This article will describe the implementation of functions to generate and validate tokens used to verify email addresses and reset passwords. I will assume you have downloaded the ASP.NET Core 3.1 - Users Without Identity Project or created a new ASP.NET Core 3.1 Razor Pages project. See Tutorial: Get started with Razor Pages in ASP.NET Core. You should review the earlier articles of the Users Without Identity Project series.

UWIP v2 implements a TokenService class to generate and validate email confirmation and password reset tokens based on the SecurityStamp. The TokenService employs a ServiceCollection extension and options to override the default values for the length of time the token is valid.

Services > Utilities > TokenService.cs:

// Copyright © Ken Haggerty (https://kenhaggerty.com)
// Licensed under the MIT License.
public interface ITokenService
{
    /// <summary>
    /// Generates a new security stamp.
    /// </summary>
    /// <returns><see cref="string" /></returns>
    string GenerateSecurityStamp();

    /// <summary>
    /// Generates a new Base64Url encoded email confirmation token for the <paramref name="appUser" />.
    /// </summary>
    /// <param name="appUser"></param>
    /// <returns><see cref="Task{TResult}" /> for <see cref="string" /></returns>
    /// <remarks>
    /// Serializes the <see cref="AppUser.Id" />, <see cref="AppUser.SecurityStamp" />,
    /// <see cref="TokenService.EmailConfirmationPurpose" />, and an expiration date time based on
    /// <see cref="TokenServiceOptions.EmailConfirmationLifeTimeMinutes" />.
    /// </remarks>
    Task<string> GenerateEmailConfirmationTokenAsync(AppUser appUser);

    /// <summary>
    /// Decodes and verifies the <paramref name="token" /> for the <paramref name="appUser" />
    /// </summary>
    /// <param name="appUser"></param>
    /// <param name="token"></param>
    /// <returns><see cref="Task{TResult}" /> for <see cref="bool" /></returns>
    /// <remarks>
    /// Deserializes the provided token and validates the <see cref="AppUser.Id" />,
    /// <see cref="AppUser.SecurityStamp" />, <see cref="TokenService.EmailConfirmationPurpose" />,
    /// and expiration date.
    /// </remarks>
    Task<bool> ValidateEmailConfirmationTokenAsync(AppUser appUser, string token);

    /// <summary>
    /// Generates a new Base64Url encoded password reset token for the <paramref name="appUser" />.
    /// </summary>
    /// <param name="appUser"></param>
    /// <returns><see cref="Task{TResult}" /> for <see cref="string" /></returns>
    /// <remarks>
    /// Serializes the <see cref="AppUser.Id" />, <see cref="AppUser.SecurityStamp" />,
    /// <see cref="TokenService.PasswordResetPurpose" />, and an expiration date time based on
    /// <see cref="TokenServiceOptions.PasswordResetLifeTimeMinutes" />.
    /// </remarks>
    Task<string> GeneratePasswordResetTokenAsync(AppUser appUser);

    /// <summary>
    /// Decodes and verifies the <paramref name="token" /> for the <paramref name="appUser" />
    /// </summary>
    /// <param name="appUser"></param>
    /// <param name="token"></param>
    /// <returns><see cref="Task{TResult}" /> for <see cref="bool" /></returns>
    /// <remarks>
    /// Deserializes the provided token and validates the <see cref="AppUser.Id" />,
    /// <see cref="AppUser.SecurityStamp" />, <see cref="TokenService.PasswordResetPurpose" />,
    /// and expiration date.
    /// </remarks>
    Task<bool> ValidatePasswordResetTokenAsync(AppUser appUser, string token);
}

public class TokenService : ITokenService
{
    private readonly string EmailConfirmationPurpose = "EmailConfirmation";
    private readonly string PasswordResetPurpose = "PasswordReset";

    private readonly int _emailConfirmationLifeTimeMinutes;
    private readonly int _passwordResetLifeTimeMinutes;
    public TokenService(IOptions<TokenServiceOptions> options)
    {
        _emailConfirmationLifeTimeMinutes = options.Value.EmailConfirmationLifeTimeMinutes;
        _passwordResetLifeTimeMinutes = options.Value.PasswordResetLifeTimeMinutes;
    }

    public string GenerateSecurityStamp()
    {
        byte[] bytes = new byte[20];
        RandomNumberGenerator.Fill(bytes);
        return Base32.ToBase32(bytes);
    }

    public async Task<string> GenerateEmailConfirmationTokenAsync(AppUser appUser)
    {
        var token = await GenerateTokenAsync(appUser, EmailConfirmationPurpose, EmailConfirmationLifeTimeMinutes)
            .ConfigureAwait(false);
        return WebEncoders.Base64UrlEncode(Encoding.UTF8.GetBytes(token));
    }

