What key is needed by an individual to digitally sign a file?

A private key is essential for an individual to digitally sign a file. This is because digital signatures work by creating a unique hash of the document and encrypting that hash with the signer's private key. The purpose of this process is to ensure both the authenticity of the signer and the integrity of the message being sent.

When someone receives a digitally signed file, they can verify the signature using the signer's public key. This process allows anyone to confirm that the file has not been altered since it was signed and that it genuinely originates from the signer, as only the corresponding private key holder could have created that particular signature. Therefore, the security and trust model of public key infrastructure (PKI) relies heavily on the discreet use of the private key for signing and the public key for verification.

This highlights the importance of safeguarding the private key, as its exposure could lead to unauthorized signing by malicious actors. In contrast, the other key types mentioned do not serve the purpose required for digital signatures.