Is this HD wallet secure?

I know about BIP 32, but I tried to do things in a simpler way and wonder if such design is secure enough.

First, we start with some master key, in this example we use output from executing SHA-256 on empty string, in real application it would be generated randomly in any secure way, for example as it is done by OpenSSL. Because if we use it directly, we would have some public key starting with "03" prefix, so we negate the result to get all public keys starting with "02", so we can make them all 256-bit. The same we can do for every public key if we ever get "03" prefix.

sha256empty: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
masterPrivateKey: 1c4f3bbd6703e3eb65040b37669046da93009b024aad0cef1b3cc57157e388ec
masterPublicKey: 02 a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd

Then, if we want to get some keys from this master public key, we simply need to add some known number to it. To generate such number, we combine this 256-bit public key with 256-bit nonce and execute SHA-256 on it.

firstHashedData:

a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd 0000000000000000000000000000000000000000000000000000000000000000

firstAddedValue: db26845476a175bd67c1e2b96812ea4aaa772f401fd23edabb98155e53d6b612
firstPrivateKey: f775c011dda559a8ccc5edf0cea331253d77ca426a7f4bc9d6d4dacfabba3efe
firstDerivedKey: 02 1dbe25ac1b430b911bda0f22d11c65a6e0fcc4861ac2a56ae8e8db27fd82ebd5

secondHashedData:

a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd 0000000000000000000000000000000000000000000000000000000000000001

secondAddedValue: 6dbcfbb893a11df2abbcf8997c10da5dcf8d14a19eca8510002188bce19d7ee4
secondPrivateKey: 8a0c3775faa501de10c103d0e2a12138628dafa3e97791ff1b5e4e2e398107d0
secondDerivedKey: 02 7367ad233c2e83f265a4751219e5ff3c4d3719e0d6f3e37147ecb011441e1749

To make it more convenient, instead of incrementing nonce, we could also place some hash of SHA-256 here, for example if we have unique usernames, we could take the hash of this username (or hash of any other meaningful data). Thoughts?