Mnemonic Phrase (BIP39) with Existing Seed Generating Library

Question

The standard BIP39 does three things:

1) Generate 256+8 entropy

2) Generate sentence

3) Derive seed from sentence (using PBKDF2 with HMAC-SHA512).

I wonder what would be the security implication of doing things the reverse way:

1) Generate entropy and use it as seed

2) Derive 256 bit array from entropy (using PBKDF2 with HMAC-SHA256/HMAC-SHA512). + 8 bit checksum

3) Use above bit array to construct sentence

Essentially, instead of creating the seed from the sentence, I create the sentence from the seed. In the last step, if HMAC-SHA512 is used, it would generate a bit array of length 512 which is too long for the sentence. To solve that I would XOR the two halves of the array. Alternately, I could just use HMAC-SHA256.

The company I work for has a library that does seed generation and this is what we are mandated to use.

Answer 1

You probably know that HMAC-SHA256/HMAC-SHA512 and thus PBKDF2 are one-way functions. There would be no way how to reconstruct the seed from the generated sentence in your setup, which is the only purpose of such sentence.