AES-256-GCM加解密适配API问题求助（.NET+BouncyCastle）

对接第三方API需要实现AES-256-GCM加解密逻辑，对方提供了PHP示例代码。我基于.NET框架用BouncyCastle库开发了对应实现，本地自行加解密测试完全正常，但调用API时一直返回“Cannot decrypt message”错误。

已经反复查阅相关文档、调试迭代多版代码，问题仍未解决，贴出当前代码寻求帮助：

.NET加密代码 #

using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Crypto.Parameters;
using System.Text;

public static string AesGcmEncrypt(string plainText, byte[] key, byte[] nonce)
{
    var gcmCipher = new GcmBlockCipher(new AesEngine());
    var aeadParams = new AeadParameters(new KeyParameter(key), 128, nonce);
    gcmCipher.Init(true, aeadParams);

    byte[] plainBytes = Encoding.UTF8.GetBytes(plainText);
    byte[] outputBuffer = new byte[gcmCipher.GetOutputSize(plainBytes.Length)];
    
    int processedLength = gcmCipher.ProcessBytes(plainBytes, 0, plainBytes.Length, outputBuffer, 0);
    gcmCipher.DoFinal(outputBuffer, processedLength);

    // 参照PHP示例拼接nonce、密文、tag
    byte[] combined = nonce.Concat(outputBuffer).ToArray();
    return Convert.ToBase64String(combined);
}

.NET解密代码 #

public static string AesGcmDecrypt(string encryptedText, byte[] key)
{
    byte[] combinedBytes = Convert.FromBase64String(encryptedText);
    // 取前12字节作为nonce（GCM标准推荐长度）
    byte[] nonce = combinedBytes.Take(12).ToArray();
    byte[] cipherWithTag = combinedBytes.Skip(12).ToArray();

    var gcmCipher = new GcmBlockCipher(new AesEngine());
    var aeadParams = new AeadParameters(new KeyParameter(key), 128, nonce);
    gcmCipher.Init(false, aeadParams);

    byte[] plainBuffer = new byte[gcmCipher.GetOutputSize(cipherWithTag.Length)];
    
    int processedLength = gcmCipher.ProcessBytes(cipherWithTag, 0, cipherWithTag.Length, plainBuffer, 0);
    gcmCipher.DoFinal(plainBuffer, processedLength);

    return Encoding.UTF8.GetString(plainBuffer);
}

对方提供的PHP示例代码片段 #

function encrypt($plaintext, $key, $nonce) {
    $ciphertext = openssl_encrypt($plaintext, 'aes-256-gcm', $key, OPENSSL_RAW_DATA, $nonce, $tag);
    return base64_encode($nonce . $ciphertext . $tag);
}

function decrypt($ciphertext, $key) {
    $data = base64_decode($ciphertext);
    $nonce = substr($data, 0, 12);
    $ciphertextPart = substr($data, 12, -16);
    $tag = substr($data, -16);
    return openssl_decrypt($ciphertextPart, 'aes-256-gcm', $key, OPENSSL_RAW_DATA, $nonce, $tag);
}

本地测试时，用我的.NET代码加密后再解密能得到原文本，但和第三方API交互就失败，有没有大佬能看出哪里存在差异？

内容的提问来源于stack exchange，提问作者Basje313