the-biergarten-app/web/backend/Infrastructure/Infrastructure.Jwt/JwtInfrastructure.cs

using System.Security.Claims;
using System.Text;
using Microsoft.IdentityModel.JsonWebTokens;
using Microsoft.IdentityModel.Tokens;
using JwtRegisteredClaimNames = System.IdentityModel.Tokens.Jwt.JwtRegisteredClaimNames;
using Domain.Exceptions;

namespace Infrastructure.Jwt;

/// <summary>
/// Generates and validates HMAC-SHA256 signed JWTs using <see cref="JsonWebTokenHandler"/>.
/// </summary>
public class JwtInfrastructure : ITokenInfrastructure
{
    /// <summary>
    /// Generates a signed JWT containing the user's ID, username, issued-at time, expiry time,
    /// and a unique token identifier (JTI), signed using HMAC-SHA256 with the provided secret.
    /// </summary>
    /// <remarks>
    /// Sets the following registered claims: <c>sub</c> (userId), <c>unique_name</c> (username),
    /// <c>iat</c> (current UTC time), <c>exp</c> (expiry), and <c>jti</c> (a newly generated GUID).
    /// </remarks>
    /// <param name="userId">The unique identifier of the user the token is issued for.</param>
    /// <param name="username">The username of the user, included as a claim.</param>
    /// <param name="expiry">The date and time at which the token expires.</param>
    /// <param name="secret">The symmetric secret used to sign the token (encoded as UTF-8 bytes).</param>
    /// <returns>The serialized, signed JWT string.</returns>
    public string GenerateJwt(
        Guid userId,
        string username,
        DateTime expiry,
        string secret
    )
    {
        var handler = new JsonWebTokenHandler();
        var key = Encoding.UTF8.GetBytes(secret);
        var claims = new List<Claim>
        {
            new(JwtRegisteredClaimNames.Sub, userId.ToString()),
            new(JwtRegisteredClaimNames.UniqueName, username),
            new(
                JwtRegisteredClaimNames.Iat,
                DateTimeOffset.UtcNow.ToUnixTimeSeconds().ToString()
            ),
            new(
                JwtRegisteredClaimNames.Exp,
                new DateTimeOffset(expiry).ToUnixTimeSeconds().ToString()
            ),
            new(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString()),
        };

        var tokenDescriptor = new SecurityTokenDescriptor
        {
            Subject = new ClaimsIdentity(claims),
            Expires = expiry,
            SigningCredentials = new SigningCredentials(
                new SymmetricSecurityKey(key),
                SecurityAlgorithms.HmacSha256
            ),
        };

        return handler.CreateToken(tokenDescriptor);
    }


    /// <summary>
    /// Validates a JWT's signature and lifetime (issuer and audience validation are disabled),
    /// using the provided secret as the HMAC-SHA256 symmetric signing key.
    /// </summary>
    /// <param name="token">The JWT string to validate.</param>
    /// <param name="secret">The symmetric secret used to verify the token's signature (encoded as UTF-8 bytes).</param>
    /// <returns>A <see cref="ClaimsPrincipal"/> wrapping the validated token's claims identity.</returns>
    /// <exception cref="UnauthorizedException">
    /// Thrown when the token is invalid, has no claims identity, is expired, or otherwise fails validation
    /// (including when validation itself throws, e.g. due to a malformed token or signature mismatch).
    /// </exception>
    public async Task<ClaimsPrincipal> ValidateJwtAsync(
        string token,
        string secret
    )
    {
        var handler = new JsonWebTokenHandler();
        var keyBytes = Encoding.UTF8.GetBytes(
            secret
        );
        var parameters = new TokenValidationParameters
        {
            ValidateIssuer = false,
            ValidateAudience = false,
            ValidateLifetime = true,
            IssuerSigningKey = new SymmetricSecurityKey(keyBytes),
        };

        try
        {
            var result = await handler.ValidateTokenAsync(token, parameters);
            if (!result.IsValid || result.ClaimsIdentity == null)
                throw new UnauthorizedAccessException();
            
            return new ClaimsPrincipal(result.ClaimsIdentity);
        }
        catch (Exception e)
        {
            throw new UnauthorizedException("Invalid token");
        }
    }
}