First of all the "seed" is bupkis because it's not actually used in the encryption - it's just used to xor the ciphertext, and then sent along with it. You can

First of all the "seed" is bupkis because it's not actually used in the encryption - it's just used to xor the ciphertext, and then sent along with it. You can trivially reverse that step and then get on attacking the rest of it. After that, the attack is elementary: You have two plaintexts, A and B. These are both xored with the same keystream X, so the attacker has access to both A ^ X and B ^ X. Xoring both cipherstreams gives you A ^ X ^ B ^ X, which is equivalent to A ^ B. Do you see the problem yet, or do I need to continue? EDIT: The real point is if you're not a crypto expert, don't write crypto code. (If you are a crypto expert, you already know better than to try writing your own crypto code). Also, don't trust obscure web pages that present crypto implementations - use a widely-known (and hence widely-vetted) implementation instead.