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Tell me if it works for you.

public string Decrypt(RSACryptoServiceProvider provider, string toDecrypt)
{
    var input = Convert.FromBase64String(toDecrypt);
    IEnumerable<byte> output = new List<byte>();
    for (var i = 0; i < input.Length; i = i + Length)
    {
        var length = Math.Max(input.Length - i, 128);
        var block = new byte[length];
        Buffer.BlockCopy(input, i, block, 0, length);
        var chunk = provider.Decrypt(block, false);
        output = output.Concat(chunk);
    }
    return Encoding.UTF8.GetString(output.ToArray());
}