// Verification method
public static string VerificationCodeCacheFormat = "vcode_cache_{0}";

public IActionResult GenerateVerificationCode()
{
    var verificationService = new VerificationCodeService();
    string code = "";
    using (var memoryStream = verificationService.CreateVerificationImage(out code))
    {
        code = code.ToLower(); // Case-insensitive verification code
        var token = Guid.NewGuid().ToString();
        var cacheKey = string.Format(VerificationCodeCacheFormat, token);
        _memoryCache.Set(cacheKey, code, new MemoryCacheEntryOptions()
            .SetSlidingExpiration(TimeSpan.FromMinutes(20)));
        
        Response.Cookies.Append("verification_token", token);
        return File(memoryStream.ToArray(), "image/png");
    }
}

// Backend validation method
public bool ValidateUserInput(string userToken, string userInput)
{
    var cacheKey = string.Format(VerificationCodeCacheFormat, userToken);
    string storedCode = "";
    
    if (!_memoryCache.TryGetValue(cacheKey, out storedCode))
        return false;
    
    if (storedCode.ToLower() != userInput.ToLower())
        return false;
    
    _memoryCache.Remove(cacheKey);
    return true;
}

Generating verifiaction codes and images

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Linq;
using System.Threading.Tasks;

namespace Security.Services
{
    /// <summary>
    /// Image verification code generator
    /// </summary>
    public class VerificationCodeService
    {
        /// <summary>
        /// Generate a random string of specified length
        /// </summary>
        /// <param name="length">Length of the string</param>
        /// <returns>Random alphanumeric string</returns>
        private string GenerateRandomString(int length)
        {
            // Character set: 0-9, a-z, A-Z
            string chars = "0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,i,j,k,l,m,n,p,q,r,s,t,u,v,w,x,y,z,A,B,C,D,E,F,G,H,I,J,K,L,M,N,P,Q,R,S,T,U,V,W,X,Y,Z";
            
            string[] charArray = chars.Split(',');
            string code = "";
            int previousValue = -1;
            Random random = new Random();
            
            for (int i = 1; i <= length; i++)
            {
                if (previousValue != -1)
                {
                    random = new Random(i * previousValue * unchecked((int)DateTime.Now.Ticks));
                }
                
                int currentValue = random.Next(61);
                
                if (previousValue == currentValue)
                {
                    return GenerateRandomString(length); // Recursively generate if duplicate
                }
                
                previousValue = currentValue;
                code += charArray[currentValue];
            }
            
            return code;
        }

        /// <summary>
        /// Create verification image from string
        /// </summary>
        /// <param name="code">Verification code string</param>
        /// <param name="length">Number of characters (default 4)</param>
        /// <returns>Memory stream containing PNG image</returns>
        public MemoryStream CreateVerificationImage(out string code, int length = 4)
        {
            code = GenerateRandomString(length);
            Bitmap image = null;
            Graphics graphics = null;
            MemoryStream memoryStream = null;
            Random random = new Random();
            
            // Color palette
            Color[] colors = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
            // Font collection
            string[] fonts = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "SimSun" };
            
            // Create image
            image = new Bitmap((int)code.Length * 18, 32);
            graphics = Graphics.FromImage(image);
            graphics.Clear(Color.White);
            
            // Draw background noise
            for (int i = 0; i < 100; i++)
            {
                int x = random.Next(image.Width);
                int y = random.Next(image.Height);
                graphics.DrawRectangle(new Pen(Color.LightGray, 0), x, y, 1, 1);
            }
            
            // Draw verification code
            for (int i = 0; i < code.Length; i++)
            {
                int colorIndex = random.Next(7);
                int fontIndex = random.Next(4);
                Font font = new Font(fonts[fontIndex], 15, FontStyle.Bold);
                Brush brush = new SolidBrush(colors[colorIndex]);
                int yPosition = 4;
                
                if ((i + 1) % 2 == 0)
                {
                    yPosition = 2;
                }
                
                graphics.DrawString(code.Substring(i, 1), font, brush, 3 + (i * 12), yPosition);
            }
            
            memoryStream = new MemoryStream();
            image.Save(memoryStream, ImageFormat.Png);
            graphics.Dispose();
            image.Dispose();
            
            return memoryStream;
        }
    }
}

The above implementation works for a single vreification code per page. Here's an improved version that supports multiple verfiication codes:

// Modified GenerateVerificationCode method to support multiple codes on a page
public IActionResult GenerateVerificationCode(string identifier = "")
{
    var verificationService = new VerificationCodeService();
    string code = "";
    using (var memoryStream = verificationService.CreateVerificationImage(out code))
    {
        code = code.ToLower();
        var token = Guid.NewGuid().ToString();
        var cacheKey = string.Format(VerificationCodeCacheFormat, token);
        _memoryCache.Set(cacheKey, code, new MemoryCacheEntryOptions()
            .SetSlidingExpiration(TimeSpan.FromMinutes(20)));
        
        Response.Cookies.Append($"verification_token_{identifier}", token);
        return File(memoryStream.ToArray(), "image/png");
    }
}

Single verification code on a page:

<p>
    Please enter the verification code: <br>
    <input type="text" id="verificationCode" class="form-control" />
    <img id="verificationImage" src="~/Home/GenerateVerificationCode" alt="Can't see clearly? Click to refresh" 
         onclick="this.src = this.src + '?'" style="vertical-align:middle;" />
</p>

Multiple verification codes on a page:

<p>
    <input name="verification_code" type="text" maxlength="5" id="otherCode" class="form-control tbText">
    <img id="otherVerificationImage" src="~/Home/GenerateVerificationCode?identifier=other" alt="Can't see clearly? Click to refresh" 
         onclick="this.src = 'GenerateVerificationCode?identifier=other&' + Math.random()" style="vertical-align:middle;" />
</p>