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The wallet was created around Feb 2013, using unknown wallet software. All that is known is a passprase, and 15 words. I suspect the passphrase was used to encrypt the wallet, and is probably irrelevant here, since the wallet file itself is lost.
All words are found in the "old" Electrum list. But the number 15 is puzzling, since no version of Electrum ever produced 15 words by default, as far as I know. I considered the possibility that this mnemonic was extended with custom words, but it looks like custom words in Electrum work only with the "new" word list.
I cloned Electrum repository from github and tried converting mnemonic to hex seed using the old list, then converting it back to mnemonic using the "new" word list. This method produces 15 words that Electrum GUI does recognize as 12 + 3 (extended), but leads to empty wallet.
My working hypothesis: there ought to be another wallet software that existed around 2013 that used Electrum's "old" word list and generated 15 words by default. Any other ideas or suggestions?
1Finally a good wallet-recovery question. A +1 from me. I hope it helps attracting people who can help you. – Christiaan Westerbeek – 2017-12-13T21:03:58.810
By any chance, could it be that 4 years ago you invented your own way of making a private key from 15 words and that you forget you did that? – Christiaan Westerbeek – 2017-12-13T21:37:33.073
Does blockchain.info ring a bell? Give it a try https://blockchain.info/wallet/forgot-password?guid=
– Chak – 2017-12-13T22:08:33.893I am aware of this tool. It is unfortunately broken (aside from violating the first rule of mnemonic - to never enter it on a web site). Try to enter 'drama' (one of the words in my mnemonic) and watch it error out. – user443854 – 2017-12-13T22:16:00.043
If your 15 words really are from blockchain.info, then this is the only way to recover it, as the old legacy mnemonic phrase is not the same as HD wallet "recovery phrase. The error you get is because you need to enter at least 3 words, I haven't looked over the whole source file, but I assume it is some kind of a checksum. Of course it has to be done online, how else is blockchain able to fetch your wallet-id and password for you? You can disregard this, if your 15 words are from somewhere else.
– Chak – 2017-12-13T23:02:57.423@Chak this is indeed from blockchain.info. I was discouraged by the errors when I entered the 15 words in the wrong order on purpose, to see if it works. In all likelihood it did not work due to checksum, like you said. When I entered all 15 words in the right order, the output matched the passphrase on record. – user443854 – 2017-12-13T23:59:17.103
@Chak, you can answer that blockchain.info circa 2013 had 15 mnemonic words, so I can accept it as correct answer. – user443854 – 2017-12-14T00:01:22.590
Glad it helped. – Chak – 2017-12-14T12:44:02.620