torzon-links.store
How it works

The 32 symbols and the four missing digits

A base32 alphabet needs exactly 32 symbols. There are 26 letters, so six digits join them, and the four that were left out were left out for a reason.

Symbols3226 letters and 6 digits
Digits excluded4Zero, one, eight and nine
Set checked▲672 charsNo excluded digit appears anywhere

The alphabet

What is allowed

Letters
a b c d e f g h i j k l m n o p q r s t u v w x y z
Digits
2 3 4 5 6 7
Excluded
0 1 8 9 and every uppercase letter
Total
32 symbols, which is exactly 5 bits

Why those four

Thirty-two is the number that matters, because five bits index exactly 32 values. Twenty-six letters leaves six slots, so six of the ten digits get in and four do not.

The four chosen for exclusion are the ones that collide visually with letters. Zero against the letter o, one against the letter l and in some faces the letter i, eight against the letter b in certain conditions, nine against the letter g. Take those out and the remaining 32 symbols are much harder to confuse with each other when a person is reading a long string off a screen.

Uppercase letters are excluded for a different reason. If both cases were allowed, two addresses differing only in capitalisation would be different addresses, and people would mistype them constantly. Restricting to lowercase removes the whole class of problem, and it also means an address can be safely lowercased anywhere without changing what it refers to.

Why the absence is a usable check

Most properties of an onion address cannot be checked by eye. This one can. If a string offered to you as an onion address contains a zero, a one, an eight, a nine or a capital letter, it is not a valid onion address. Not a suspicious one, not an old format, malformed.

That is a genuinely useful test because it takes a second and produces a definite answer. It is also completely one-sided. Passing it tells you nothing whatsoever, because every valid address passes it and so does every forged address, which is generated the same way. The alphabet check rules things out and never rules anything in.

Across the 672 characters of the twelve addresses on this site, no excluded symbol appears anywhere. Every record page reports the check as a pass, and the frequency table shows the same thing from the other direction, with no rows existing for symbols that never occur.

What a stray character would look likeIf an address here contained an illegal character, its record page would show the alphabet metric in red with a downward arrow and name the offending characters. None of the twelve does.

Questions

Can an onion address contain a capital letter?

No. The encoding is lowercase only. A string with capitals in it can be lowercased and may then be valid, but as written it is not in the correct form.

Why is the letter o allowed if zero is not?

Because only one of each confusable pair needed to go, and letters are more numerous and more useful than digits in an alphabet of this kind. Removing the digit keeps 26 letters intact.