The sixteen character onion address

Addresses and identifiers

An older generation of onion address was sixteen characters long instead of fifty six, and today the length is useful mainly as a rough date stamp on whatever you found it in.

The shape

The short form used the same alphabet as the current one and the same suffix. Only the width differed, and the width is the whole story. Sixteen characters, then a dot, then onion. No punctuation inside, no upper case in normal use, no groups and no fields, exactly as with the longer form.

What the short form lacked is more interesting than what it had. It carried no marker announcing which generation of the format it belonged to, so nothing inside the string identified it as short apart from being short. It also carried no internal check of the kind the current form uses, which means a mistyped character in a short address had a much better chance of producing another string that was still shaped like an address. Nothing about the sixteen characters ever pushed back on a typo.

Total width: 16 characters, then a dot, then onion
Alphabet: the same base32 set as the current form, set out on the alphabets page
Case: insensitive, written lower case
Version marker: none anywhere in the string
Internal check: nothing comparable to the current form
Status today: the format is no longer answered, so nothing you do with one will reach a service

xxxxxxxxxxxxxxxx.onion
sixteen x characters, one dot, then the word onion

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.onion
the current form, for width comparison only

Two skeletons at the same scale. Nothing here is an address; every x is a placeholder.

Where you still meet one

The short form turns up in places that preserve text without updating it. An old bookmark file. A note somebody wrote down once and has copied forward ever since. A directory page nobody has edited in years. A screenshot pasted into a conversation, then pasted onward from the conversation. A printed card. A forum thread that was answered, closed and left alone.

The common thread is that all of these are archives rather than sources. They preserve a string that was accurate when it was captured, and they have no mechanism for noticing that it stopped being accurate. That is a property of the container, not of the person who handed it to you, and it is the reason a short address is a much better clue about the age of a document than about the honesty of anyone in the chain.

How to check it by eye

Sizing an old reference at a glance

Read the suffix. If the string ends in .onion, you are in this territory rather than looking at an ordinary domain name.
Judge the width against a word you know. Sixteen characters is about the length of a long English word, and the current form is more than three times that.
If it is short, stop checking the characters. There is nothing further to learn from the alphabet once the width has settled the generation.
Look at what is around it instead. Dates on the page, dead links beside it, an interface style that has fallen out of use: the container is now the evidence, not the string.

What a wrong shape means

For this card, wrong has two senses worth separating. A string that is short but breaks the alphabet, say by containing a hyphen or a capital run, is simply a mangled copy of an old address, and there is nothing to recover from it. A string that is neither sixteen nor fifty six characters is not an onion address of either generation, and is most often two fragments that have been joined by a line wrap.

Meeting a correctly built short address is the more common case, and it is not really an error at all. It tells you the note you are holding was written some time ago and has been carried forward without review. That is worth knowing about the note. It is not worth reading as a signal about the person who sent it, since somebody quoting an old page has no more information than the old page had.

An old reference

examplexxxxxxxxx.onion
sixteen characters: the older generation

A current reference

awazonth6ocz5cyos63czmhtsglqr7ydkdcc4lopux7nxbauoo2qmvyd.onion
fifty six characters: the generation in use

Only the width separates them, and the width alone decides which generation you are reading. The short string on the left spells example and is padded with x, so it is a placeholder rather than anything that ever existed.

What a correct shape does not prove

A short address can be perfectly built by the rules of its own generation. Even so:

It does not indicate deception. Stale notes outnumber traps by an enormous margin, and copying an old page forward is the least suspicious activity in the world.
It does not tell you the service ever belonged to the market, or to anyone in particular. The short form had no more to say about ownership than the current one does.
It does not date the note precisely. All you learn is that the source predates the change of format, which is a wide window, not a year.
It does not mean the rest of the document is stale. A page can carry one fossil string in a paragraph nobody revisited while the surrounding text was rewritten last week.
It does not point at a replacement. There is no arithmetic that turns a short address into the current address of the same service, and any page that claims to perform that conversion is claiming something the format cannot support.

What this card is not

Everything about the form in use now sits on the onion address card, including the recognition properties that make the current shape easy to pick out. If the string you are examining is not sixteen characters and not fifty six, the more likely explanation is that a line break split it, which is covered on the lost line breaks card. If it came wrapped in a link with a path attached, read the mirror link card first, because the host may not be the part you think it is.