How Can I Migrate My Mcrypt-Based PHP Code to Openssl in PHP 7.2 and Beyond?

Preparing for the Removal of Mcrypt in PHP 7.2

As PHP 7.2 draws nearer, the deprecation of the Mcrypt extension becomes imminent. While Openssl serves as a viable alternative, navigating this transition can be challenging, particularly for those less familiar with cryptography.

One of the hurdles to overcome involves converting code relying on Mcrypt's Rijndael-256 CBC mode with IV preservation to Openssl's equivalent. At present, such a direct conversion is not feasible. The reason lies in the fundamental difference between Rijndael-256 and AES-256.

Mcrypt's Rijndael-256 implementation operates on a 256-bit block size, whereas Openssl's AES-256 adheres to a 128-bit block size. As a result, this difference precludes direct substitution of one for the other. This limitation necessitates the re-encryption of data to achieve compatibility with AES-256.

Beyond the incompatibility issue, the existing code presents several additional security concerns:

• Lack of Authentication: The absence of HMACs leaves the data vulnerable to tampering.
• Improper Padding: The use of zero bytes for padding fails to conform to industry best practices, such as PKCS#5 padding, which is required for secure block mode encryption.
• Byte Safety: The reliance on mb_substr() for string manipulation compromises byte safety.

Fortunately, Openssl supports PKCS#5 padding natively. However, it is strongly recommended to employ robust encryption libraries, such as defuse/php-encryption, which address these concerns and provide a higher level of data protection.

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