pem格式（现在实际的pem已经消失了）大部分（包括这里）是用base64编码的，带有换行符和 ----BEGIN x----- 以及 -----END x----- 添加行；看到了吗https://en.wikipedia.org/wiki/privacy-enhanced_mail .
bouncy提供了一组处理pem的相关类：

org.bouncycastle.util.io.pem.PemObject obj = new org.bouncycastle.util.io.pem.PemReader(reader).readPemObject();
// where reader is a java.io.Reader that reads the PEM such as a FileReader on a file 
if( !obj.getType().equals("PKCS7") ) throw error; // or maybe CMS -- optional 
... CMSEnvelopedDataParser (obj.getContent()) and on from there

或者你自己做的q&d方法是这样的：

StringBuilder b64 = new StringBuilder();
try( Reader rdr = new BufferedReader (new FileReader (filename)) ){
    for( String line; (line = rdr.readLine()) != null; )
        if( !line.startsWith ("-----") ) b64.append (line);
        // else optionally check for errors?
} // or use a Stream/filter/collect if you prefer 
byte[] der = Base64.getDecoder().decode(pem);
... CMSEnvelopedDataParser (der) and on from there