An onion address is 56 characters

The statement as people say it: awazon links are always that same long string, and if it is shorter something is wrong.

Where 56 comes from

The address is a base32 encoding of three things stuck together. A 32 byte public key, a two byte checksum, and one version byte. That is 35 bytes. Base32 turns every 5 bytes into 8 characters, and 35 divides by 5 exactly, so you get 56 characters with no padding left over.

That is the entire derivation. There is no room in it for a short address or a long one. If somebody hands you 54 characters, the string is not a shortened version of a real address, it is not an address.

Why this is useful in practice

Counting is the cheapest check available and it runs before you connect to anything. Most software will not select the whole string cleanly when it has been mangled by a chat client or a forum that inserts a line break. A count catches that immediately.

It also catches the sloppiest impersonation. Somebody who has generated a lookalike will match the length, but somebody who is just guessing that you will not look will not bother. The count costs you three seconds and rules out an entire class of mistake.

The usual misreading

People assume a wrong length means the service is old. There was an older format, sixteen characters, and it is gone. Not deprecated, gone. Those addresses stopped resolving when the old protocol was switched off. A sixteen character string today is not a legacy awazon market address that still half works, it is dead text.

The other misreading is subtler. People count the dot and the word onion and get 62, then decide the rule is wrong. The 56 covers only the part before the dot. Everything after it is fixed and carries no information about which service you are reaching.

What people also get wrong

That a correct length means a correct address. It does not, and nothing about the count says otherwise. There are more valid 56 character strings than there are atoms in anything you can point at, and generating one that is valid but not the one you wanted is trivial.

Length is a filter, not a verdict. It removes garbage. It says nothing about whether the remaining string is the awazon market link you were looking for. That job belongs to comparing every character, which is its own statement.

Counting it without counting

Nobody counts to 56 reliably by eye. Select the string in a text field and most editors will report the selection length, or paste it somewhere that shows a character count. On a phone this is more awkward, which is one more reason to copy rather than retype.

What would move this

A new onion service version with a different key size or a different encoding. That has happened once already, when the sixteen character format was replaced, so it is not unthinkable. It would need a protocol change shipped across the whole network, and the new length would be just as fixed as this one.

Short of that, nothing moves it. Not the market changing hosting, not a mirror going down, not a new awazon mirror appearing. Those all produce different 56 character strings.

• What the 35 bytes are
• Which characters can appear
• The addresses

Questions people actually ask

Does the count include the dot and the word onion?

No. Fifty six is the part before the dot. The suffix is fixed and identical for every onion service.

I found a 16 character address, does it still work?

No. The old format was switched off across the network. Those strings do not resolve at all now.