【PHP8.x】sodium_crypto_core_ristretto255_sub()関数の使い方

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1<?php
2
3/**
4 * Demonstrates the use of sodium_crypto_core_ristretto255_sub
5 * to subtract two Ristretto255 points.
6 *
7 * This function is part of the libsodium extension and performs
8 * elliptic curve point arithmetic. Specifically, it calculates P - Q,
9 * where P and Q are Ristretto255 points represented as 32-byte strings.
10 */
11function demonstrateRistretto255Subtraction(): void
12{
13    // Ensure the Sodium extension is loaded.
14    // This is crucial for all sodium_* functions to work.
15    if (!extension_loaded('sodium')) {
16        die('Error: The Sodium extension is not enabled. Please check your php.ini configuration.');
17    }
18
19    echo "--- Demonstrating Ristretto255 Point Subtraction (P - Q) ---\n\n";
20
21    // 1. Obtain the Ristretto255 base point (generator point).
22    // This is a fixed, standard 32-byte point on the curve.
23    $basePoint = sodium_crypto_core_ristretto255_base();
24    echo "Base Point (Generator): " . bin2hex($basePoint) . "\n";
25
26    // 2. Generate two random scalars.
27    // Scalars are 32-byte secret numbers used for point multiplication.
28    // SODIUM_CRYPTO_CORE_RISTRETTO255_SCALARBYTES is a constant for the scalar size (32 bytes).
29    $scalar1 = random_bytes(SODIUM_CRYPTO_CORE_RISTRETTO255_SCALARBYTES);
30    $scalar2 = random_bytes(SODIUM_CRYPTO_CORE_RISTRETTO255_SCALARBYTES);
31
32    echo "Scalar 1 (for Point P): " . bin2hex($scalar1) . "\n";
33    echo "Scalar 2 (for Point Q): " . bin2hex($scalar2) . "\n\n";
34
35    // 3. Multiply the base point by each scalar to derive two distinct Ristretto255 points.
36    // These derived points (Point P and Point Q) are valid inputs for subtraction.
37    $pointP = sodium_crypto_core_ristretto255_scalar_mul($basePoint, $scalar1);
38    $pointQ = sodium_crypto_core_ristretto255_scalar_mul($basePoint, $scalar2);
39
40    echo "Point P (Base * Scalar 1): " . bin2hex($pointP) . "\n";
41    echo "Point Q (Base * Scalar 2): " . bin2hex($pointQ) . "\n\n";
42
43    // 4. Perform the point subtraction: Result = P - Q.
44    // The `sodium_crypto_core_ristretto255_sub` function takes two 32-byte Ristretto255
45    // points and returns their difference, which is also a 32-byte Ristretto255 point.
46    try {
47        $resultPoint = sodium_crypto_core_ristretto255_sub($pointP, $pointQ);
48        echo "Resulting Point (P - Q): " . bin2hex($resultPoint) . "\n\n";
49
50        // Optional: Verify the subtraction by adding Q back to the result.
51        // (P - Q) + Q should theoretically equal P.
52        $verificationPoint = sodium_crypto_core_ristretto255_add($resultPoint, $pointQ);
53        echo "Verification: (P - Q) + Q = " . bin2hex($verificationPoint) . "\n";
54
55        if ($pointP === $verificationPoint) {
56            echo "Verification successful: (P - Q) + Q is identical to P.\n";
57        } else {
58            echo "Verification failed: (P - Q) + Q is NOT identical to P. This indicates an issue.\n";
59        }
60
61    } catch (SodiumException $e) {
62        // Catch any errors that might occur during cryptographic operations.
63        echo "An error occurred during the Ristretto255 subtraction: " . $e->getMessage() . "\n";
64    }
65}
66
67// Execute the demonstration function when the script runs.
68demonstrateRistretto255Subtraction();

PHPのsodium_crypto_core_ristretto255_sub関数は、Ristretto255楕円曲線暗号の文脈で、2つのRistretto255ポイントの減算を実行するための機能です。この関数は、libsodiumという暗号ライブラリのPHP拡張機能の一部として提供されており、セキュリティを考慮したアプリケーション開発で利用されます。

引数には、減算の対象となる2つのRistretto255ポイントを文字列型で指定します。具体的には、最初の引数$pが引かれる点、次の引数$qが引く点となります。これらのポイントはそれぞれ32バイトのバイナリ文字列として表現される必要があります。関数は、$pから$qを減算した結果の新しいRistretto255ポイントを、同じく32バイトのバイナリ文字列として戻り値で返します。

サンプルコードでは、まずlibsodium拡張が有効であることを確認し、sodium_crypto_core_ristretto255_base関数を用いてRistretto255楕円曲線の基準点（ベースポイント）を取得しています。次に、random_bytesで生成したランダムなスカラー値を基に、sodium_crypto_core_ristretto255_scalar_mul関数を使って2つの異なるRistretto255ポイント（$pointPと$pointQ）を生成しています。これらの生成されたポイントがsodium_crypto_core_ristretto255_sub関数の引数として渡され、PからQを引いた結果のポイントが$resultPointに格納されます。最後に、sodium_crypto_core_ristretto255_add関数を使用して、減算結果に$pointQを加算し、その結果が元の$pointPと一致するかを検証することで、減算処理の正当性を確認しています。この関数は、鍵交換やデジタル署名といった高度な暗号処理の基盤となる数学的演算の一つです。

