hh - Humanized Hash (Kotlin)

hh turns a blockchain address, a public key or any hash into a small deterministic picture that a person can compare at a glance: a 4 x 4 matrix of solid squares, circles and triangles in four colours. It exists to catch address poisoning and clipboard substitution, which work because people check only the first and last characters of a long string. The colours are chosen so that people with a colour vision deficiency can tell them apart as well.

0x1234567890abcdef00112233445566778899aabb	0x12345678f1e2d3c4b5a69788796a5b4c8899aabb

The two addresses agree in their first and last eight hex digits. Their pictures are unrelated.

This is the Kotlin implementation. It depends on the Kotlin standard library only, runs on the JVM and on Android, and produces, byte for byte, the output of the C++ reference implementation hh-cpp, which owns the specification and the golden vectors. testdata/ is a byte-identical copy of those vectors; testdata/SOURCE names the hh-cpp release they came from.

Implementations

Every implementation produces the same pictures, tags and encoded files, byte for byte, and its tests check it against a copy of the golden vectors of hh-cpp.

Language	Repository	Package	Install
C++17, C ABI	hh-cpp, the reference: specification and golden vectors	CMake hh::hh, pkg-config hh (releases)	CMake FetchContent or find_package(hh)
Kotlin and Java: JVM, Android	hh-kotlin (this repository)	Maven Central io.github.censync:hh	implementation("io.github.censync:hh:1.1.0")
TypeScript and JavaScript: browsers, Node.js, Deno, Bun	hh-ts	npm @censync/hh	npm install @censync/hh
Go	go-hh	github.com/censync/go-hh	go get github.com/censync/go-hh
Python	hh-python	PyPI humanized-hash	pip install humanized-hash

A longer example: Sui

A Sui address has 64 hex digits, and nobody reads 64 digits. The second address below differs from the first in one digit, the third in two; the changed digits are marked. In the text they are easy to miss. The pictures and the tags are unrelated, because every cell depends on every bit of the input.

Picture	Address	Tag
	0xeab3150efcb34ff74930d8f3d491be109070a39e4d380de7737aff5c72a0b6b2	B6P-65H
	0xeab3150efcb34ff74930d8f8d491be109070a39e4d380de7737aff5c72a0b6b2	Q60-QKR
	0xeab3150efcb34ff74930d8f3d491be109010a39e4d380dc7737aff5c72a0b6b2	ZSJ-7BK

What a forger pays, by calculation. One current GPU tries about 1.4 billion addresses per second; a try against hh also has to compute the stretched base digest, which leaves about 680 000 tries per second. The figures are the expected search times on one such GPU for a typical picture (SECURITY.md of hh-cpp has the reasoning).

The forged address has to match	Tries	One GPU
the first 4 and the last 4 hex digits	2^32	3 seconds
the first 6 and the last 6 hex digits	2^48	2.3 days
the first 8 and the last 8 hex digits	2^64	420 years
the universal picture, with two cells allowed to differ	2^52, stretched	210 years
the universal picture, in every cell	2^68, stretched	14 million years
the ends of the text and the picture	the product of the two
the keyed picture	cannot be searched: without the key the picture cannot be computed

A lookalike of the text is cheap, which is why address poisoning works. A lookalike of the picture is not, and the two costs multiply. A picture that looks the same is still strong evidence rather than proof; the tag or the full address is the check that is certain.

Properties

• Two modes. A universal picture is the same for everyone and is what two people compare. A keyed picture is computed with a 32-byte secret of the wallet: an attacker who does not hold the key cannot compute, and therefore cannot grind, a lookalike. Inside an application keyed pictures are the default.
• Deterministic to the byte. Integer arithmetic only. The same input gives the same pixels and the same PNG, BMP and JPEG bytes as hh-cpp, on every platform.
• Frozen. The algorithm has no version and never changes; a picture that a user has learned stays the same for ever. Library releases follow SemVer and never alter the output.
• No dependencies beyond kotlin-stdlib: no java.awt, javax.imageio, java.util.zip or android.*. SHA-256, HMAC, PBKDF2, deflate and the image encoders are part of the library.
• Made for colour vision deficiency. About one man in twelve does not see colours the way the rest do. The four colours were chosen for them: the palette was searched so that every pair stays apart under simulated protanopia, deuteranopia and tritanopia, and every colour keeps a contrast of 3:1 on white and on dark surfaces. Shape carries most of the information, so a picture still works in greyscale (the measurements are in docs/design of hh-cpp).
• Pixels, not pictures. The library returns pixel arrays and encoded files; making a Bitmap or an ImageBitmap of them is one line in the host.

Where it is used

hh answers one question: is this the same address as the one I mean? Wherever a person has to answer it from a long string, a picture answers it faster and more reliably than the first and last characters do.

• Sending and confirming. The picture of the recipient stands next to the address field and on the confirmation screen. A swapped or mistyped address changes it completely.
• Address books and account lists. Every saved payee and every account of the user carries its picture, so a list is scanned instead of read; 32 to 48 px is enough for recognition.
• Two devices of one user. An offline signer and the online device show the same picture for the same address, so the two screens are compared at a glance instead of 64 characters.
• Support, screenshots and voice. The six-character tag (TKS-PVH) travels through chat and over the phone; the picture travels in a screenshot.
• Documents and messages from a server. The encoders return PNG, BMP or JPEG bytes, so a backend puts the picture into a receipt, an invoice or an email without a graphics library.
• Anything that is a hash, not only an address. An SSH or PGP key fingerprint, a TLS certificate pin, an API key, the checksum of a backup or of a firmware image.

