Possible to Digitally Sign with .crt or .cer?

Here’s the distinction, step by step, in plain terms:

1. What’s inside a .crt or .cer file

  • A .crt or .cer file usually contains only the public portion of a certificate.
  • That means:
    • The public key (used for verifying signatures or encrypting data).
    • The identity information (subject name, issuer, validity period, extensions, etc.).
    • Possibly the issuer chain (intermediate/root certificates).
  • Importantly: there is no private key in these files.

    That’s why they can be shared freely with anyone (for example, sending your SSL certificate to a browser or to an API client).

2. Why a private key is needed for signing

  • Digital signing works like this:
    1. Your application creates a hash (digest) of the data.
    2. The hash is encrypted with the private key of your certificate.
    3. That encrypted digest becomes the digital signature.
  • Later, anyone can verify it using the public key (from your .crt or .cer file).
  • Without the private key, step #2 cannot happen. The certificate alone only lets you verify signatures, not create them.

3. How certificates with private keys are stored

  • If you need both the certificate and the private key in one file, they’re usually in formats such as:
    • PKCS#12 / PFX (.p12, .pfx) → contains cert + private key (encrypted).
    • PEM (.pem) → can contain both, depending on how it’s exported.
  • On Windows, they may also be stored in the Windows Certificate Store (with the private key marked as “exportable” or not).

4. In Chilkat specifically

To sign data with Chilkat:

  • You must load a certificate object (Cert) that has an associated private key.
  • For example, using:
    • Cert.LoadPfxFile or LoadPfxData (when you have a .pfx file with private key).
    • Cert.LoadFromSmartcard (if the private key is inside an HSM or smartcard).
    • Cert.LoadByCommonName (from Windows Certificate Store or Apple KeyStore, if the private key is present).
  • If you try to use just a .crt/.cer file, Chilkat won’t be able to sign, because there’s no private key.

Summary:

  • .crt / .cer = certificate only, public key only.
  • To sign, you need a certificate with its private key (from .pfx, .p12, smartcard, HSM, or OS store).
  • In Chilkat, signing methods will fail unless the loaded Cert object has an associated private key.

Would you like me to also show you a Chilkat code snippet (for example in C# or VBA) that demonstrates the difference between loading a .cer versus a .pfx when trying to sign?