using frombase64transform cryptostream?

i have .net twine base64 encoded illustration an array encrypted bytes. assembled this:

string encrypt( twine summary )
{
    cryptostream cryptostream = ...
    streamwriter tide = new streamwriter( cryptostream );
    ...
    relapse convert.tobase64string( ... );
}

now i wish decryption duty like

string decrypt( twine cypher )
{
    textreader reader = new stringreader( cypher );
    byte[] aegis = new byte[ cypher.length ];
    for( int i = 0; i < cypher.length; ++i )
    {
        buffer[ i ] = (byte) reader.read();
    }
    frombase64transform transformer = new frombase64transform();
    memorystream tender = new memorystream
    ( 
        transformer.transformfinalblock( buffer, 0, buffer.length )
    );
    ...
}

is there proceed frombase64transform directly cryptostream (as .net support suggests), instead manually converting twine bytes, following manually decoding bytes, finally decrypting decoded bytes?