Restructure solution layout by module

src/__Libraries/StellaOps.Cryptography/PasswordHashing.cs

@@ -0,0 +1,81 @@
+using System;
+
+namespace StellaOps.Cryptography;
+
+/// <summary>
+/// Supported password hashing algorithms.
+/// </summary>
+public enum PasswordHashAlgorithm
+{
+    Argon2id,
+    Pbkdf2
+}
+
+/// <summary>
+/// Options describing password hashing requirements.
+/// Values follow OWASP baseline guidance by default.
+/// </summary>
+public sealed record PasswordHashOptions
+{
+    /// <summary>
+    /// Algorithm to use when hashing new passwords.
+    /// </summary>
+    public PasswordHashAlgorithm Algorithm { get; init; } = PasswordHashAlgorithm.Argon2id;
+
+    /// <summary>
+    /// Memory cost in KiB (default 19 MiB).
+    /// </summary>
+    public int MemorySizeInKib { get; init; } = 19 * 1024;
+
+    /// <summary>
+    /// Iteration count / time cost.
+    /// </summary>
+    public int Iterations { get; init; } = 2;
+
+    /// <summary>
+    /// Parallelism / degree of concurrency.
+    /// </summary>
+    public int Parallelism { get; init; } = 1;
+
+    /// <summary>
+    /// Validates the option values and throws when invalid.
+    /// </summary>
+    public void Validate()
+    {
+        if (MemorySizeInKib <= 0)
+        {
+            throw new InvalidOperationException("Password hashing memory cost must be greater than zero.");
+        }
+
+        if (Iterations <= 0)
+        {
+            throw new InvalidOperationException("Password hashing iteration count must be greater than zero.");
+        }
+
+        if (Parallelism <= 0)
+        {
+            throw new InvalidOperationException("Password hashing parallelism must be greater than zero.");
+        }
+    }
+}
+
+/// <summary>
+/// Abstraction for password hashing implementations.
+/// </summary>
+public interface IPasswordHasher
+{
+    /// <summary>
+    /// Produces an encoded hash for the supplied password.
+    /// </summary>
+    string Hash(string password, PasswordHashOptions options);
+
+    /// <summary>
+    /// Verifies the supplied password against a stored hash.
+    /// </summary>
+    bool Verify(string password, string encodedHash);
+
+    /// <summary>
+    /// Detects when an existing encoded hash no longer satisfies the desired options.
+    /// </summary>
+    bool NeedsRehash(string encodedHash, PasswordHashOptions desired);
+}