What is a characteristic of HMAC hashing?

The characteristic of HMAC (Hash-based Message Authentication Code) hashing that is highlighted in the correct answer is its reliance on a secret value known only to the transmitter. HMAC is designed to provide both data integrity and authenticity, which is achieved through the combination of a cryptographic hash function and a secret key.

In HMAC, the secret key is used in conjunction with the message to produce a hash value, ensuring that only those with knowledge of the key can generate the correct hash output for a given message. This makes it significantly more secure than standard hashing methods, as an attacker would need access to the secret key in order to produce a valid hash, thwarting attempts at tampering or forging messages.

This focus on a secret key directly supports the applications of HMAC in protocols requiring message integrity and authentication, such as secure communications and data integrity verification. The other options do not describe the fundamental nature of HMAC hashing accurately; for example, HMAC does not use public keys, it does not inherently generate longer hash values than standard hashing by design, and while any hashing can be susceptible to brute-force attacks depending on the length of the input and the computational power available, this is not a characteristic unique to HMAC itself.