var certificate = FindCertificate(cert.Thumbprint, X509FindType.FindByThumbprint);
var password = Encoding.Unicode.GetString(Decrypt(Convert.FromBase64String(cert.Password)));
var signingCert = new X509Certificate2(certificate.RawData, password); // password - пароль к eToken
var content = new ContentInfo(data);
var signedMessage = new SignedCms(content, true);
var signer = new CmsSigner(signingCert);
signedMessage.ComputeSignature(signer); // вот тут валится исключение
var signedBytes = signedMessage.Encode();
return signedBytes;

System.Security.Cryptography.CryptographicException: Key does not exist.

   at System.Security.Cryptography.Pkcs.SignedCms.ComputeSignature(CmsSigner signer, Boolean silent)
   at System.Security.Cryptography.Pkcs.SignedCms.ComputeSignature(CmsSigner signer)

var password = "1q2w3e4r";
var pars = new CspParameters(75, null, "RaUser-a9832a39-661c-46c2-b805-30591a997633");
pars.Flags = CspProviderFlags.NoFlags;
var s = new SecureString();
foreach (var c in password)
{
s.AppendChar(c);
}
pars.KeyPassword = s;
var content = new ContentInfo(data);
var signedMessage = new SignedCms(content, true);
var signer = new CmsSigner(pars); // вот тут опять валится то же самое исключение