How to decrypt string from openssl_encrypt in c#

There's some junk in here I didn't clean up, so please excuse that. You have two things to change. Search the text in the parentheses to find where the fix is.

• (Fix 1) I read somewhere that the php method truncates keys that are longer than they should be. Your key was 64 chars but should be 32 chars.
• (Fix 2) The PaddingMode looks like it should be PKCS7

UPDATE

The code remains the same, but I added one more piece that I forgot I removed:

• (Fix 3) The key should be text, not hex.

using System;
using System.Security.Cryptography;
using System.IO;
using System.Linq;
using System.Collections.Generic;
                    // https://stackoverflow.com/questions/68673772/how-to-decrypt-string-from-openssl-encrypt-in-c-sharp
//https://stackoverflow.com/questions/19719294/decrypt-string-in-c-sharp-that-was-encrypted-with-php-openssl-encrypt
// https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography.rijndaelmanaged?view=net-5.0
public class Program
{
    // full key 1234567890123456789012345678901234567890123456789012345678901234
    
    // Fix 1: php truncates a key longer than what is supported, so I chopped this off to 32
    private static string key = "12345678901234567890123456789012";
    private static string originalText = "does this work";
    private static string encryptedValue = "a64d63874ba9ee8f5a5028cb40ab70a4yFFQwLuOHeWouyfp0dyrnw==";
    
    public static void Main()
    {
        // Console.WriteLine(OpenSslDecrypt(encryptedValue, key));
        Console.WriteLine("decrypted text: " + DecryptPHP(encryptedValue));
    }
    
    public static byte[] FromHex(string hex)
    {
        hex = hex.Replace("-", "");
        byte[] raw = new byte[hex.Length / 2];
        for (int i = 0; i < raw.Length; i++)
        {
            raw[i] = Convert.ToByte(hex.Substring(i * 2, 2), 16);
        }
        return raw;
    }
    
    public static string DecryptPHP(string encrypted)
    {
        var hexSalt = encryptedValue.Substring(0, 32);
        byte[] stringSalt = FromHex(hexSalt);
        
        var saltBytes = FromHex(hexSalt);
        Console.WriteLine("salt length bytes: " + saltBytes.Length);
        var unsaltedEncryptedValue = encryptedValue.Substring(32);
        
        Console.WriteLine("salt: " + hexSalt);
        Console.WriteLine("unsaltedEncryptedValue: " + unsaltedEncryptedValue);

        //get the encrypted data and decrypt
        byte[] unsaltedEncryptedBytes = Convert.FromBase64String(unsaltedEncryptedValue);
        // byte[] unsaltedEncryptedBytes = System.Text.ASCIIEncoding.UTF8.GetBytes(unsaltedEncryptedValue);

        // Fix 3: key should be text, not hex.
        // var _key = FromHex(key);
        byte[] keyBytes = System.Text.ASCIIEncoding.UTF8.GetBytes(key);
        
        return DecryptStringFromBytesAes(unsaltedEncryptedBytes, keyBytes, saltBytes);
    }

    public static string DecryptStringFromBytesAes(byte[] cipherText, byte[] key, byte[] iv)
    {
        // Check arguments.
        if (cipherText == null || cipherText.Length <= 0)
            throw new ArgumentNullException("cipherText");
        if (key == null || key.Length <= 0)
            throw new ArgumentNullException("key");
        if (iv == null || iv.Length <= 0)
            throw new ArgumentNullException("iv");

        // Declare the RijndaelManaged object
        // used to decrypt the data.
        RijndaelManaged aesAlg = null;

        // Declare the string used to hold
        // the decrypted text.
        string plaintext;

        // Create a RijndaelManaged object
        // with the specified key and IV.
        //  Fix 2: PaddingMode should be PKCS7, based on my tests. It gets rid of the extra ? chars.
        aesAlg = new RijndaelManaged { Mode = CipherMode.CBC, Padding = PaddingMode.PKCS7, KeySize = 256, BlockSize = 128, Key = key, IV = iv };

        // Create a decrytor to perform the stream transform.
        ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
        // Create the streams used for decryption.
        using (MemoryStream msDecrypt = new MemoryStream(cipherText))
        {
            using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
            {
                using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                {
                    // Read the decrypted bytes from the decrypting stream
                    // and place them in a string.
                    plaintext = srDecrypt.ReadToEnd();
                    srDecrypt.Close();
                }
            }
        }

        return plaintext;
    }
}

dotnetfiddle link: https://dotnetfiddle.net/s2tfmM