<p>12-word BIP39 phases only have a 4-bit checksum. In other words, 8 bits of the last word are still &quot;data&quot;.</p>

CC BY-SA 4.0
2022-08-27T13:31:06.160Z
Thanks for helping out… can you please expand on this? Why are `occur` and `surge` considered valid checksums for preceding 11 words?
sunknudsen
CC BY-SA 4.0
2022-08-27T13:32:43.073Z
Your premise is wrong. It's not the whole word that is a checksum. The word is an encoding of 12 bits of data. 8 of those bits are data. The other 4 bits are checksum.
Pieter Wuille
CC BY-SA 4.0
2022-08-27T13:37:22.287Z
Can you please include example of how checksum is derived in answer? Having a hard time understanding how two different words can yield same checksum even if only part of work is checksum (perhaps a naive understanding of your answer).
sunknudsen
CC BY-SA 4.0
2022-08-27T13:39:48.657Z
@sunknudsen if you add a 4 bit checksum to some data there’s still a 1/16 chance that random data will validate the checksum
Mike D
CC BY-SA 4.0
2022-08-27T13:39:49.137Z
They may or may not be the same checksum - I haven't checked. The point is that part of the last word is part of the phrase the checksum is computed over. There shouldn't be anything surprising over the fact that two phrases can differ.
Pieter Wuille
CC BY-SA 4.0
2022-08-27T13:40:36.157Z
Got it… just stumbled upon https://bitcoin.stackexchange.com/questions/69957/bip39-manual-phrase-calculations-how-are-multiple-checksums-valid thanks to your feedback. Guess we can close this one?
sunknudsen
CC BY-SA 4.0
2022-08-27T13:41:40.567Z
That's a better answer!
Pieter Wuille
CC BY-SA 4.0
2022-08-27T13:29:45.827Z
2022-08-27T13:29:45.827Z
Pieter Wuille
<p>My understanding is that 12th word is a checksum… so why are following mnemonics both valid?</p>
<blockquote>
<p>gather video album produce report surge grant buzz buzz jaguar modify occur</p>
</blockquote>
<blockquote>
<p>gather video album produce report surge grant buzz buzz jaguar modify surge</p>
</blockquote>

<mnemonic-seed><checksum>
Are BIP39 checksum collisions a thing?
sunknudsen
# Are BIP39 checksum collisions a thing?

(Asked by: username_0 on 2022-08-27)

My understanding is that 12th word is a checksum… so why are following mnemonics both valid?

> gather video album produce report surge grant buzz buzz jaguar modify occur

> gather video album produce report surge grant buzz buzz jaguar modify surge

**Tags:** mnemonic-seed, checksum


---

## 1 answers

### Answer by username_1 on 2022-08-27. Score: 2

12-word BIP39 phases only have a 4-bit checksum. In other words, 8 bits of the last word are still "data".

> **Comments:**
> **username_0**: Thanks for helping out… can you please expand on this? Why are `occur` and `surge` considered valid checksums for preceding 11 words?  
> **username_1**: Your premise is wrong. It's not the whole word that is a checksum. The word is an encoding of 12 bits of data. 8 of those bits are data. The other 4 bits are checksum.  
> **username_0**: Can you please include example of how checksum is derived in answer? Having a hard time understanding how two different words can yield same checksum even if only part of work is checksum (perhaps a naive understanding of your answer).  
> **username_2**: @username_0 if you add a 4 bit checksum to some data there’s still a 1/16 chance that random data will validate the checksum  
> **username_1**: They may or may not be the same checksum - I haven't checked. The point is that part of the last word is part of the phrase the checksum is computed over. There shouldn't be anything surprising over the fact that two phrases can differ.  
> **username_0**: Got it… just stumbled upon https://bitcoin.stackexchange.com/questions/69957/bip39-manual-phrase-calculations-how-are-multiple-checksums-valid thanks to your feedback. Guess we can close this one?  
> **username_1**: That's a better answer!  

---

