How grinding works
Generate a keypair. Encode the public key. Look at the result. If it does not start the way you wanted, throw it away and generate another. There is no cleverer method, because the encoding is designed so that you cannot work backwards from a desired output to a key.
So the question is how many keys you expect to throw away. Each position in the address holds one of 32 symbols, and each is effectively independent, so demanding one specific character costs you 32 attempts on average. Demanding two costs 1024. Demanding six costs a bit over a billion, and demanding twelve costs a number with eighteen digits in it.
The shape of the curve
| Characters chosen | Expected attempts | Practical difficulty |
|---|---|---|
| 1 | 32 | instant |
| 2 | about 1 thousand | instant |
| 4 | about 1 million | seconds |
| 5 | about 34 million | seconds to a minute |
| 6 | about 1 billion | minutes |
| 8 | about 1 trillion | a serious job, but doable |
| 10 | about 1 quadrillion | a large budget and patience |
| 12 | about 1 quintillion | out of reach |
| 56 | the whole key space | the thing the design prevents |
The exact timings depend entirely on hardware and are not worth pretending to know, so the right column describes effort rather than clock time. The shape is the point. The first few characters are free, the middle of the range is a project, and beyond that the curve leaves the building.
Why this matters for reading an address
The characters that cost nothing to control are the characters people check. That is the whole problem in one sentence. A reader who compares four or five leading characters is comparing precisely the part that anyone could have arranged deliberately over a coffee break.
The characters nobody can control are the ones nobody looks at. A forger has no more say over the middle of their address than they do over the weather, because getting a chosen middle is at least as expensive as getting a chosen prefix of the same length, and they gain nothing by paying for it since no reader will check.
That asymmetry is the reason this site prints the middle twenty characters as a named field on every record page, and it is the argument made in full on where a forgery differs.
What it means for the one vanity address here
One of the twelve opens with the market name, which is a six character prefix and therefore an ordinary job rather than an impressive one. It proves the person who generated it wanted that prefix. It does not prove who they were. The vanity split page covers how the rest of the set relates to it.
Questions
Could someone generate an address identical to an existing one?
No. That would mean finding a specific private key, which is the problem the whole system is built on being impossible. Grinding gets you a chosen beginning and never a chosen whole.
How many leading characters are safe to rely on?
None, in the sense of relying on them alone. Any prefix short enough for a person to compare comfortably is inside the range that is cheap to grind.