Phalcon: Non-constant-time HMAC verification in `Encryption\Crypt::decrypt` (timing side-channel)

Summary

Phalcon\Encryption\Crypt provides authenticated encryption: when useSigning is enabled (the default), encrypt() appends an HMAC tag and decrypt() verifies it before returning the plaintext. The verification compares the attacker-supplied tag against the freshly computed HMAC using PHP/Zephir identity comparison (!==), which the Zephir compiler lowers to !ZEPHIR_IS_IDENTICAL(...) — a byte-wise memcmp that returns early on the first differing byte. The comparison time therefore depends on how many leading bytes of the supplied tag are correct, a classic MAC-verification timing side-channel. Every other secret/MAC comparison in the framework uses the constant-time hash_equals() (zephir_hash_equals) — the CSRF token check (Security::checkToken) and the JWT signature check (Signer\Hmac::verify); Crypt::decrypt is the lone deviation.

Details

Vulnerable code

phalcon/Encryption/Crypt.zep:246 (Zephir source):

if true === this->useSigning {
    // Checks on the decrypted message digest using the HMAC method.
    if digest !== hash_hmac(hashAlgorithm, padded, decryptKey, true) {
        throw new Mismatch("Hash does not match.");
    }
}

Generated C --> ext/phalcon/encryption/crypt.zep.c:364-367:

ZEPHIR_CALL_FUNCTION(&_8$$7, "hash_hmac", NULL, 245, &hashAlgorithm, &padded, &decryptKey, &__$true);
...
if (!ZEPHIR_IS_IDENTICAL(&digest, &_8$$7)) {                 // <-- non-constant-time
    ZEPHIR_THROW_EXCEPTION_DEBUG_STR(..., "Hash does not match.", "phalcon/Encryption/Crypt.zep", 247);

ZEPHIR_IS_IDENTICAL --> zephir_is_identical() (ext/kernel/operators.c:472) --> Zend is_identical_function --> for equal-length strings a memcmp that exits on the first mismatching byte (data-dependent timing).

Impact

The HMAC is the integrity/authentication tag of Phalcon's authenticated-encryption scheme. A successful timing attack (Keyczar/CVE-2009-0654-style: fix the IV+ciphertext so the target tag is constant, then recover it byte-by-byte from response timing) yields a tag the attacker can attach to a chosen IV+ciphertext so that decrypt() accepts it as authentic, defeating the integrity guarantee. Combined with CFB malleability (flipping a ciphertext byte flips the corresponding plaintext byte), an attacker who recovers the forging capability can tamper with the decrypted contents the application trusts (e.g. encrypted cookies carrying authorization/identity state). There is no confidentiality break by itself.