Add pure Go DES crypt(3) password hashing support

[fpesce]

Pull Request: Add Pure Go Traditional DES crypt(3) Support

Target Repository: github.com/GehirnInc/crypt
Base Branch: master
Feature Branch: feature/des-crypt
Author: Francois Pesce
License: BSD-2-Clause

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Title

Add pure Go traditional DES crypt(3) password hashing support

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Description

Summary

This pull request adds support for the classic 13-character UNIX DES-based crypt(3) password hashing algorithm in pure Go.

While GehirnInc/crypt supports MD5, SHA-256, SHA-512, and Apache APR1 password hashing schemes, it currently lacks support for the foundational 13-character DES algorithm. Following the closure and rejection of crypt(3) password hashing proposals in golang.org/x/crypto (Proposal #14274), third-party libraries in the Go ecosystem serve as the primary solution for UNIX credential verification. This PR completes the functional scope of GehirnInc/crypt to provide full POSIX crypt(3) compatibility without Cgo dependencies.

What Changed

1. New des_crypt Subpackage:
   • Pure Go implementation of 25-round DES crypt(3) Feistel cipher based on my (not AI, this was done 7 years ago for a personal project) implementation (github.com/fpesce/go-des-crypt), contributed under BSD 2-Clause.
   • Implements crypt.Crypter interface (Generate, Verify, Cost, SetSalt).
   • Strict 13-character hash validator (Validate(hash string) bool) ensuring standard 64-symbol alphabet (./0-9A-Za-z) and checking the 2-bit zero-mask on the 13th character for 64-bit DES block compliance.
   • Automatic 2-character cryptographically secure random salt generation via crypto/rand when salt is empty.
   • Accepts both 2-character salts and 13-character full hashes as salt arguments (POSIX idiom).
   • Strict POSIX password truncation to 8 bytes and zero-padding for < 8 bytes.
   • Constant-time verification via crypto/subtle.ConstantTimeCompare.
2. Root crypt Package Updates:
   • Added crypt.DES constant to Crypt enum.
   • Added RegisterFallback(c Crypt, f func() Crypter, validator func(string) bool) to support prefixless algorithm registration.
   • Updated NewFromHash() and IsHashSupported() to check modular prefixes first, falling back to registered non-prefix validators (allowing transparent auto-detection of 13-character DES hashes).
3. Tests & Documentation:
   • Exhaustive unit tests with standard test vectors ("foob" / "ar" -> "arlEKn0OzVJn.", "test" / "PQ" -> "PQl1.p7BcJRuM").
   • Integration tests in crypt_test.go verifying auto-detection and API availability.
   • Updated README.rst with algorithm list, examples, and security advisories.

Security Consideration

Traditional DES crypt(3) provides only 56 bits of key strength and is cryptographically obsolete for modern credential storage. This implementation is provided strictly for legacy password verification, retro-computing, and data migration.

Verification & Testing

go test -v -race ./...

All packages pass with 100% test coverage for new components and zero data races.