Scrypt is a hashing algorithm first published in 2009 to address the security of passwords stored in a database. Generating such a hash is computationally intensive, meaning it takes a "long" time to generate a hash. This is fast enough for the sign-in process but makes it extremely costly to attempt brute-force to crack an existing password's hash.

SCrypt Modified is a fork of this algorithm. From its documentation, the intentions of the fork are unclear.

While official SCrypt takes password and salt to generate the hash, modified has a few additional steps:

1. Generate derivedKey by creating a Scrypt hash where password is utf8-encoded, and salt is base64-decoded salt + base64-decoded salt separator
2. Generate hash by encrypting base64-decoded signerKey with aes-256-ctr where key is the first 32 characters of derivedKey, while using an empty initialization vector of length 16
3. Final hash must be base64-encoded

You can find the Appwrite implementation of this algorithm in Scryptmodified.php.

You can also find more information about this Scrypt modified fork here.