Three rules keep it honest: the picture complements the text check and never replaces it; inside one application keyed pictures are the default and universal pictures are what is shared with others; a picture that backs a decision is at least 64 dp and stands beside the picture it is compared with. docs/INTEGRATION.md has the rest.

Quick start

dependencies {
    implementation("io.github.censync:hh:1.1.0")
}

import io.github.censync.hh.*

val digest = BaseDigest.ofHex("0x5aAeb6053F3E94C9b9A09f33669435E7Ef1BeAed")  // slow: cache it
val fingerprint = Fingerprint.universal(digest)      // or Fingerprint.keyed(digest, key)
val image = fingerprint.render(128)                  // 128 x 128 pixels

val bitmap = Bitmap.createBitmap(image.toArgb(), image.width, image.height, Bitmap.Config.ARGB_8888)
val png: ByteArray = image.encodePng()
val tag: String = fingerprint.tag                    // "TKSPVH", shown as TKS-PVH

Invalid arguments throw HhException, an IllegalArgumentException that carries the HhErrorCode of the specification; the functions that take outside data have ...OrNull companions. Text goes in as a String (BaseDigest.ofText) or, when it is UTF-8 bytes already, verbatim (BaseDigest.ofUtf8). Consumers need Kotlin 1.9 or newer (the jar carries Kotlin 2.0 metadata) and a JVM 8 or Android API 24 runtime.

From Java the API reads the same: the factories are static, the constants are fields and default arguments are overloads.

BaseDigest digest = BaseDigest.ofHex("0x5aAeb6053F3E94C9b9A09f33669435E7Ef1BeAed");
HhImage image = Fingerprint.universal(digest).render(128, RenderOptions.TRANSPARENT);
byte[] png = image.encodePng();

A decision (confirming a payment, verifying a pasted address) should be backed by a picture of at least 64 dp, better 96 dp, next to the picture it is compared with. Smaller pictures are for recognition in lists. See docs/INTEGRATION.md for Android, Compose and desktop recipes and for the product rules, and SECURITY.md of hh-cpp for what a picture proves and what it does not.

Looks

The cells, the palette and the geometry are fixed; the host chooses the shape, the background and the frame, in either mode. Every picture below is the address of the quick start, rendered at 128 px.

Shape	Opaque white	Light blue E8EEF7	Transparent	Keyed, transparent
Square
Round

• Background. Any colour with any transparency. Outside rounded corners and outside the disc the picture is transparent anyway, so a transparent background takes whatever is behind it: the two transparent columns above are the same bytes on a light page and on a dark one.
• Contrast. An opaque background is refused below 2:1 against a palette colour, and the contrast report gives the WCAG ratio so that a host can warn below 3:1. White scores 300, the light blue above 257, 121212 scores 300; mid greys and saturated surfaces are what to avoid.
• Frames are open to both modes. Every style that fits the shape works for universal and keyed pictures alike. By default a universal picture has no frame and a keyed square gets rounded corners; a host that marks its keyed pictures picks one style and keeps it everywhere, and names the mode in the caption, since a frame alone proves nothing.
• The round shape inscribes the same grid in a circle, so its cells are about a third smaller; give it a third more pixels.

val options = RenderOptions(
    shape = Shape.ROUND,
    backgroundRgb = 0xE8EEF7,
    backgroundAlpha = 0,       // transparent; the colour then does not matter
    frame = FrameStyle.TICKS,  // any style of the shape, in either mode
)

val report = options.measureContrast(pageRgb = 0xFFFFFF)
if (report.figuresX100 < 300) { /* warn */ }

A complete program

A JVM command line program that writes the picture of an address to a PNG file and prints its tag. Gradle takes hh and the Kotlin standard library from Maven Central.

settings.gradle.kts:

rootProject.name = "hh-example"

build.gradle.kts:

plugins {
    kotlin("jvm") version "2.4.20"
    application
}

repositories {
    mavenCentral()
}

dependencies {
    implementation("io.github.censync:hh:1.1.0")
}

application {
    mainClass = "MainKt"
}

src/main/kotlin/Main.kt:

import io.github.censync.hh.BaseDigest
import io.github.censync.hh.Fingerprint
import java.io.File

fun main() {
    val digest = BaseDigest.ofHex("0x5aAeb6053F3E94C9b9A09f33669435E7Ef1BeAed")
    val fingerprint = Fingerprint.universal(digest)
    File("address.png").writeBytes(fingerprint.render(128).encodePng())
    val tag = fingerprint.tag
    println("${tag.take(3)}-${tag.drop(3)}")
}

gradle run prints TKS-PVH and writes address.png, byte for byte the file testdata/golden/evm-1-universal-128.png that every implementation reproduces. A Maven project declares the same coordinates, and kotlin-stdlib comes with them:

<dependency>
    <groupId>io.github.censync</groupId>
    <artifactId>hh</artifactId>
    <version>1.1.0</version>
</dependency>

Building

JDK 11 or newer; the Gradle wrapper fetches Gradle 8.9.

./gradlew build                                  # compiles and runs every test
tools/crosscheck.sh <path to hh_cli of hh-cpp>   # differential test against hh-cpp
./gradlew :benchmark:run                         # stretching benchmark on the JVM
benchmark/run-on-device.sh                       # the same on a connected Android device

Module	Purpose
hh	the published library, io.github.censync:hh
cli	command line tool for the differential test; never published
benchmark	stretching benchmark for the JVM and Android devices; never published

The rules for patches are in CONTRIBUTING.md, the releases in CHANGELOG.md.

License

MIT, see LICENSE. Copyright (c) 2026 Dmitry Mandrika. CenSync