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The main purpose of using a message digest in digital signatures is to speed up the encryption process. When a message is signed, it involves creating a hash value (or message digest) of the original data. This hash is a fixed-size string that represents the original message, regardless of its length. By using the message digest instead of the entire message, the cryptographic process becomes more efficient.

Hash functions are designed to be computationally fast while still providing a unique representation of the data. This means that signing a message using its digest requires significantly less computational power and time compared to encrypting the entire message. It allows for quicker verification and reduces the complexity of the operations involved in digital signatures, making the process more efficient while maintaining the integrity of the data being transmitted.

In contrast, the other options do not fully capture the role of a message digest in this context. For example, although message digests contribute to security, their primary function in this scenario is related to efficiency, rather than enhancing confidentiality or avoiding public key cryptography entirely.