Provide the ability to set password hash algorithm parameters (#22942)

This PR refactors and improves the password hashing code within gitea
and makes it possible for server administrators to set the password
hashing parameters

In addition it takes the opportunity to adjust the settings for `pbkdf2`
in order to make the hashing a little stronger.

The majority of this work was inspired by PR #14751 and I would like to
thank @boppy for their work on this.

Thanks to @gusted for the suggestion to adjust the `pbkdf2` hashing
parameters.

Close #14751

---------

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/auth/password/hash/pbkdf2.go

@@ -0,0 +1,67 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package hash
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"strings"
+
+	"code.gitea.io/gitea/modules/log"
+
+	"golang.org/x/crypto/pbkdf2"
+)
+
+func init() {
+	Register("pbkdf2", NewPBKDF2Hasher)
+}
+
+// PBKDF2Hasher implements PasswordHasher
+// and uses the PBKDF2 key derivation function.
+type PBKDF2Hasher struct {
+	iter, keyLen int
+}
+
+// HashWithSaltBytes a provided password and salt
+func (hasher *PBKDF2Hasher) HashWithSaltBytes(password string, salt []byte) string {
+	if hasher == nil {
+		return ""
+	}
+	return hex.EncodeToString(pbkdf2.Key([]byte(password), salt, hasher.iter, hasher.keyLen, sha256.New))
+}
+
+// NewPBKDF2Hasher is a factory method to create an PBKDF2Hasher
+// config should be either empty or of the form:
+// "<iter>$<keyLen>", where <x> is the string representation
+// of an integer
+func NewPBKDF2Hasher(config string) *PBKDF2Hasher {
+	// This default configuration uses the following parameters:
+	// iter=10000, keyLen=50.
+	// This matches the original configuration for `pbkdf2` prior to storing parameters
+	// in the database.
+	// THESE VALUES MUST NOT BE CHANGED OR BACKWARDS COMPATIBILITY WILL BREAK
+	hasher := &PBKDF2Hasher{
+		iter:   10_000,
+		keyLen: 50,
+	}
+
+	if config == "" {
+		return hasher
+	}
+
+	vals := strings.SplitN(config, "$", 2)
+	if len(vals) != 2 {
+		log.Error("invalid pbkdf2 hash spec %s", config)
+		return nil
+	}
+
+	var err error
+	hasher.iter, err = parseIntParam(vals[0], "iter", "pbkdf2", config, nil)
+	hasher.keyLen, err = parseIntParam(vals[1], "keyLen", "pbkdf2", config, err)
+	if err != nil {
+		return nil
+	}
+
+	return hasher
+}