ALTCHA PHP Library

July 20, 2026 · View on GitHub

A lightweight PHP library for creating and verifying ALTCHA challenges using key derivation functions (PBKDF2, Argon2id, Scrypt).

Installation

composer require altcha-org/altcha

Usage

<?php

require 'vendor/autoload.php';

use AltchaOrg\Altcha\Altcha;
use AltchaOrg\Altcha\CreateChallengeOptions;
use AltchaOrg\Altcha\SolveChallengeOptions;
use AltchaOrg\Altcha\VerifySolutionOptions;
use AltchaOrg\Altcha\Payload;
use AltchaOrg\Altcha\Algorithm\Pbkdf2;

$pbkdf2 = new Pbkdf2();
$altcha = new Altcha(
    hmacSignatureSecret: 'secret',
    hmacKeySignatureSecret: 'key-secret', // optional, enables fast verification path
);

// Create a new challenge
// Modify the cost and counter depending on the algorithm
$challenge = $altcha->createChallenge(new CreateChallengeOptions(
    algorithm: $pbkdf2,
    cost: 5000,
    counter: random_int(5000, 10000),
    expiresAt: time() + 600,
));

// Solve the challenge (client-side in production)
$solution = $altcha->solveChallenge(new SolveChallengeOptions(
    algorithm: $pbkdf2,
    challenge: $challenge,
));

// Verify the solution (server-side)
if ($solution !== null) {
    $payload = new Payload($challenge, $solution);
    $result = $altcha->verifySolution(new VerifySolutionOptions(
        algorithm: $pbkdf2,
        payload: $payload,
    ));

    if ($result->verified) {
        echo "Solution verified!\n";
    }
}

VerifySolutionOptions::$payload also accepts the raw base64-encoded string posted by the widget, or a decoded associative array — no manual parsing required:

// $_POST['altcha'] is the base64-encoded payload string from the widget
$result = $altcha->verifySolution(new VerifySolutionOptions(
    payload: $_POST['altcha'],
    algorithm: $pbkdf2,
));

API

`Altcha`

$altcha = new Altcha(
    hmacSignatureSecret: 'secret',
    hmacKeySignatureSecret: 'key-secret', // enables fast verification path
    hmacAlgorithm: HmacAlgorithm::SHA256, // default
);

`Altcha::createChallenge(CreateChallengeOptions $options): Challenge`

Creates a new challenge.

`CreateChallengeOptions`

Parameter	Type	Default	Description
`algorithm`	`DeriveKeyInterface`	required	Key derivation algorithm
`cost`	`int`	required	Iterations/time cost
`counter`	`?int`	`null`	Counter for deterministic mode
`data`	`?array`	`null`	Custom metadata
`expiresAt`	`?int`	`null`	Unix timestamp for expiration
`keyLength`	`int`	`32`	Derived key length in bytes
`keyPrefixLength`	`int`	`keyLength / 2`	Key prefix length in bytes
`memoryCost`	`?int`	`null`	Memory cost (Argon2id/Scrypt)
`nonce`	`?string`	`null`	Custom nonce (hex)
`parallelism`	`?int`	`null`	Parallelism factor (Scrypt)
`salt`	`?string`	`null`	Custom salt (hex)

When counter is provided and hmacKeySignatureSecret is set, the challenge includes a keySignature for fast verification (skips re-derivation).

Returns: Challenge with parameters (ChallengeParameters) and signature.

`Altcha::solveChallenge(SolveChallengeOptions $options): ?Solution`

Iterates counter values to find a derived key matching the challenge prefix.

`SolveChallengeOptions`

Parameter	Type	Default	Description
`algorithm`	`DeriveKeyInterface`	required	Key derivation algorithm
`challenge`	`Challenge`	required	The challenge to solve
`start`	`int`	`0`	Initial counter value
`step`	`int`	`1`	Counter increment per iteration
`timeout`	`float`	`30.0`	Timeout in seconds

Returns: Solution with counter, derivedKey (hex), and time (seconds). Returns null if no match is found.

`Altcha::verifySolution(VerifySolutionOptions $options): VerifySolutionResult`

Verifies a solution against its challenge.

`VerifySolutionOptions`

Parameter	Type	Default	Description
`algorithm`	`DeriveKeyInterface`	required	Key derivation algorithm
`payload`	`Payload\|string\|array`	required	Challenge + solution pair — a `Payload` object, a raw base64-encoded payload string (as posted by the widget), or a decoded associative array. Throws `InvalidArgumentException` if a string/array can't be parsed into a valid payload.

`VerifySolutionResult`

Property	Type	Description
`verified`	`bool`	Whether the solution is valid
`expired`	`bool`	Whether the challenge has expired
`invalidSignature`	`?bool`	Whether the challenge signature is invalid
`invalidSolution`	`?bool`	Whether the derived key doesn't match
`time`	`float`	Verification time in seconds

Key Derivation Algorithms

All algorithms implement DeriveKeyInterface.

PBKDF2

use AltchaOrg\Altcha\Algorithm\Pbkdf2;
use AltchaOrg\Altcha\HmacAlgorithm;

new Pbkdf2();                        // PBKDF2/SHA-256
new Pbkdf2(HmacAlgorithm::SHA384);  // PBKDF2/SHA-384
new Pbkdf2(HmacAlgorithm::SHA512);  // PBKDF2/SHA-512

Argon2id

Requires ext-sodium (typically bundled with PHP).

use AltchaOrg\Altcha\Algorithm\Argon2id;

new Argon2id();

Uses memoryCost from challenge options.

