How to add file name to archive during compression using the Gzip class?

Question

I use Gzip to compress data to encryption.

Gzip gz;
gz.Put(file,size)
gz.MessageEnd();
gz.Get(file,gz.MaxRetrievable());

I want the generated gzip file to include the original file name in the metadata. How to do this using the Crypto ++ interface?

Answer 1

I want the generated gzip file to include the original file name in the metadata. How to do this using the Crypto ++ interface?

You cannot be out of the box. This is apparently a limitation of Crypto ++. (But see below.)

From RFC 1952 there is clearly a field for him:

  (if FLG.FNAME set)

     +=========================================+
     |...original file name, zero-terminated...| (more-->)
     +=========================================+

But Crypto ++ does not allow you to install it (from gzip.c source code ):

void Gzip::WritePrestreamHeader()
{
    m_totalLen = 0;
    m_crc.Restart();

    AttachedTransformation()->Put(MAGIC1);
    AttachedTransformation()->Put(MAGIC2);
    AttachedTransformation()->Put(DEFLATED);
    AttachedTransformation()->Put(0);       // general flag
    AttachedTransformation()->PutWord32(0); // time stamp
    byte extra = (GetDeflateLevel() == 1) ? FAST : ((GetDeflateLevel() == 9) ? SLOW : 0);
    AttachedTransformation()->Put(extra);
    AttachedTransformation()->Put(GZIP_OS_CODE);
}

And Crypto ++ silently discards it if it is present during unpacking ( Gunzip- the GZIP decompressor) (from the source code of gzip.c ):

void Gunzip::ProcessPrestreamHeader()
{
    ...

    if (flags & EXTRA_FIELDS)   // skip extra fields
    {
        word16 length;
        if (m_inQueue.GetWord16(length, LITTLE_ENDIAN_ORDER) != 2) throw HeaderErr();
        if (m_inQueue.Skip(length)!=length) throw HeaderErr();
    }

    if (flags & FILENAME)   // skip filename
        do
            if(!m_inQueue.Get(b)) throw HeaderErr();
        while (b);

    if (flags & COMMENTS)   // skip comments
        do
            if(!m_inQueue.Get(b)) throw HeaderErr();
        while (b);
}

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Gzip Gunzip:

word32 m_filetime;
std::string m_filename;
std::string m_comment;

-, m_filetime ( Gzip Gunzip). , Gzip ctors:

Gzip(const NameValuePairs &parameters, BufferedTransformation *attachment=NULL)
    : Deflator(parameters, attachment), m_filetime(0) {}

-, Gzip:

void SetFiletime(word32 filetime) { m_filetime = filetime; }
void SetFilename(const std::string& filename) { m_filename = filename; }
void SetComment(const std::string& comment) { m_comment = comment; }

-, Gunzip:

word32 GetFiletime() const { return m_filetime; }
const std::string& GetFilename() const { return m_filename; }
const std::string& GetComment() const { return m_comment; }

-, Gzip::WritePrestreamHeader cpp:

void Gzip::WritePrestreamHeader()
{
    m_totalLen = 0;
    m_crc.Restart();

    int flags = 0;
    if(!m_filename.empty())
        flags |= FILENAME;
    if(!m_comment.empty())
        flags |= COMMENTS;

    AttachedTransformation()->Put(MAGIC1);
    AttachedTransformation()->Put(MAGIC2);
    AttachedTransformation()->Put(DEFLATED);
    AttachedTransformation()->Put((byte)flags);    // general flag
    AttachedTransformation()->PutWord32(m_filetime, LITTLE_ENDIAN_ORDER);    // time stamp

    byte extra = (GetDeflateLevel() == 1) ? FAST : ((GetDeflateLevel() == 9) ? SLOW : 0);
    AttachedTransformation()->Put(extra);        
    AttachedTransformation()->Put(GZIP_OS_CODE);

    if(!m_filename.empty())
        AttachedTransformation()->Put((const unsigned char*)m_filename.data(), m_filename.size() +1);

    if(!m_comment.empty())
        AttachedTransformation()->Put((const unsigned char*)m_comment.data(), m_comment.size() +1);
}

-, Gzip::WritePoststreamTail cpp:

void Gzip::WritePoststreamTail()
{
    SecByteBlock crc(4);
    m_crc.Final(crc);
    AttachedTransformation()->Put(crc, 4);
    AttachedTransformation()->PutWord32(m_totalLen, LITTLE_ENDIAN_ORDER);

    m_filetime = 0;

    m_filename.erase(0);
    m_filename.reserve(16);

    m_comment.erase(0);
    m_comment.reserve(32);
}

-, Gunzip::ProcessPrestreamHeader cpp:

void Gunzip::ProcessPrestreamHeader()
{
    m_length = 0;
    m_crc.Restart();

    m_filetime = 0;

    m_filename.erase(0);
    m_filename.reserve(16);

    m_comment.erase(0);
    m_comment.reserve(32);

    byte buf[6];
    byte b, flags;

    if (m_inQueue.Get(buf, 2)!=2) throw HeaderErr();
    if (buf[0] != MAGIC1 || buf[1] != MAGIC2) throw HeaderErr();
    if (!m_inQueue.Get(b) || (b != DEFLATED)) throw HeaderErr();     // skip CM flag
    if (!m_inQueue.Get(flags)) throw HeaderErr();
    if (flags & (ENCRYPTED | CONTINUED)) throw HeaderErr();
    if (m_inQueue.GetWord32(m_filetime, LITTLE_ENDIAN_ORDER) != 4) throw HeaderErr();
    if (m_inQueue.Skip(2)!=2) throw HeaderErr();    // Skip extra flags and OS type

    if (flags & EXTRA_FIELDS)   // skip extra fields
    {
        word16 length;
        if (m_inQueue.GetWord16(length, LITTLE_ENDIAN_ORDER) != 2) throw HeaderErr();
        if (m_inQueue.Skip(length)!=length) throw HeaderErr();
    }

    if (flags & FILENAME)   // extract filename
    {
        do
        {
            if(!m_inQueue.Get(b)) throw HeaderErr();
            if(b) m_filename.append( 1, (char)b );
        }
        while (b);
    }

    if (flags & COMMENTS)   // extract comments
    {
        do
        {
            if(!m_inQueue.Get(b)) throw HeaderErr();
            if(b) m_comment.append( 1, (char)b );
        }
        while (b);
    }
}

---

, :

try {

    Gzip zipper(new FileSink("gzip-test.gz", true));
    zipper.SetFilename("test-filename.txt");
    zipper.SetComment("This is a test of filenames and comments");

    string data = "abcdefghijklmnopqrstuvwxyz";
    zipper.Put( (unsigned char*) data.c_str(), data.size());
    zipper.MessageEnd();        
}
catch(CryptoPP::Exception& ex)
{
    cerr << ex.what() << endl;
}

gzip-test.gz. test-filename.txt, This is a test of filenames and comments.

:

---

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---

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string s1, s2;
string data = "abcdefghijklmnopqrstuvwxyz";

Gzip zipper(new StringSink(s1));
zipper.SetFilename("test-filename.txt");
zipper.SetComment("This is a test of filenames and comments");

zipper.Put( (unsigned char*) data.c_str(), data.size());
zipper.MessageEnd();

Gunzip unzipper(new StringSink(s2));
unzipper.Put( (unsigned char*) s1.data(), s1.size());
unzipper.MessageEnd();

cout << "Filename: " << unzipper.GetFilename() << endl;
cout << "Comment: " << unzipper.GetComment() << endl;
cout << "Data: " << s2 << endl;

.

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