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1<?php
2
3/**
4 * Demonstrates the usage of sodium_crypto_core_ristretto255_sub.
5 *
6 * This function performs the subtraction of two Ristretto255 points, P and Q (P - Q).
7 * Ristretto255 is an elliptic curve group used in advanced cryptographic operations
8 * like key exchange or digital signatures.
9 *
10 * This sample generates two points and subtracts them, then verifies the result
11 * by an equivalent scalar subtraction.
12 *
13 * Note: The keyword `/usr/lib64/php/modules/sodium.so undefined symbol crypto_core_ristretto255_sub`
14 * indicates that the 'sodium' PHP extension, or specifically the 'ristretto255'
15 * functionality within it, might not be properly installed or is from an older version
16 * that does not support these functions (Ristretto255 functions were added in PHP 8.1).
17 * If you encounter this error, ensure your PHP version is 8.1 or higher and
18 * the 'sodium' extension is correctly installed and up-to-date with the 'libsodium' library.
19 */
20function demonstrateRistretto255Subtraction(): void
21{
22    // 1. Generate two random scalars.
23    // In elliptic curve cryptography, scalars act like private numbers or keys.
24    // Each scalar is a 32-byte string.
25    $scalar1 = sodium_crypto_core_ristretto255_scalar_random();
26    $scalar2 = sodium_crypto_core_ristretto255_scalar_random();
27
28    // 2. Get the Ristretto255 base point (G).
29    // This is a standard, fixed, and publicly known point on the Ristretto255 curve.
30    $basePointG = sodium_crypto_core_ristretto255_base();
31
32    // 3. Compute two Ristretto255 points, P and Q.
33    // Points are generated by multiplying the base point G with a scalar.
34    // P = scalar1 * G
35    // Q = scalar2 * G
36    $pointP = sodium_crypto_core_ristretto255_scalar_mul($scalar1, $basePointG);
37    $pointQ = sodium_crypto_core_ristretto255_scalar_mul($scalar2, $basePointG);
38
39    echo "Generated Point P (scalar1 * G): " . bin2hex($pointP) . "\n";
40    echo "Generated Point Q (scalar2 * G): " . bin2hex($pointQ) . "\n";
41
42    // 4. Perform the subtraction: R = P - Q.
43    // This operation computes a new point R which is the cryptographic difference of P and Q.
44    $pointR = sodium_crypto_core_ristretto255_sub($pointP, $pointQ);
45
46    echo "Result Point R (P - Q):          " . bin2hex($pointR) . "\n";
47
48    // 5. (Optional) Verify the result by an equivalent scalar operation.
49    // P - Q is mathematically equivalent to (scalar1 - scalar2) * G.
50    // First, subtract the scalars to get a difference scalar.
51    $scalarDiff = sodium_crypto_core_ristretto255_scalar_sub($scalar1, $scalar2);
52    // Then, multiply this difference scalar with the base point G.
53    $expectedR = sodium_crypto_core_ristretto255_scalar_mul($scalarDiff, $basePointG);
54
55    echo "Expected Point R ((scalar1 - scalar2) * G): " . bin2hex($expectedR) . "\n";
56
57    // Compare the two results to ensure consistency.
58    if ($pointR === $expectedR) {
59        echo "Verification successful: P - Q matches (scalar1 - scalar2) * G.\n";
60    } else {
61        echo "Verification failed: P - Q does NOT match (scalar1 - scalar2) * G.\n";
62    }
63}
64
65// Execute the demonstration function.
66demonstrateRistretto255Subtraction();

このサンプルコードは、PHPのsodium_crypto_core_ristretto255_sub関数を使ったRistretto255楕円曲線上の点の減算処理を示しています。Ristretto255は、鍵交換やデジタル署名といった高度な暗号操作に用いられる楕円曲線群の一種です。

sodium_crypto_core_ristretto255_sub関数は、二つのRistretto255点 $pと$q（いずれも32バイトのバイナリデータ）を受け取り、点$pから点$qを減算した結果の新しい点（これも32バイトのバイナリデータ）を返します。

コードではまず、二つのスカラー（暗号的な秘密の値）を生成し、それぞれをRistretto255の基準点（ベースポイント）に乗算することで、二つの点PとQを計算しています。次に、sodium_crypto_core_ristretto255_sub関数を用いてPからQを減算し、結果の点Rを得ています。さらに、スカラーの減算結果をベースポイントに乗算した点と比較することで、この減算結果が数学的に正しいことを検証しています。

もし、/usr/lib64/php/modules/sodium.so undefined symbol crypto_core_ristretto255_subというエラーに遭遇した場合、それはPHPのsodium拡張、またはその基盤となるlibsodiumライブラリがRistretto255関数をサポートしていない古いバージョンである可能性が高いです。この機能はPHP 8.1以降で導入されたため、PHPのバージョンを8.1以上に更新し、sodium拡張も最新の状態に保つことで解決できる場合があります。