Lets say my 21 word paper would be exposed. Would it be hard for the persona that has this 21 word seed to crack the wallet ?

If the position of these words is known, then yes. Definetely crackable. Within a short timeframe.

However, if the attacker had no clue as to the position of the missing words... ie. you'd taken 3 words from position 3, 17 and 23, but there was no indication of position/order of words on the paper (ie. they're not numbered)... then things become a lot more complicated. The number of combinations the attacker would need to test increases by several orders of magnitude and it would require a lot more work. It's possible the time required would extend to hours or days or years. I'm not not sure of the actual math... possibly something like:

(2048 * 24) * (2048 * 24) * (2048 * 24) = 118,747,260,000,000+ combinations (note, I'm very tired, so this math is probably really wrong

)

If an attacker had 3 (out of 24) words without knowing any particular spot/order the amount of combinations would be:

Amount of 'iterations' when choosing 3 position out of 24 to guess (24 choose 3) =

**2024** (without any order of these 3 words) -> (2024 * 3!) =

**12 144** (considering all possible orders)

Now for each of these iteration you need to check 21 positions with 2048 words (2048

^{21}) =

**3,45087317 * 10**^{69} combinations.

To sum it up: 12 144 * 3,45087317 * 10

^{69} =

**4,19074038 × 10**^{73} would be the amount of combination.

Thats just slightly below the 2,96427748 × 10

^{79} combinations from the full 24 words. Still considered safe.

Just saw its mentioned the other way around

With

** 21 out of 24** words known you have

**2024** possibilities to chose the position of the word -> multiplied with 3! ->

**12 144** combinations to check.

Now each of those combinations has 2048

^{3} possibilities to get 'filled' -> 2048

^{3} * 12 144 =

**104 316 165 685 248** combinations an attacker had to check.

I would say this is definetely possible to crack.

If you aren't sure whether your seed got compromised (or if you are sure it got somehow lost/compromised) you should create a new wallet/seed ASAP.