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Simplify header-reading logic and support JET3 code pages

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Using the notes and RC4 key provided in the HACKING file, decrypt the
database definition page all at once instead of decrypting individual
fields with ad-hoc keys. Use the newly decrypted header to access the
database code page at offset 0x3C, and use this numeric value to
initialize the iconv converter with an appropriate charset name for
popular windows code pages. More encodings can be added later, with
the eventual goal of getting rid of the MDB_JET3_CHARSET environment
variable.

Note that individual columns can have their own code pages but this
issue is not addressed.

An extra field is added to the MdbFile structure - because this
struct is allocated internally, this should not break the public
ABI.

Finally, only set the db_passwd field if it's a JET3 database (see #144)
This commit is contained in:
Evan Miller
2020-12-20 18:02:23 -05:00
parent a8414720e4
commit 134306d1af
3 changed files with 36 additions and 26 deletions
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32
src/libmdb/file.c
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@@ -168,9 +168,6 @@ static char *mdb_find_file(const char *file_name)
* Return value: The handle on success, NULL on failure
*/
static MdbHandle *mdb_handle_from_stream(FILE *stream, MdbFileFlags flags) {
int key[] = {0x86, 0xfb, 0xec, 0x37, 0x5d, 0x44, 0x9c, 0xfa, 0xc6, 0x5e, 0x28, 0xe6, 0x13, 0xb6};
int j, pos;
MdbHandle *mdb = (MdbHandle *) g_malloc0(sizeof(MdbHandle));
mdb_set_default_backend(mdb, "access");
mdb_set_date_fmt(mdb, "%x %X");
@@ -216,27 +213,22 @@ static MdbHandle *mdb_handle_from_stream(FILE *stream, MdbFileFlags flags) {
mdb_close(mdb);
return NULL;
}
RC4_KEY rc4_key;
unsigned int tmp_key = 0x6b39dac7;
RC4_set_key(&rc4_key, 4, (unsigned char *)&tmp_key);
RC4(&rc4_key, mdb->f->jet_version == MDB_VER_JET3 ? 126 : 128, mdb->pg_buf + 0x18);
mdb->f->code_page = mdb_get_int16(mdb->pg_buf, 0x3c);
mdb->f->db_key = mdb_get_int32(mdb->pg_buf, 0x3e);
/* I don't know if this value is valid for some versions?
* it doesn't seem to be valid for the databases I have
*
* f->db_key ^= 0xe15e01b9;
*/
mdb->f->db_key ^= 0x4ebc8afb;
/* fprintf(stderr, "Encrypted file, RC4 key seed= %d\n", mdb->f->db_key); */
if (mdb->f->jet_version == MDB_VER_JET3) {
/* JET4 needs additional masking with the DB creation date, currently unsupported */
/* Bug - JET3 supports 20 byte passwords, this is currently just 14 bytes */
memcpy(mdb->f->db_passwd, mdb->pg_buf + 0x42, sizeof(mdb->f->db_passwd));
}
/* write is not supported for encrypted files yet */
mdb->f->writable = mdb->f->writable && !mdb->f->db_key;
/* get the db password located at 0x42 bytes into the file */
for (pos=0;pos<14;pos++) {
j = mdb_get_int32(mdb->pg_buf, 0x42+pos);
j ^= key[pos];
if ( j != 0)
mdb->f->db_passwd[pos] = j;
else
mdb->f->db_passwd[pos] = '\0';
}
mdb_iconv_init(mdb);
return mdb;