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When using PHP's hash function to process data, hash_final is a often overlooked but critical function. It is usually used with hash_init and hash_update to incrementally calculate the hash of large data blocks. However, many developers are prone to making some mistakes when using hash_final , resulting in incorrect calculation results or abnormal program behavior. This article will list several common errors and give the correct way to use them.
Many developers mistakenly believe that they can call hash_final multiple times to get the hash result. But in reality, hash_final destroys the hash context and cannot be used again once called.
Error example:
$ctx = hash_init('sha256');
hash_update($ctx, 'Hello, world!');
$hash1 = hash_final($ctx);
$hash2 = hash_final($ctx); // mistake：The context has been destroyed
Fix method:
If you need to preserve the hash context, it is recommended to use hash_copy to clone the context.
$ctx = hash_init('sha256');
hash_update($ctx, 'Hello, world!');
$ctx_copy = hash_copy($ctx);
$hash1 = hash_final($ctx);
$hash2 = hash_final($ctx_copy); // correct
When processing multi-section data, some developers mistakenly call hash_update only on part of the data, ignoring the rest, resulting in the hash value inconsistent with expectations.
Error example:
$data1 = 'Part1';
$data2 = 'Part2'; // Forgot to update this part
$ctx = hash_init('sha256');
hash_update($ctx, $data1);
$hash = hash_final($ctx);
Fix method:
Make sure all data fragments are submitted to the hash context using hash_update .
$ctx = hash_init('sha256');
hash_update($ctx, $data1);
hash_update($ctx, $data2);
$hash = hash_final($ctx);
Some beginners will pass the output of hash_final to the hash() function again for a "quadratic hash", without realizing that this is usually redundant unless specific scenarios (such as building HMACs or iterative hashs).
Error example:
$ctx = hash_init('sha256');
hash_update($ctx, 'Example');
$intermediate = hash_final($ctx);
$final = hash('sha256', $intermediate); // Usually not necessary
Repair suggestions:
Unless you do need to do extra processing on the intermediate results, a hash_final is enough at once.
$ctx = hash_init('sha256');
hash_update($ctx, 'Example');
$hash = hash_final($ctx); // correct
hash_final returns a hexadecimal encoded string by default. If you want to get the original binary data, the second parameter should be set to true . Ignoring this can lead to mistakenly treating the original data as a string, resulting in errors in subsequent processing such as storage or comparison.
Example:
$ctx = hash_init('sha256');
hash_update($ctx, 'BinaryTest');
$raw_hash = hash_final($ctx, true);
file_put_contents('https://gitbox.net/storage/hash.bin', $raw_hash); // correct写入原始数据
hash_final is a powerful but easily misused function. When developers use it, they need to pay special attention to the context life cycle, data integrity and output encoding method. The rational use of hash_init , hash_update and hash_final can effectively process large files, segmented data and more complex encryption processes. When dealing with security-related hash logic, rigor is the first priority.
If you need to use hash functions in a production environment, it is recommended to combine PHP's hash_hmac or OpenSSL extension to further enhance data integrity verification and security.
