C#

AES-128-CBC Decryption

Encrypting a string using AES-128-CBC with phpseclib:

use phpseclib3\Crypt\AES;

$cipher = new AES('cbc');
$cipher->setKey(str_repeat('a', 16));
$cipher->setIV(str_repeat('b', 16));

echo bin2hex($cipher->encrypt('test'));

Decryption with C#:

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

class Test
{
    // from https://stackoverflow.com/a/311179/569976
    public static byte[] StringToByteArray(String hex)
    {
        int NumberChars = hex.Length;
        byte[] bytes = new byte[NumberChars / 2];
        for (int i = 0; i < NumberChars; i += 2)
            bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
          return bytes;
    }

    static void Main()
    {
        var ciphertext = StringToByteArray("10f42fd95857ed2775cfbc4b471bc213");

        var aes = new AesManaged();
        //aes.Mode = CipherMode.CBC;
        var key = Encoding.ASCII.GetBytes("aaaaaaaaaaaaaaaa");
        var iv = Encoding.ASCII.GetBytes("bbbbbbbbbbbbbbbb");

        ICryptoTransform decryptor = aes.CreateDecryptor(key, iv);
        MemoryStream msDecrypt = new MemoryStream(ciphertext);
        CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read);
        StreamReader srDecrypt = new StreamReader(csDecrypt);
        String plaintext = srDecrypt.ReadToEnd();

        Console.WriteLine(plaintext);
    }
}

RSA Decryption

Encryption with PHP:

use phpseclib3\Crypt\PublicKeyLoader;

$key = PublicKeyLoader::load('-----BEGIN RSA PUBLIC KEY-----
MEgCQQCo9+BpMRYQ/dL3DS2CyJxRF+j6ctbT3/Qp84+KeFhnii7NT7fELilKUSnx
S30WAvQCCo2yU1orfgqr41mM70MBAgMBAAE=
-----END RSA PUBLIC KEY-----');
$key = $key->withPadding(RSA::ENCRYPTION_PKCS1);
echo base64_encode($key->encrypt('test'));

Decryption with C#

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

class Test
{
    static void Main()
    {
        var key = @"<RSAKeyPair>
  <Modulus>qPfgaTEWEP3S9w0tgsicURfo+nLW09/0KfOPinhYZ4ouzU+3xC4pSlEp8Ut9FgL0AgqNslNaK34Kq+NZjO9DAQ==</Modulus>
  <Exponent>AQAB</Exponent>
  <P>oxK/MgGeeLui385KJ7ZOYktjhLBNAB69fKwTZFsUNh0=</P>
  <Q>AQlBGkUJzJ26e/ZsQ1w3+gFNHDf0TwY2/akhw3FmRxs1</Q>
  <DP>bYSzn3Py6AasNj6nEtCfB+i1p3F35TK/87DlPSrmAgk=</DP>
  <DQ>yS4RaI9YG8EWx/2w0T67ZUVAw8eOMB6BIUg0Xcu+3ok=</DQ>
  <InverseQ>E6z/k6I+ChN1LLttwX0galITxmAYrOBhBVl433tgTTQ=</InverseQ>
  <D>IJLixBy2qpFoS4DSmoEmo3qGy0t6z09AIJtH+5OeRV1be+N4cDYJKffGzDa88vQENZiRm0GRq6a+HPGQMd2kTQ==</D>
</RSAKeyPair>";

        var ciphertext = "L812/9Y8TSpwErlLR6Bz4J3uR/T5YaqtTtB5jxtD1qazGPI5t15V9drWi58colGOZFeCnGKpCrtQWKk4HWRocQ==";

        var ciphertextBytes = Convert.FromBase64String(ciphertext);

        var rsa = RSA.Create();
        rsa.FromXmlString(key);

        var plaintextBytes = rsa.Decrypt(ciphertextBytes, RSAEncryptionPadding.Pkcs1);

        var plaintext = Encoding.Default.GetString(plaintextBytes);

        Console.WriteLine(plaintext);
    }
}

Note that rsa.FromXmlString(key) only supports XML formatted keys (as the name implies lol).

If you're using .NET Core 3.0+ / are not using Mono, ImportRSAPrivateKey can be used to load PKCS1 formatted keys, using the following (untested) changes to the above (using the phpBB MOD Text Template):

