requests. An encoder without good knowledge of the provenance of header fields <del> might instead introduce a penalty for bad guesses, such that attempts to guess a header field value results in all values being removed from consideration in all future requests, effectively preventing further guesses. Simply removing values from the dynamic table can be ineffectual if the attacker has a reliable way of causing values to be reinstalled.  For example, a request to load an image in a web browser typically includes the Cookie header field (a potentially highly valued target for this sort of attack), and web sites can easily force an image to be loaded, thereby refreshing the entry in the dynamic table. </del> <ins> might instead introduce a penalty for a header field with many different values, such that a large number of attempts to guess a header field value results in the header field no more being compared to the dynamic table entries in future messages, effectively preventing further guesses. Simply removing entries corresponding to the header field from the dynamic table can be ineffectual if the attacker has a reliable way of causing values to be reinstalled.  For example, a request to load an image in a web browser typically includes the Cookie header field (a potentially highly valued target for this sort of attack), and web sites can easily force an image to be loaded, thereby refreshing the entry in the dynamic table. </ins> This response might be made inversely proportional to the length of <del> the header field.  Marking as inaccessible might occur for shorter values more quickly or with higher probability than for longer values. Implementations might also choose to protect certain header fields that are known to be highly valued, such as the Authorization or Cookie header fields, by disabling or further limiting compression. </del> <ins> the header field value.  Marking as inaccessible might occur for shorter values more quickly or with higher probability than for longer values. </ins> 7.1.3. <ins> Implementations can also choose to protect sensitive header fields by not compressing them and instead encoding their value as literals. </ins> Refusing to generate an indexed representation for a header field is only effective if compression is avoided on all hops.  The never indexed literal (see literal.header.never.indexed) can be used to signal to intermediaries that a particular value was intentionally <del> sent as a literal.  An intermediary MUST NOT re-encode a value that uses the never indexed literal with a representation that would index it. </del> <ins> sent as a literal. An intermediary MUST NOT re-encode a value that uses the never indexed literal representation with another representation that would index it.  If HPACK is used for re-encoding, the never indexed literal representation MUST be used. The choice to use a never indexed literal representation for a header field depends on several factors.  Since HPACK doesn't protect against guessing an entire header field value, short or low-entropy values are more readily recovered by an adversary.  Therefore, an encoder might choose not to index values with low entropy. An encoder might also choose not to index values for header fields that are considered to be highly valuable or sensitive to recovery, such as the Cookie or Authorization header fields. On the contrary, an encoder might prefer indexing values for header fields that have little or no value if they were exposed.  For instance, a User-Agent header field does not commonly vary between requests and is sent to any server.  In that case, confirmation that a particular User-Agent value has been used provides little value. Note that these criteria for deciding to use a never indexed literal representation will evolve over time as new attacks are discovered. </ins> 7.2.