eSign image with Digital signature Leave feedback

An Image Digital Signature uses steganography (LSB - Least Significant Bit technique) to embed digital signatures directly into raster image pixels. This method allows you to add invisible, password-protected signatures to images while maintaining image quality. The signature data is embedded into the least significant bits of image pixels, making the signature undetectable to the naked eye.

• Invisible Signatures: Signatures are embedded directly into image pixels using steganography
• Password Protection: Signatures are protected with a password (minimum 4 characters)
• Format Support: Works with PNG and JPG image formats
• Quality Preservation: Image quality remains virtually unchanged
• Size Requirements: Images must meet minimum size requirements (8x8 pixels, 16,384 total pixels)

• Minimum Width: 8 pixels
• Minimum Height: 8 pixels
• Minimum Total Pixels: 16,384 pixels (width × height)

These are the limits below which signing is refused. An image that only just exceeds them may still be too small to hold a signature that verifies, so use images well above the minimum and check the result with image digital signature verification.

• Minimum Length: 4 characters
• Case Sensitive: Yes
• Special Characters: Allowed

• PNG - Supported
• JPG/JPEG - Supported
• BMP - Not supported for digital steganography signatures

GroupDocs.Signature provides the ImageDigitalSignOptions class (package com.groupdocs.signature.options.sign) to specify settings for digital signatures in images using steganography.

Here are the steps to sign an image with a digital signature:

• Create a new instance of the Signature class and pass the source image path as a constructor parameter
• Instantiate the ImageDigitalSignOptions object and set the password with setPassword (minimum 4 characters), or pass the password to the constructor
• Call the sign method of the Signature class instance and pass the ImageDigitalSignOptions to it

This example shows how to sign an image with a digital signature using steganography.

Signature signature = new Signature("image.png");
try
{
    // Create sign options with password
    ImageDigitalSignOptions signOptions = new ImageDigitalSignOptions();
    signOptions.setPassword("MySecurePassword123");

    // Sign the image
    signature.sign("signed_image.png", signOptions);

    System.out.println("Image signed successfully. File saved at signed_image.png");
}
finally
{
    signature.dispose();
}

Note

The returned SignResult does not list the embedded image digital signature in getSucceeded(). To confirm that the image carries the signature, verify it with ImageDigitalVerifyOptions.

To control the format of the output image, pass ImageSaveOptions (package com.groupdocs.signature.options.saveoptions.imagessaveoptions) to the sign method:

Signature signature = new Signature("image.jpg");
try
{
    ImageDigitalSignOptions signOptions = new ImageDigitalSignOptions("MySecurePassword123");

    // Save the signed image as PNG
    ImageSaveOptions saveOptions = new ImageSaveOptions();
    saveOptions.setFileFormat(ImageSaveFileFormat.Png);

    signature.sign("signed_image.png", signOptions, saveOptions);
}
finally
{
    signature.dispose();
}

You can also sign images using streams instead of file paths:

try (InputStream inputStream = new FileInputStream("image.png");
     OutputStream outputStream = new FileOutputStream("signed_image.png"))
{
    Signature signature = new Signature(inputStream);
    try
    {
        ImageDigitalSignOptions signOptions = new ImageDigitalSignOptions();
        signOptions.setPassword("MySecurePassword123");

        signature.sign(outputStream, signOptions);
        System.out.println("Image signed successfully.");
    }
    finally
    {
        signature.dispose();
    }
}

The library automatically validates password length and image size requirements. Both errors reach your code as GroupDocsSignatureException:

try
{
    Signature signature = new Signature("image.png");
    try
    {
        ImageDigitalSignOptions signOptions = new ImageDigitalSignOptions();
        signOptions.setPassword("MyPassword123"); // Minimum 4 characters required

        signature.sign("signed_image.png", signOptions);
        System.out.println("Image signed successfully!");
    }
    finally
    {
        signature.dispose();
    }
}
catch (GroupDocsSignatureException ex)
{
    // "Password must be at least 4 characters long." or
    // "Image is too small for digital steganography signature. ..."
    System.out.println("Signing error: " + ex.getMessage());
}

• PNG: Supported
• JPG/JPEG: Supported
• BMP: Not supported
• Other formats: Not supported

• Images smaller than 8x8 pixels cannot be signed
• Images with fewer than 16,384 pixels cannot be signed

• Image compression (especially JPG) may affect signature detection
• Image editing may corrupt embedded signatures
• Always keep original signed images for verification
• The signature is embedded invisibly in the image pixels

This guide demonstrates how to use GroupDocs.Signature to sign images with digital signatures using steganography. It covers loading images, configuring password-protected digital signatures that are embedded invisibly into image pixels, and saving signed images. The feature provides invisible authentication while maintaining image quality and requires minimum size requirements for images.

To learn more about document eSign features, please refer to the advanced usage section.

You may easily run the code above and see the feature in action in our GitHub examples:

• GroupDocs.Signature for Java examples, plugins, and showcase
• GroupDocs.Signature for .NET examples, plugins, and showcase
• Document Signature for Java App Dropwizard UI Example
• Document Signature for Java Spring UI Example
• Document Signature for .NET MVC UI Example
• Document Signature for .NET App WebForms UI Example

Along with the full-featured Java library, we provide simple but powerful free online apps.

To sign PDF, Word, Excel, PowerPoint, and other documents you can use the online apps from the GroupDocs.Signature App Product Family.