What is the purpose of a "salt" in hashing?

The role of a "salt" in hashing is primarily to enhance security by mitigating rainbow table attacks. Rainbow tables are precomputed tables used to reverse cryptographic hash functions, making it easier for an attacker to crack passwords. By adding a unique salt to each password before hashing, the resultant hash values become unique even for identical passwords. This means that an attacker cannot use a precomputed rainbow table effectively, as each hash will correspond to a different salt. Consequently, the use of salts significantly strengthens the security of hashed data by ensuring that attackers must compute hashes for each individual password with its unique salt, rather than relying on precomputed values.

The other options address different aspects of hashing but do not accurately define the primary purpose of salt in hashing practices. While increasing the size of the output might occur due to the addition of a salt, it is not the primary function. Similarly, improving hashing speed and efficiency is not achieved by using a salt; in fact, it may slightly decrease speed due to the additional processing required. Substituting characters in the original data is also not relevant to the purpose of a salt.