public class JWTSettings
    {
        public string Private_Key { get; set; } // это мой приватный ключ для подписи токенов
        public double AccessToken_LifeTime { get; set; }
        public double RefreshToken_LifeTime { get; set; }
        public string Issuer { get; set; }
    }

$ openssl genrsa -out jwtRS256.key 4096
$ openssl rsa -in jwtRS256.key -pubout -out jwtRS256.key.pub

        public void CreateTokensV2()
        {  
...
            var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_settings.Key)); 

            var creds = new SigningCredentials(key, SecurityAlgorithms.HmacSha256);
...
         }

        public static void ValidateTokenAsync(string token, string key)
        {
            try
            {
                new JwtSecurityTokenHandler().ValidateToken(token, 
                    GetTokenValidationParameters(key),  out var securityToken);
            }
            catch (SecurityTokenExpiredException)
            {
                throw new MyException(new ErrorCodes().TOKEN_EXPIRED);
            }
.....
         }

        private static TokenValidationParameters GetTokenValidationParameters(string key)
        {

            return new TokenValidationParameters
            {
                ValidateIssuerSigningKey = true,                
                IssuerSigningKey = new  SymmetricSecurityKey(Encoding.UTF8.GetBytes(key)), 

                ValidateIssuer = true,
                ValidIssuer = settings.Issuer,

                ValidateAudience = false,

                ValidateLifetime = true,

                ClockSkew = TimeSpan.Zero
            };
        }

      
            RSACryptoServiceProvider publicAndPrivate = new RSACryptoServiceProvider();
            publicAndPrivate.ImportParameters(rsaParams_Private);

            var expiresAccessToken = DateTime.UtcNow.AddMinutes(_settings.AccessTokenLifeTime);

            JwtSecurityToken jwtToken_Access = new JwtSecurityToken(
                audience: null,
                issuer: _settings.Issuer,
                claims: new List<Claim>() {
                        new Claim(JwtRegisteredClaimNames.Jti, GuidAccessToken.ToString()),
                        new Claim(JwtRegisteredClaimNames.Typ, TokenTypes.Access.ToString()),
                        new Claim(JwtRegisteredClaimNames.Exp, expiresAccessToken.ToString()),
                        new Claim(JwtRegisteredClaimNames.UniqueName, "12345")
                },
                lifetime: new Lifetime(DateTime.UtcNow, DateTime.UtcNow.AddHours(1)),
                signingCredentials: new SigningCredentials(new RsaSecurityKey(publicAndPrivate)
                                                             , SecurityAlgorithms.RsaSha256));

            JwtSecurityTokenHandler tokenHandler_Access = new JwtSecurityTokenHandler();

            string tokenString = tokenHandler_Access.WriteToken(jwtToken_Access);

            Console.WriteLine("Token string: {0}", tokenString);