.NET 4.8中如何实现RSA OAEP-SHA256加密？

问题背景 #

你在.NET 8中使用的RSA加密代码依赖RSA.Create()和RSAEncryptionPadding.OaepSHA256，但移植到.NET 4.8时，原生RSACryptoServiceProvider仅支持OAEP-SHA1，使用第三方扩展类尝试SHA256填充时抛出Specified padding mode is not valid for this algorithm错误。

解决方案 #

方案1：使用.NET 4.8原生RSA基类实现 #

.NET 4.8中RSA基类已支持灵活的加密填充配置，默认实例为RSACng（基于Cryptography Next Generation），可直接支持OAEP-SHA256，无需依赖第三方库。

完整代码实现

首先定义KeyPair类（若未定义可补充）：

public class KeyPair
{
    public string PublicKey { get; set; }
    public string PrivateKey { get; set; }
}

加解密及密钥生成方法：

using System;
using System.Security.Cryptography;
using System.Text;

public static class RsaOaepSha256Helper
{
    // 公钥加密（OAEP-SHA256）
    public static string EncryptWithPublicKey(string text, string publicKeyBase64)
    {
        using (RSA rsa = RSA.Create())
        {
            rsa.ImportRSAPublicKey(Convert.FromBase64String(publicKeyBase64), out _);
            byte[] dataBytes = Encoding.UTF8.GetBytes(text);
            byte[] encryptedBytes = rsa.Encrypt(dataBytes, RSAEncryptionPadding.OaepSHA256);
            return Convert.ToBase64String(encryptedBytes);
        }
    }

    // 私钥解密（OAEP-SHA256）
    public static string DecryptWithPrivateKey(string cipherTextBase64, string privateKeyBase64)
    {
        using (RSA rsa = RSA.Create())
        {
            rsa.ImportRSAPrivateKey(Convert.FromBase64String(privateKeyBase64), out _);
            byte[] encryptedBytes = Convert.FromBase64String(cipherTextBase64);
            byte[] decryptedBytes = rsa.Decrypt(encryptedBytes, RSAEncryptionPadding.OaepSHA256);
            return Encoding.UTF8.GetString(decryptedBytes);
        }
    }

    // 生成2048位RSA密钥对（兼容OAEP-SHA256）
    public static KeyPair GenerateKeyPair()
    {
        using (RSA rsa = RSA.Create(2048))
        {
            return new KeyPair
            {
                PublicKey = Convert.ToBase64String(rsa.ExportRSAPublicKey()),
                PrivateKey = Convert.ToBase64String(rsa.ExportRSAPrivateKey())
            };
        }
    }
}

方案2：使用Bouncy Castle库实现 #

若因环境限制无法使用RSACng，可通过Bouncy Castle库实现OAEP-SHA256加解密。

步骤1：安装NuGet包

安装BouncyCastle（推荐1.8.9及以上版本）：

Install-Package BouncyCastle

步骤2：代码实现

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

public static class RsaBouncyCastleHelper
{
    // 公钥加密（OAEP-SHA256）
    public static string EncryptWithPublicKey(string text, string publicKeyBase64)
    {
        byte[] publicKeyBytes = Convert.FromBase64String(publicKeyBase64);
        AsymmetricKeyParameter publicKey = PublicKeyFactory.CreateKey(publicKeyBytes);
        
        IBufferedCipher cipher = CipherUtilities.GetCipher("RSA/NONE/OAEPWithSHA256AndMGF1Padding");
        cipher.Init(true, publicKey);
        
        byte[] dataBytes = Encoding.UTF8.GetBytes(text);
        byte[] encryptedBytes = cipher.DoFinal(dataBytes);
        return Convert.ToBase64String(encryptedBytes);
    }

    // 私钥解密（OAEP-SHA256）
    public static string DecryptWithPrivateKey(string cipherTextBase64, string privateKeyBase64)
    {
        byte[] privateKeyBytes = Convert.FromBase64String(privateKeyBase64);
        AsymmetricKeyParameter privateKey = PrivateKeyFactory.CreateKey(privateKeyBytes);
        
        IBufferedCipher cipher = CipherUtilities.GetCipher("RSA/NONE/OAEPWithSHA256AndMGF1Padding");
        cipher.Init(false, privateKey);
        
        byte[] encryptedBytes = Convert.FromBase64String(cipherTextBase64);
        byte[] decryptedBytes = cipher.DoFinal(encryptedBytes);
        return Encoding.UTF8.GetString(decryptedBytes);
    }

    // 生成RSA密钥对
    public static KeyPair GenerateKeyPair()
    {
        RsaKeyPairGenerator generator = new RsaKeyPairGenerator();
        generator.Init(new KeyGenerationParameters(new SecureRandom(), 2048));
        AsymmetricCipherKeyPair keyPair = generator.GenerateKeyPair();
        
        byte[] publicKeyBytes = SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(keyPair.Public).GetEncoded();
        byte[] privateKeyBytes = PrivateKeyInfoFactory.CreatePrivateKeyInfo(keyPair.Private).GetEncoded();
        
        return new KeyPair
        {
            PublicKey = Convert.ToBase64String(publicKeyBytes),
            PrivateKey = Convert.ToBase64String(privateKeyBytes)
        };
    }
}

错误原因说明 #

你之前使用的扩展类基于RSACryptoServiceProvider，该类在.NET 4.8中底层依赖Windows CryptoAPI旧实现，仅支持SHA1的OAEP填充，因此无法识别OaepSHA256参数。改用RSA.Create()（默认返回RSACng实例）或Bouncy Castle即可避开这一限制。

内容的提问来源于stack exchange，提问作者cjsmith87