You don't need any ASP.NET Core stuff. A simple approach would be:
Nu-get the Packages
System.IdentityModel.Tokens.Jwt,
Microsoft.IdentityModel.Tokens
Set up some validation parameters:
var validationParameters = new TokenValidationParameters
{
RequireExpirationTime = true,
ValidateLifetime = true,
IssuerSigningKeys = keys, // Your public keys.
ValidAudience = "my valid audience",
ValidIssuer = "my valid issuer"
}
Call ValidateToken to get a ClaimsPrincipal with claims and stuff.
token is your JWT string, e.g. parsed from Authorization HTTP header.
var handler = new JwtSecurityTokenHandler();
handler.ValidateToken(token, validationParameters, out SecurityToken validatedToken);
Using JsonWebKeySet from the above IdentityModel.Tokens package, you can automagically obtain keys from an OpenID Connect configuration:
https://github.com/IdentityModel/IdentityModel/blob/master/source/IdentityModel.Shared/Jwt/JsonWebKeySet.cs