I wrote a cryptographic function that encrypts a plaintext by using a series of SHA-512 hashes that are used to encrypt the data via a simple byte-to-byte XOR. In order to prevent various attacks, I implemented CBC by hashing the current hash value and the previously enciphered block together. That resulting hash is then used to encipher the next 64-byte block of plaintext via simple XOR. The initial key is a SHA-512 hash of the passphrase or other key. I added several thousand rounds of hashing, and I still need to implement a random 64-byte pad to the front of the code to eliminate some guessing, but as a method is this a sound approach?
I can't imagine it being incredibly insecure, but I am interested in the methods one would use to crack the code.