Perl DES PCBC as protection against decryption/crypt analysis Is error propagation in DES PCBC a good method to prevent decryption/crypt analysis by third parties? 

If I had a very large file encrypted with Perl's DES with the PCBC option, and then removed the first megabyte of the resulting ciphertext would this render the remaining ciphertext 100% immune to crypt analysis ? Why or why not? Suppose that the missing 1 megabyte file disappeared forever would the ciphertext never be able to be decrypted? I'm assuming that too much information is being lost by not having the first megabyte of the ciphertext for it ever to be decrypted or any further information about the plaintext to be discovered.

What if the data was encrypted first in AES-CBC for more protection and then re-encrypted with DES PCBC for the intended error propagation? Could this weaken it or would it strengthen it slightly?

I am considering using the PERL DES cipher with the PCBC option since this seems to be the easiest and fastest way to implement this.
This is actually a good question, about how a mode of operation will affect analysis of a data stream.

In regards to an implementation of AES-PCBC, if you have AES-ECB you can build a wrapper for PCBC around it with the appropriate block size. It is not too difficult, but unnecessary...

In regards to security analysis, PCBC is no more resistant than CBC with an unknown Initialization Vector, since upon decryption the only difference between the modes is that the previous block input is only 1 value.



This will make decryption slower since it can not be decoded in parallel, but brute forcing the key space CAN be done in parallel. Once the correct combination of mode input and key are found, the entire remaining data stream will be correctly decrypted. The brute force of the IV/prior block is faster than the key, since no key change is required. DES is a 64-bit block, which means it has a small IV, and that makes your Key/IV search combination theoretically feasible with appropriate hardware (not now, but in the future yes).

The error propagation aspect of the mode was desired before widespread use of message authentication codes, modern cryptography sees error propagation through a mode as unnecessary. There are other applications that may prefer error propagation, but data at rest is not one of them. The downsides of error propagating modes (mostly performance) do not make up for any perceived advantage in security.

A bad mode is bad, but a good mode does not make a good cipher any better, and DES just isn't good enough anymore.