online signing services simplifies this by handling the cryptography for you.
  • For legal purposes, ensure your certificate comes from a recognized CA, not a self-signed one.
  • Keep your private key secure—losing it or having it stolen compromises everything signed with it.