    public async Task<bool> ValidateEmailConfirmationTokenAsync(AppUser appUser, string token)
    {
        token = Encoding.UTF8.GetString(WebEncoders.Base64UrlDecode(token));
        return await ValidateTokenAsync(appUser, EmailConfirmationPurpose, token).ConfigureAwait(false);
    }

    public async Task<string> GeneratePasswordResetTokenAsync(AppUser appUser)
    {
        var token = await GenerateTokenAsync(appUser, PasswordResetPurpose, PasswordResetLifeTimeMinutes)
            .ConfigureAwait(false);
        return WebEncoders.Base64UrlEncode(Encoding.UTF8.GetBytes(token));
    }

    public async Task<bool> ValidatePasswordResetTokenAsync(AppUser appUser, string token)
    {
        token = Encoding.UTF8.GetString(WebEncoders.Base64UrlDecode(token));
        return await ValidateTokenAsync(appUser, PasswordResetPurpose, token).ConfigureAwait(false);
    }

    private static async Task<string> GenerateTokenAsync(AppUser appUser, string purpose, int lifeTimeMinutes)
    {
        if (appUser == null) throw new ArgumentNullException(nameof(appUser));

        var expirationDate = DateTimeOffset.UtcNow + TimeSpan.FromMinutes(lifeTimeMinutes);

        using MemoryStream memoryStream = new MemoryStream();
        using (StreamWriter streamWriter = new StreamWriter(memoryStream))
        {
            await streamWriter.WriteLineAsync(expirationDate.ToString()).ConfigureAwait(false);
            await streamWriter.WriteLineAsync(appUser.Id.ToString()).ConfigureAwait(false);
            await streamWriter.WriteLineAsync(appUser.SecurityStamp).ConfigureAwait(false);
            await streamWriter.WriteLineAsync(purpose).ConfigureAwait(false);
        }
        return Convert.ToBase64String(memoryStream.ToArray());
    }

    private static async Task<bool> ValidateTokenAsync(AppUser appUser, string purpose, string token)
    {
        try
        {
            using MemoryStream memoryStream = new MemoryStream(Convert.FromBase64String(token));
            using StreamReader streamReader = new StreamReader(memoryStream);

            var expirationDateString = await streamReader.ReadLineAsync().ConfigureAwait(false);
            var expirationDate = DateTimeOffset.Parse(expirationDateString);
            if (expirationDate < DateTimeOffset.UtcNow) return false;

            var readerAppUserId = await streamReader.ReadLineAsync().ConfigureAwait(false);
            var compareAppUserId = int.Parse(readerAppUserId);
            if (compareAppUserId != appUser.Id) return false;

            var securityStamp = await streamReader.ReadLineAsync().ConfigureAwait(false);
            if (!string.Equals(securityStamp, appUser.SecurityStamp)) return false;

            var readerPurpose = await streamReader.ReadLineAsync().ConfigureAwait(false);
            if (!string.Equals(readerPurpose, purpose)) return false;

            if (!streamReader.EndOfStream)return false;

            return true;
        }
        catch
        {
            // Do not leak exception
        }
        return false;
    }
}

Entities > TokenServiceOptions.cs:

// Copyright © Ken Haggerty (https://kenhaggerty.com)
// Licensed under the MIT License.
 public class TokenServiceOptions
{
    /// <summary>
    /// The number of minutes a new email confirmation token is valid.
    /// </summary>
    public int EmailConfirmationLifeTimeMinutes { get; set; } = 240;
    /// <summary>
    /// The number of minutes a new password reset token is valid.
    /// </summary>
    public int PasswordResetLifeTimeMinutes { get; set; } = 60;
}

Services > Extensions > TokenServiceExtension.cs:

// Copyright © Ken Haggerty (https://kenhaggerty.com)
// Licensed under the MIT License.
public static class TokenServiceExtension
{
    /// <summary>
    /// Adds a singleton service for email confirmation and password reset tokens to the specified <see cref="IServiceCollection" />.
    /// </summary>
    /// <param name="serviceCollection"><see cref="IServiceCollection" /></param>
    /// <param name="options"><see cref="TokenServiceOptions" /></param>
    /// <returns>
    /// The <see cref="IServiceCollection" /> so that additional calls can be chained.
    /// </returns>
    /// <remarks>
    /// TokenServiceOptions defaults: EmailConfirmationLifeTimeMinutes = 240, PasswordResetLifeTimeMinutes = 60
    /// </remarks>
    public static IServiceCollection AddTokenService(this IServiceCollection serviceCollection,
        Action<TokenServiceOptions> options = null)
    {
        serviceCollection.AddSingleton<ITokenService, TokenService>();

        if(options != null)
            serviceCollection.Configure(options);

        return serviceCollection;
    }
}

The TokenServiceExtension is employed in Startup. ConfigureServices.

Startup > ConfigureServices:

services.AddTokenService(options =>
{
    options.EmailConfirmationLifeTimeMinutes = 120;
});

Update 02/23/2021

I updated the article links.