A seal is a hash written as one word of 26 characters from five scripts, with a check: change one character and it no longer reads. Paste a hash or a seal on either side. The City writes the other form, checks it, says where it sits in its chains if it is on record, and lays the two side by side: the characters they share, the bits they share, and where both land on the City's 64 states. Try change one character to watch the check refuse a seal. Seals come from Sigil, an open encoding by John Servil. Everything runs in your browser; nothing is sent anywhere.
32 bytes become 26 characters in base 1,353, the number first multiplied by the prime 22,381. One misread or changed character, or two swapped, cannot decode; a reader unsure of a character can try its readings and exactly one decodes. No two characters look alike, so a forger who wants a look-alike must match 10.4 bits per compared character. Seals are written with Sigil (github.com/johnservil/sigil, Apache-2.0 or MIT, pinned at 8bd993a; try its own playground); Ethiopic and Canadian syllabics glyphs from Noto Sans (SIL OFL 1.1).
On record means: a step root, an entry root, a ledger or a lever leaf of one of the City's chains, as listed in seals.json at the last build. Merkle leaves inside an entry (its records, files and links) are checked on the chain's own page.