To check an RSA1024-SHA1 signature of a string using a certificate (.crt file), you need to follow these steps:

Load the certificate file using X509Certificate2 class:

var certificate = new X509Certificate2("certificate.crt");

Decode the signature from Base64 string to byte array:

byte[] signatureBytes = Convert.FromBase64String(signatureBase64);

Get the RSA public key from the certificate:

var rsaPublicKey = (RSACryptoServiceProvider)certificate.PublicKey.Key;

Compute the SHA1 hash of the input string:

var sha1 = SHA1.Create();
byte[] hash = sha1.ComputeHash(Encoding.UTF8.GetBytes(inputString));

Verify the signature using the RSACryptoServiceProvider class:

bool isSignatureValid = rsaPublicKey.VerifyHash(hash, CryptoConfig.MapNameToOID("SHA1"), signatureBytes);

The VerifyHash method checks whether the specified signature matches the hash of the input string using the specified hash algorithm (in this case, SHA1). If the signature is valid, the method returns true; otherwise, it returns false. Here's the complete code:

var certificate = new X509Certificate2("certificate.crt");
byte[] signatureBytes = Convert.FromBase64String(signatureBase64);
var rsaPublicKey = (RSACryptoServiceProvider)certificate.PublicKey.Key;
var sha1 = SHA1.Create();
byte[] hash = sha1.ComputeHash(Encoding.UTF8.GetBytes(inputString));
bool isSignatureValid = rsaPublicKey.VerifyHash(hash, CryptoConfig.MapNameToOID("SHA1"), signatureBytes);

Replace certificate.crt with the path to your certificate file, and signatureBase64 with the Base64-encoded signature string. Also, replace inputString with the string you want to verify the signature for.