Scrypt

Requires ext-scrypt (php-scrypt).

use AltchaOrg\Altcha\Algorithm\Scrypt;

new Scrypt();

Uses memoryCost (r, default: 8) and parallelism (p, default: 1) from challenge options.

`HmacAlgorithm`

Enum used for HMAC signing and PBKDF2 hash selection:

HmacAlgorithm::SHA256 - SHA-256
HmacAlgorithm::SHA384 - SHA-384
HmacAlgorithm::SHA512 - SHA-512

`ServerSignature::verifyServerSignature(string|array $data, string $hmacKey, HmacAlgorithm $hmacAlgorithm = HmacAlgorithm::SHA256): ServerSignatureVerification`

Verifies a server signature payload.

`ServerSignatureVerification`

Property	Type	Description
`verified`	`bool`	Whether the signature is valid, not expired, and solution verified
`expired`	`bool`	Whether the verification data has expired
`invalidSignature`	`bool`	Whether the HMAC signature is invalid
`invalidSolution`	`bool`	Whether the solution was not verified
`time`	`float`	Verification time in seconds
`verificationData`	`?ServerSignatureVerificationData`	Parsed verification data

Verification data is parsed generically from URL-encoded params with auto-detected types (bool, int, float, string, array for fields/reasons). Access values via property or array syntax:

use AltchaOrg\Altcha\ServerSignature;

$result = ServerSignature::verifyServerSignature($payload, 'server-secret');

if ($result->verified) {
    $result->verificationData->expire;          // int
    $result->verificationData->score;           // float
    $result->verificationData->verified;        // bool
    $result->verificationData->fields;          // array
    $result->verificationData->classification;  // string
    $result->verificationData['email'];         // array access also works
}

`ServerSignature::verifyFieldsHash(array $formData, array $fields, string $fieldsHash, HmacAlgorithm $hmacAlgorithm = HmacAlgorithm::SHA256): bool`

Verifies the hash of specific form fields.

$isValid = ServerSignature::verifyFieldsHash(
    formData: ['name' => 'John', 'email' => 'john@example.com'],
    fields: ['name', 'email'],
    fieldsHash: hash('sha256', "John\njohn@example.com"),
);

`Sentinel::verify(VerifyServerOptions $options): VerifyServerResult`

Verifies a payload remotely by calling ALTCHA Sentinel's POST /v1/verify/signature API, instead of verifying the HMAC signature locally. Useful when Sentinel issues and signs challenges directly, so your server doesn't need to hold the HMAC secret at all.

use AltchaOrg\Altcha\Sentinel;
use AltchaOrg\Altcha\VerifyServerOptions;

$result = Sentinel::verify(new VerifyServerOptions(
    payload: $_POST['altcha'], // raw base64 string, or a decoded array
    url: 'https://sentinel.example.com/v1/verify/signature',
    secret: $sentinelApiKeySecret, // optional, checked against the payload's API key
    timeout: 10.0,
    retries: 2,
));

if ($result->verified) {
    // ...
}

`VerifyServerOptions`

Parameter	Type	Default	Description
`payload`	`string\|array`	required	The payload to verify, as received from the client (raw base64 string or decoded array)
`url`	`string`	required	Full URL of the Sentinel `/v1/verify/signature` endpoint
`secret`	`?string`	`null`	API key secret checked against the payload's API key
`httpClient`	`?HttpClientInterface`	`null`	Custom HTTP client. Defaults to a built-in stream-based client requiring no extra extension
`headers`	`array<string, string>`	`[]`	Additional headers to send with the request
`timeout`	`float`	`10.0`	Per-attempt request timeout in seconds
`retries`	`int`	`0`	Number of retry attempts after the first try
`retryDelay`	`int`	`300`	Base delay in milliseconds between retries
`retryBackoff`	`RetryBackoff`	`RetryBackoff::Exponential`	Backoff strategy (`Fixed` or `Exponential`) applied to `retryDelay`

A 4xx/non-2xx HTTP response and network errors are retried up to retries times (with backoff), except HTTP 400, which is treated as a definitive verification failure and returned immediately.

`VerifyServerResult`

Property	Type	Description
`verified`	`bool`	Whether the payload was successfully verified
`apiKey`	`?string`	API key associated with the verification
`reason`	`?string`	Reason or error message if verification failed
`verificationData`	`?ServerSignatureVerificationData`	Verification data returned by Sentinel

To use your own HTTP client (e.g. Guzzle, Symfony HttpClient) instead of the built-in stream-based one, implement AltchaOrg\Altcha\Http\HttpClientInterface and pass it via httpClient:

use AltchaOrg\Altcha\Http\HttpClientInterface;
use AltchaOrg\Altcha\Http\HttpResponse;

class MyHttpClient implements HttpClientInterface
{
    public function send(string $url, string $method, array $headers, string $body, float $timeout): HttpResponse
    {
        // ... perform the request with your client of choice ...
        return new HttpResponse($statusCode, $responseBody);
    }
}

`Payload`

Wraps a Challenge and Solution pair for verification and serialization.

$payload = new Payload($challenge, $solution);

$payload->toArray();   // array
$payload->toJson();    // JSON string
$payload->toBase64();  // base64-encoded JSON

V1 API

The V1 API is available under the AltchaOrg\Altcha\V1 namespace. It uses simple hash-based challenges (SHA-1, SHA-256, SHA-512) instead of key derivation functions.

Tests

vendor/bin/phpunit tests

License

MIT