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How to Recover Blockchain.com Legacy Wallets with an Unexpected Number of Mnemonic Words (13, 16, 22 words)

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7 min read

Imagine knowing that you have Bitcoin stored in an old Blockchain.info or Blockchain.com wallet — but you can’t remember your Wallet ID and password. Then, one day you find a series of words written on a piece of paper — and you realize that these are mnemonic words! These words can grant you access to your wallet! But, something’s off — you’re expecting 12 words — or maybe 24 words — but instead, you have 16 words. Or maybe 9 words. Or maybe 22.

What does is mean when your mnemonic is an unexpected length? Can you still recover a with an unexpected number of words, such as 13 words instead of the standard 12 or 24?

Fear not! In this post, we’ll dive into the world of mnemonic words, recovery phrases, and how to tackle the challenge of recovering wallets with a strange number of mnemonic words.

Understanding Mnemonic Phrases and Recovery Seeds

Modern wallet backups rely heavily on recovery seeds. Backups for older wallets relied heavily on mnemonic phrases. People use these terms (and others, such as “mnemonics”, “seed phrase” and “recovery phrase”) interchangeably — but, there is an important distinction between them.

What are Recovery Seeds?

A recovery seed is a form of wallet backup that can recreate the private keys, the public keys and every public address of a crypto asset. The word “seed” in the term “recovery seed” refers to the fact that private keys are literally derived from (aka “grown” from) the words in the recovery seed. Having control of the recovery seed means that years after you created your wallet you can import your recovery seed into a new wallet, and have control of your funds.

Recovery seeds were proposed in 2013 the 39th Bitcoin Improvement Proposal — and that’s why you’ll occasionally see these referred to as “BIP39” recovery seeds. These phrases consist of a set of seemingly random words chosen from a predefined word list that contains 2048 words. But, the first block was mined in the Bitcoin blockchain in 2009, more than 4 years earlier — and between 2009 and 2013 (actually more like 2014 – 2016, as BIP39 was being implemented and tested in the most popular self-custody wallets) no one had BIP39 recovery seeds.

What are Mnemonic Phrases?

The term “mnemonic phrase” is a broader concept that refers to a series of words that can give you control of a crypto wallet.

In fact, a recovery seed is a specific kind of mnemonic phrase. But, all mnemonic phrases are not recovery seeds — certain wallets provided mnemonic phrases years before the proposal for recovery seeds was formalized and implemented in wallets.

In particular, Blockchain.info (now Blockchain.com) wallets had their own recovery phrases since 2011, and while those phrases can help you get access to your Blockchain wallet, they don’t guarantee access. These Blockchain.info mnemonics are sometimes called “legacy” mnemonics.

Other wallets, notably the open source Electrum wallet, also offered recovery phrases that were not BIP39 recovery seeds — although these phrases worked differently from Blockchain.info’s phrases.

And Now, Let’s Make Matters Even More Confusing!

Guess what? Most of the time, you can’t tell from the length of a recovery phrase alone whether it is a Blockchain mnemonic phrase or a BIP39 recovery seed. You typically need to look at whether the words exist in the BIP39 word list in combination with the length of the phrase to know for sure. Better yet, you need to actually import the phrase into a wallet, and see if it recovers your funds.

Why a BIP39 Recovery seed might be a different length than 12 or 24 words

Even though standard BIP39 recovery seed lengths are 12 or 24 words long, the BIP39 proposal allows for 12, 15, 18, 21 and 24 word recovery seeds.

In addition, the proposal allows people to add an additional word to their recovery seed to create what is often referred to as a passphrase or a “plausible deniability” wallet. So, you could have written down 13 words on a piece of paper, the first 12 of which are BIP39 words and the 13th could be essentially anything — and that could represent a BIP39 seed with a passphrase.

And, finally, sometimes there might be a word missing in the written version of their seed — so they might have a BIP39 seed that is only 11 or 23 words long.

