bpo-42981: Clarify error messages raised by _hashlib_scrypt_impl()

[illia-v]

The changes clarify that INT_MAX is still a valid value for maxmem and dklen, and that 1 is not a valid value for n despite it is a power of 2.

https://bugs.python.org/issue42981

[tiran]

In my opinion it does not make any sense to make the error messages more explicit. hashlib.scrypt() is a low-level interface. I expect that users are somewhat familiar with the algorithm. The additional checks are primarily to avoid overflows in C.

n=1 is nonsensical. Any value remotely close to INT_MAX is nonsensical. scrypt with n=2, r=1, p=INT_MAX would require 500 GB (!) of virtual memory.

[illia-v]

I see your point.

I expect that users are somewhat familiar with the algorithm.

Your expectations are reasonable, but people may start exploring the algorithm from calling the Python function. I did so and was confused when it failed with "n must be a power of 2" after I set n to 1.

scrypt with n=2, r=1, p=INT_MAX would require 500 GB (!) of virtual memory.

With INT_MAX equal to 32767, it fits the default 32 MiB limit. If it is 2147483647 or 9223372036854775807, the parameters look invalid.
RFC 7914 says that "p is a positive integer less than or equal to ((2^32-1) * 32) / (128 * r)." That means that 1073741823 is the larges possible p and the number is twice smaller than 2147483647, is not it?
Anyway, I didn't change any checks related to p.

[tiran]

The value of INT_MAX depends on OS and platform. It's much larger on 64bit machines:

>>> import _testcapi
>>> _testcapi.INT_MAX
2147483647

[illia-v]

The value of INT_MAX depends on OS and platform.

I looked at OpenSSL's code to find whether max values of maxmem and dklen in their implementation depend on a platform too. It looks like they do, but are SIZE_MAX that is much bigger.
Password and salt length seems to be limited to SIZE_MAX too.

https://github.com/openssl/openssl/blob/90cebd1b216e0a160fcfd8e0eddca47dad47c183/crypto/evp/pbe_scrypt.c#L158-L161
https://github.com/openssl/openssl/blob/90cebd1b216e0a160fcfd8e0eddca47dad47c183/crypto/evp/pbe_scrypt.c#L227-L229