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I see no obvious security flaws, but it seems inefficient and is nonstandard. You would be better off always using the root key as the HMAC key and basing the HMAC message/data on only the key index (or other identity) rather than previous keys. That way you can create any of the keys independently. That is essentially what HKDF does in HKDF-Expand. If you ...


SHA-256 - or any cryptographically secure hash - relies on the internal construction of the hash to have the one-way property. This one-way property is maintained for any kind and size of input. However, if the input domain is small enough then it may be able to brute force the hash value. As SHA-256 is not keyed anybody can perform the calculations. So all ...


SHA256 gives 256 bits, which is 32 bytes, not 64 "characters" (please use the well-defined bits and bytes). When written in hexadecimal notation, you need 64 characters from 0-9a-f, but the length of the hash is 256 bits. Yes, you could invert the hash. Unfortunately, a straightforward lookup table would be too large to store, but you can still do brute ...


Use SHA-256. Truncating the hash does reduce its collision resistance, but you can't "lose entropy" if you're only keeping 128 bits in each pool anyway.

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