Value Error: Invalid data

[gakar06]

When I try to use this script on data I get the following error:

ValueError : Invalid data, illegal PKCS#7 padding. Could be using a wrong key.

What does the format need to be of the key and data coming into the xxtea.decrypt function? As of right now, the key is 16 bytes and the data is 256 bytes.

Thanks,
Chris

[ifduyue]

@gakar06 Hello, did you use this lib to encrypt and decrypt? This xxtea lib may not be compatible with other ones.

[ifduyue]

What does the format need to be of the key and data coming into the xxtea.decrypt function? As of right now, the key is 16 bytes and the data is 256 bytes.

key needs to be 16-byte long, data can be of any length.
Could you provide a minimal script to reproduce the problem?

[gakar06]

What is the purpose of doing the PKCS#7 padding and removal in the XXTEA code? Just curious. The issue seems to occurring within xxtea.c in the xxtea_decrypt function. At the bottom, the ELSE of

if (rc >= 0) {
/* Remove PKCS#7 padded chars /
Py_SIZE(retval) = rc;
}
else {
/ Illegal PKCS#7 padding */
PyErr_SetString(PyExc_ValueError, "Invalid data, illegal PKCS#7 padding. Could be using a wrong key.");
goto cleanup;
}

If I comment out the ELSE, the decrypt works. Any ideas why? What is the PKCS used for?

Thanks!
Chris

[ifduyue]

It's because XXTEA algorithm works on two or more 32-bit integers.
If data length is not a multiple of 32-bit, we have to add paddings, and remove these paddings at the decryption.

[ifduyue]

xxtea 0.1.5 doesn't add paddings, let's see the problem:

pip install xxtea==0.1.5

>>> import os
>>> import xxtea
>>> key = os.urandom(16)
>>> xxtea.encrypt('\0'*3, key)
'1aa3918940169fe3'
>>> xxtea.decrypt(_, key)
''

[ifduyue]

Going to close this right now. Feel free to reopen it if there are any problems