How hard to break a cipher if it has a different key for each word?

Recently I watched the movie 'The Prestige' again but this time the cipher diary impressed me very well. I like to create my own cipher for my diary because I have some time to kill everyday after deactivating facebook.

So If I create my own substitution table like 'Vigenère_cipher', and use a different key for every page or every word (somehow I manage to remember the key and decipher it), Is it still possible to crack this cipher in anyway?

I read about it some articles, Crackers can detect text patterns and it is easy for them to crack if long text is ciphered using the same key, I think using different keys will reduce this kind of pattern analysis. But I am not an expert in this field, so you can tell me about the possibilities of cracking a cipher written in this way.

NOTE: I am not using this kind of thing to cipher any sensitive data, others data, I am just want to know the possibilities of this thing. I maybe in a wrong thought about all these because I'm not a expert in this field.

• I think this question has not been answered yet. Oct 4, 2019 at 14:36

A Vigenère cypher can be unbreakable if, and only if:

1. the key is random,
2. the key is at least as long as the message being encrypted
3. the key is used only once.

In that case a Vigenère cypher is mathematically equivalent to a One Time Pad, and the same mathematical proof of unbreakability applies.

You proposal is closer to being unbreakable than standard Vigenère, but it is not an OTP either. Hence your proposal is not unbreakable, but is more difficult to break than a standard Vigenère.

• I would say that the question has not been answered yet. Oct 4, 2019 at 14:37
• The answer to "how hard" depends on a lot more detail than is provided. How big is the key? How often is it reused? How many messages are sent before the key is changed? How big are those messages? Oct 4, 2019 at 15:07
• Yes, of course. The question asks for a response that addresses those possibilities. Oct 4, 2019 at 15:16
• @Patriot Thanks for your interest, but I believe that if the author can decode it in a way, others can do the same. The only difference is how hard it is going to be to break the cipher. I would love to hear your thoughts on this and change my accepted answer if you can give me a more convincing or correct answer. Oct 5, 2019 at 21:09