##
## Maximum password lengths vary depending on kernel, hash type, and encoding
##

See https://hashcat.net/faq/lengths

##
## Generic hash modes only support salt lengths up to 256
##

This limitation on salt lengths only affects generic hash modes, such as md5(pass.salt).

Dedicated hash modes allow unlimited salt length support.

##
## File and folder names including UTF-16 characters are not supported
##

UTF-16 is mostly seen on Windows. UTF-8 (as mostly used on Linux and macOS) are fine.

Important: That does not mean UTF-16 file content, which is fully supported.

It only means the filename itself.

##
## Hashing algorithms that internally use UTF-16 characters could in special cases lead to false negatives
##

For optimized kernels only (-O): Since hashcat v6.2.1 there's true UTF16 support for pure kernels:

The UTF-16 conversion implementation used within the kernel code is very elementary and for performance
reasons does not respect all complicated encoding rules required to correctly convert, for instance, ASCII
or UTF-8 to UTF-16LE (or UTF-16BE).

The implementation most likely fails with multi-byte characters, because we basically add a zero byte every
second byte within the kernel conversion code.

Since hashcat v6.2.1 there's true UTF16 support for pure kernels.

##
## The use of --keep-guessing eventually skips reporting duplicate passwords
##

This does not mean that valid passwords are skipped; they are always reported.

Only if you hit the same password twice for the same hash the password may be shown only once.

If --keep-guessing is not used, this can not occur.

This limitation cannot be fixed, because it would require too much device (GPU/CPU) memory.

If we wanted to report back all possible password candidates executed in a single kernel invocation, it would require this much memory:

Number-of-MCU * Max-threads-per-device * Max-accel * Max-inner-loops * sizeof (plain_t)

For example, on a Vega64: 64 * 512 * 1024 * 1024 * 20 = 687,194,767,360 bytes

##
## Hashcat GPU memory usage may be limited by maximum allocation sizes of OpenCL drivers
##

Most hashcat hash modes only use a single OpenCL allocation.

The size of this allocation is limited by GPU drivers / OpenCL runtimes.

Only a few modes (like scrypt) make more than one allocation.

##
## The maximum number of functions per rule is limited to 31
##

This makes the size of one rule 128 byte.

On the other hand, there is a 25% OpenCL single allocation memory limit.

A typical GPU of today has 8GB = 2GB/128 = 16M rules max

If hashcat supported more functions per rule, it would be limited to fewer rules.

This is a trade-off game.

##
## Position identifiers in rules are limited to 36
##

The upper limit of maximum 36 positions for various rule functions (0-9, A-Z) was a design decision by the original authors of the rule engine.