So, just to make life interesting, here are the BIP39 seed lengths that you plausibly could see in the wild: 11, 12, 13, 15, 16, 18, 19, 21, 22, 23, 24, 25!

Why a Blockchain.info Mnemonic Phrase might be almost ANY length

A Blockchain.info mnemonic phrase simply encodes a password (in version 2) and a password and Wallet ID (in later versions). So, the longer the password that needs to be encoded, the longer the mnemonic phrase.

We have seen mnemonic phrases in the wild as short as 9 words and as long as 26. And, we have seen both 12 and 24 word Blockchain legacy mnemonic phrases!

As an example, in March of 2021, Melissa wrote to us with what appeared to be a 12 word recovery seed. But, only 5 of the 12 words were included BIP39 word list. Guess what — her Blockchain legacy mnemonic just happened to have exactly 12 words in it!

She actually had a v2 Blockchain legacy mnemonic which encoded her password into 12 words. (Unfortunately, though, she had changed her password since the original encoding, so the password revealed by the legacy mnemonic was no longer the password that protected her wallet. Despite these ups and downs, we did manage to crack her new password, so she eventually regained control of her funds).

How to Use a Blockchain Legacy Mnemonic

If you need to recover your Blockchain wallet, you need a few separate pieces of information:

  • Your Wallet ID (which includes 32 alphanumeric characters and 4 dashes. It takes the following format:
  • xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
  • Your password
  • Control of the email address associated with your wallet (if you have registered an email with your wallet)

If your legacy mnemonic was encoded using the version 2 algorithm, it will only include the password that you defined at the time that you created your password. If you have changed your password to something new, your legacy mnemonic won’t store that new password.

If you haven’t stored your Wallet ID, but you did associate an email with your wallet, then you can retrieve the Wallet ID using the instructions in our article on cracking the password for your Blockchain wallet.

If your legacy mnemonic was encoded using the version 3 algorithm (or later versions), your mnemonic phrase will include both your password and your Wallet ID.

In either case, if you no longer control the email address associated with your wallet, you can follow the instructions in this article on what to do if you’re not receiving authorization emails.

What Happens if Blockchain’s Legacy Mnemonic Tool Doesn’t Work?

Sometimes when you enter your legacy mnemonic into Blockchain’s tool, you’ll get an error back.  These errors may include:

  • “Unknown word” followed by one or more of the words in your mnemonic
  • “Invalid Checksum”

When you get those errors, you have an error in the way that you have entered your words in the tool. So the first thing you should check is that you entered the words exactly as they are written down. If you did, then unfortunately, you wrote down the words incorrectly. You might have written an incorrect (or misspelled) word, written words in the wrong order, or omitted a word.

If you get either of these errors, check against the v2 and v3 dictionaries linked to above, and make sure that your words are in the dictionary.

If that doesn’t solve the problem, then we have written software to brute force every possible combination of either the wrong or missing words. Please contact us.

Why All of This Matters

On a quiet morning in February 2022, we received a note from Emma that caught our attention. It was a time of mourning for her, as she had just lost her father. While sorting through his belongings, a discovery piqued her curiosity: a Blockchain wallet and a 20-word mnemonic phrase that seemed to whisper of hidden secrets.

With a mix of reverence and intrigue, we began the meticulous task of cross-referencing each word with the v2 and v3 dictionaries. The words fit perfectly, like pieces in a puzzle, yet the puzzle didn’t fit — the mnemonic did not release the password to the wallet.

Determined to uncover the full picture, we developed a brute force tool. This digital detective tirelessly tried combinations, searching for the word that would complete the sequence. After a week of continuous effort, our persistence paid off. The missing word emerged, a key turning in a long-locked door.

With this final piece, we successfully retrieved the Wallet ID and password. Emma’s story was no longer just about the loss of her father; it was also about reconnecting with a part of him that continued to live on through this unexpected digital legacy.

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