I am attempting to come up with a way of memorizing a seed that could lead to any number of brain wallets for bitcoin. I need multiple wallets because a) I don't want all my eggs in one basket, and b) I want to be able to import small amounts from time to time.
Let's say I came up with a passphrase, and assume it's long enough to be secure. If I was to concatenate a value at the end of the phrase, and increment this each time, it could lead to an infinite amount of brainwallets, ie.
SHA256(passphrase.'1') SHA256(passphrase.'2') etc...
The only disadvantage I see with this approach is that if somebody, at some point in the future was to stumble upon the seed to the private key (keylogger, brute force etc.), it would be trivial to access all the other wallets.
So I was considering using two rounds of SHA256 instead, but adding a second phrase to the hash value of the first round, like this -
SHA256(SHA256(passphrase.'1').'second phrase') SHA256(SHA256(passphrase.'2').'second phrase')
I would the use the resulting hash as the input to the brain wallet.
This would mean concatenating ASCII text to the hex of the first SHA256 operation. The reason that I don't want to use the actual numeric value is that I need something that can be done easily over a unix command line (I'm not sure how to do 256-bit math operations). I don't want any custom software involved because there's not that much room in my brain. This would hopefully eliminate the worry that somebody could reverse-engineer the process by which I generate the brain wallets.
I'm worried that if I use the ASCII text to generate a second key, I'm somehow compromising the SHA256 process and making everything less secure.
Is there reason for me to be worried or is my approach secure?