BIP39 - Conversion from *PUB Mnemonic(24) to *PUB Mnemonic(25) with passphrase

[ragnar dannesk gold]

Is it possible to convert an X/Y/Z-PUB Key without a seed extension to what the X/Y/Z-Pub Key would be for the same seed with a Mnemonic added?

In other words, without access to the seed, can a Master Public Key be converted to what said Master Public Key would be if a (known) passphrase were added to the seed without knowing the seed itself?


Edit - For the sake of what I'm trying to do:

I have access to 3 things:

A) YPub - Which I know was created with 24 seed words
B) Another YPub - Which I know was created with 24 seed words and the 25th word
C) The 25th word itself


What I am trying to do, is use the first YPub (A) as well as the 25th word (C) to confirm whether the seed from A and B are the same, without the seed itself.

So without the seed itself, can I determine if A + C = B

[bob123]

Is it possible to convert an X/Y/Z-PUB Key without a seed extension to what the X/Y/Z-Pub Key would be for the same seed with a Mnemonic added?

This is only possible through bruteforcing the 25th word to the (known) mnemonic seed.

In other words, without access to the seed, can a Master Public Key be converted to what said Master Public Key would be if a (known) passphrase were added to the seed without knowing the seed itself?

No. Without access to the seed, you can not create a xpub at all.

As far as i understand you, you are worried about the security of the 25th word ?
As long as your seed is kept secret, noone will gain information about your xpub/xpriv/addresses/etc..  The xpub of your seed without the 25th word doesn't make a difference here.

[ragnar dannesk gold]

Hi Bob,

Thanks for taking the time to reply.

For the sake of what I'm trying to do:

I have access to 3 things:

A) YPub - Which I know was created with 24 seed words
B) Another YPub - Which I know was created with 24 seed words and the 25th word
C) The 25th word itself


What I am trying to do, is use the first YPub (A) as well as the 25th word (C) to confirm whether the seed from A and B are the same, without the seed itself.

So without the seed itself, can I determine if A + C = B

[Abdussamad]

It can't be done. mnemonic -> seed -> xprv is the order of things. mnemonic > seed involves hash functions and the extension is used as a salt to that so it isn't possible to go from seed > mnemonic. in your case you don't even have the binary seed or the xprv. you have the xpub and reversing that to get the xprv would require breaking ecdsa as well.