Can someone guess your seed phrase or private key?

No, nobody can realistically guess a properly generated seed phrase or private key. The number of possible combinations is so large that trying them all would take longer than the age of the universe, according to wallet maker Trezor. The protection holds only if the words were picked at random by good software and never shared.

How many seed phrase combinations are there?

A seed phrase is built under BIP-39, a standard published in 2013. It starts with a block of random data, called entropy, of 128 to 256 bits. A short checksum is added, and the result is split into 11-bit pieces. Each piece picks one word from a list of 2,048.

The standard's table sets the sizes:

• 12 words come from 128 bits of entropy plus a 4-bit checksum.
• 24 words come from 256 bits of entropy plus an 8-bit checksum.

Twelve words drawn from 2,048 give about 5.4 x 10^39 possible orders. Because the last word must match the checksum, only about one in 16 of those is a valid phrase. That leaves 2^128, roughly 3.4 x 10^38 valid 12-word phrases. A 24-word phrase has 2^256.

Can you guess a bitcoin private key?

The odds are similar. The Bitcoin Wiki says any 256-bit number from 1 up to a fixed limit just under 2^256 is a valid private key. That is about 1.16 x 10^77 possible keys. A key generated at random is one point in that space, and there is no shortcut for finding it from the address, as explained in how cryptography secures crypto.

Is a 12 or 24 word seed phrase safer?

On paper, 24 words is far stronger. In practice, Trezor says 128 bits is already in the same range as the strength of a Bitcoin private key, and calls the extra strength of 24 words much more than needed. It says the longer phrase on its older Model One mainly supported a recovery method that involved typing words on a computer. Newer Trezor models use a 20-word format instead, covered in our guide to Shamir and multi-share backups.

An optional passphrase adds another secret on top of either length.

When can a seed phrase be cracked?

Guessing becomes possible when the randomness was weak. In 2023, a research team called Milk Sad traced a wave of thefts to Libbitcoin Explorer, a wallet tool whose seed command used a weak random number generator. The flaw is tracked as CVE-2023-39910, and attackers drained funds from affected wallets in July 2023. Trezor makes the same point from the other side, noting that its estimate holds only when the words are truly random.

The same math also explains why a lost phrase stays lost, as covered in what happens if you lose your seed phrase. A phrase with one or two missing words is a different case, because only the gaps need to be searched. Most real thefts skip the math entirely and trick people into typing their words into fake sites, a pattern described in crypto phishing and wallet drainers.