HMAC Truncation

demo.php

<?php
function canonicalize(string ...$pieces) {
    $output []= pack('J', count($pieces));
    foreach ($pieces as $piece) {
        $output []= pack('J', mb_strlen($piece, '8bit'));
        $output []= $piece;
    }
    return implode($output);
}

function encrypt(string $plaintext, string $key, array $identity): string
{
    $nonce = random_bytes(24);
    $identity []= $nonce;
    $output = sodium_crypto_aead_xchacha20poly1305_ietf_encrypt(
        $plaintext,
        canonicalize(...$identity),
        $nonce,
        $key
    );
    return sodium_bin2hex($nonce . $output);
}

function decrypt(string $ciphertext_hex, string $key, array $identity): string
{
    $ciphertext_raw = sodium_hex2bin($ciphertext_hex);
    $nonce = mb_substr($ciphertext_raw, 0, 24, '8bit');
    $encrypted = mb_substr($ciphertext_raw, 24, null, '8bit');
    $identity []= $nonce;
    return sodium_crypto_aead_xchacha20poly1305_ietf_decrypt(
        $encrypted,
        canonicalize(...$identity),
        $nonce,
        $key
    );
}

$k1 = random_bytes(32);
$k2 = random_bytes(32);

$rows = [
    ['name' => 'foo', 'email' => 'foo@example.com'],
    ['name' => 'bar', 'email' => 'bar@example.com']
];

$encrypted = [];
$decrypted = [];
foreach ($rows as $i => $row) {
    if (!isset($encrypted[$i])) {
        $encrypted[$i] = [];
    }
    $encrypted[$i]['name'] = encrypt($row['name'], $k1, ['users', 'name']);
    $encrypted[$i]['email'] = encrypt($row['email'], $k1, ['users', 'email']);
    $encrypted[$i]['email_idx'] = substr(hash_hmac('sha256', $row['email'], $k1), 48);

    if (!isset($decrypted[$i])) {
        $decrypted[$i] = [];
    }
    $decrypted[$i]['name'] = decrypt($encrypted[$i]['name'], $k1, ['users', 'name']);
    $decrypted[$i]['email'] = decrypt($encrypted[$i]['email'], $k1, ['users', 'email']);
}

var_dump($encrypted, $decrypted);

output.txt

array(2) {
  [0]=>
  array(3) {
    ["name"]=>
    string(86) "1744440ec2d27b4a8c7d092e2367daa4c1f3d33a5f8aabde7221a05796120c4df078b5fc99cee6a65ddc31"
    ["email"]=>
    string(110) "444ca3f4092215884d04f360bc593a76a6776bdbbabc7950eb1129d182aac703c02368d617380883ec8fd43703f3f585492c142097ebb5"
    ["email_idx"]=>
    string(16) "28e9ef076233d870"
  }
  [1]=>
  array(3) {
    ["name"]=>
    string(86) "4b09b22685b7c5c4c06a3c1d3deb7f731f0f00f0b10dd94f50cf8dccd76e7e7f8b018734facf5f663f2727"
    ["email"]=>
    string(110) "8843320551b538f8fe3aa0e20d841fa2cc1fc12f10a771c063bef16d35bf14ccc4c3dee7e8cfae626641d252d475b515327e29e79965ca"
    ["email_idx"]=>
    string(16) "04e48ca810a8b1ae"
  }
}
array(2) {
  [0]=>
  array(2) {
    ["name"]=>
    string(3) "foo"
    ["email"]=>
    string(15) "foo@example.com"
  }
  [1]=>
  array(2) {
    ["name"]=>
    string(3) "bar"
    ["email"]=>
    string(15) "bar@example.com"
  }
}