TokenValidator.cs

// Microsoft.IdentityModel.Protocols.OpenIdConnect
// System.IdentityModel.Tokens.Jwt

/// <summary>
/// Token validator
/// </summary>
public class TokenValidator
{
    private readonly TokenValidationParameters _tokenValidationParameters;

    /// <summary>
    /// Validate authenticated user/application using open ID connection configuration
    /// </summary>
    /// <param name="authority">The authority</param>
    /// <param name="clientId">The client ID</param>
    public TokenValidator(string authority, string clientId)
    {
        if (string.IsNullOrWhiteSpace(authority)) throw new ArgumentNullException(nameof(authority));
        if (string.IsNullOrWhiteSpace(clientId)) throw new ArgumentNullException(nameof(clientId));

        var configurationManager = new ConfigurationManager<OpenIdConnectConfiguration>(authority + "/.well-known/openid-configuration", new OpenIdConnectConfigurationRetriever());
        var configuration = AsyncHelper.RunSync(async () => await configurationManager.GetConfigurationAsync(CancellationToken.None));

        _tokenValidationParameters =
            new TokenValidationParameters
            {
                ValidAudience = clientId,
                ValidIssuer = configuration.Issuer,
                IssuerSigningKeys = configuration.SigningKeys,
                ValidateIssuerSigningKey = true,
            };
    }

    /// <summary>
    /// Validate token
    /// </summary>
    /// <param name="token"></param>
    /// <returns></returns>
    public TokenValidationResult ValidateToken(string token)
    {
        var claimsPrincipal = new JwtSecurityTokenHandler().ValidateToken(token, _tokenValidationParameters, out SecurityToken securityToken);

        return new TokenValidationResult(claimsPrincipal, securityToken);
    }
}