#
#-----[ FIND ]------------------------------------------
#
using System.Text;
#
#-----[ AFTER, ADD ]------------------------------------
#
using System.Text.RegularExpressions;
#
#-----[ FIND ]------------------------------------------
#
        var key = @"<RSAKeyPair>
  <Modulus>qPfgaTEWEP3S9w0tgsicURfo+nLW09/0KfOPinhYZ4ouzU+3xC4pSlEp8Ut9FgL0AgqNslNaK34Kq+NZjO9DAQ==</Modulus>
  <Exponent>AQAB</Exponent>
  <P>oxK/MgGeeLui385KJ7ZOYktjhLBNAB69fKwTZFsUNh0=</P>
  <Q>AQlBGkUJzJ26e/ZsQ1w3+gFNHDf0TwY2/akhw3FmRxs1</Q>
  <DP>bYSzn3Py6AasNj6nEtCfB+i1p3F35TK/87DlPSrmAgk=</DP>
  <DQ>yS4RaI9YG8EWx/2w0T67ZUVAw8eOMB6BIUg0Xcu+3ok=</DQ>
  <InverseQ>E6z/k6I+ChN1LLttwX0galITxmAYrOBhBVl433tgTTQ=</InverseQ>
  <D>IJLixBy2qpFoS4DSmoEmo3qGy0t6z09AIJtH+5OeRV1be+N4cDYJKffGzDa88vQENZiRm0GRq6a+HPGQMd2kTQ==</D>
</RSAKeyPair>";
#
#-----[ REPLACE WITH ]----------------------------------
#
        var key = @"-----BEGIN RSA PRIVATE KEY-----
MIIBOgIBAAJBAKj34GkxFhD90vcNLYLInFEX6Ppy1tPf9Cnzj4p4WGeKLs1Pt8Qu
KUpRKfFLfRYC9AIKjbJTWit+CqvjWYzvQwECAwEAAQJAIJLixBy2qpFoS4DSmoEm
o3qGy0t6z09AIJtH+5OeRV1be+N4cDYJKffGzDa88vQENZiRm0GRq6a+HPGQMd2k
TQIhAKMSvzIBnni7ot/OSie2TmJLY4SwTQAevXysE2RbFDYdAiEBCUEaRQnMnbp7
9mxDXDf6AU0cN/RPBjb9qSHDcWZHGzUCIG2Es59z8ugGrDY+pxLQnwfotadxd+Uy
v/Ow5T0q5gIJAiEAyS4RaI9YG8EWx/2w0T67ZUVAw8eOMB6BIUg0Xcu+3okCIBOs
/5OiPgoTdSy7bcF9IGpSE8ZgGKzgYQVZeN97YE00
-----END RSA PRIVATE KEY-----";
#
#-----[ FIND ]------------------------------------------
#
        rsa.FromXmlString(key);
#
#-----[ REPLACE WITH ]----------------------------------
#
        var rx = new Regex("-+[^-]+-+");
        key = rx.Replace(key, "")
                .Replace("\r", "")
                .Replace("\n", "");
        var keyBytes = Convert.FromBase64String(key);

        rsa.ImportRSAPrivateKey(keyBytes, out _);

ImportPkcs8PrivateKey can be used to load PKCS8 formatted keys.

Now let us consider OAEP decryption. Here's the phpseclib to generate OAEP encrypted strings:

use phpseclib3\Crypt\PublicKeyLoader;

$key = PublicKeyLoader::load('-----BEGIN RSA PUBLIC KEY-----
MEgCQQCo9+BpMRYQ/dL3DS2CyJxRF+j6ctbT3/Qp84+KeFhnii7NT7fELilKUSnx
S30WAvQCCo2yU1orfgqr41mM70MBAgMBAAE=
-----END RSA PUBLIC KEY-----');
$key = $key
    //->withPadding(RSA::ENCRYPTION_OAEP)
    ->withHash('sha1')
    ->withMGFHash('sha1');
echo base64_encode($key->encrypt('test'));

^{(sha1 is being used because the key is a 512-bit key from Sample RSA Keys; 512-bits is used for brevity but because it's 512-bits sha256 can't be used per the max size formulas discussed at RSA::ENCRYPTION_OAEP)}

Here are the modifications we need to do to the above C# program to perform OAEP decryption:

#
#-----[ FIND ]------------------------------------------
#
        var ciphertext = "L812/9Y8TSpwErlLR6Bz4J3uR/T5YaqtTtB5jxtD1qazGPI5t15V9drWi58colGOZFeCnGKpCrtQWKk4HWRocQ==";
#
#-----[ REPLACE WITH ]----------------------------------
#
        var ciphertext = "XZ/zwGTyLwm3S1A2N7vg87qlpqtqB4sLo5t6BlayLIfW3kICEiol46ryz/oSsOypAH6GKxcn5TvQF/gJvMOFBg==";
#
#-----[ FIND ]------------------------------------------
#
        var plaintextBytes = rsa.Decrypt(ciphertextBytes, RSAEncryptionPadding.Pkcs1);
#
#-----[ REPLACE WITH ]----------------------------------
#
        var plaintextBytes = rsa.Decrypt(ciphertextBytes, RSAEncryptionPadding.OaepSHA1);

If you want or need to use non-matching MGFHash and Hash algorithms in OAEP then you'll need to use BouncyCastle.

RSA Signature Verification

Signature creation with PHP:

use phpseclib3\Crypt\PublicKeyLoader;

$key = PublicKeyLoader::load('-----BEGIN RSA PRIVATE KEY-----
MIIBOgIBAAJBAKj34GkxFhD90vcNLYLInFEX6Ppy1tPf9Cnzj4p4WGeKLs1Pt8Qu
KUpRKfFLfRYC9AIKjbJTWit+CqvjWYzvQwECAwEAAQJAIJLixBy2qpFoS4DSmoEm
o3qGy0t6z09AIJtH+5OeRV1be+N4cDYJKffGzDa88vQENZiRm0GRq6a+HPGQMd2k
TQIhAKMSvzIBnni7ot/OSie2TmJLY4SwTQAevXysE2RbFDYdAiEBCUEaRQnMnbp7
9mxDXDf6AU0cN/RPBjb9qSHDcWZHGzUCIG2Es59z8ugGrDY+pxLQnwfotadxd+Uy
v/Ow5T0q5gIJAiEAyS4RaI9YG8EWx/2w0T67ZUVAw8eOMB6BIUg0Xcu+3okCIBOs
/5OiPgoTdSy7bcF9IGpSE8ZgGKzgYQVZeN97YE00
-----END RSA PRIVATE KEY-----')
    //->withHash('sha256')
    ->withPadding(RSA::SIGNATURE_PKCS1);

echo base64_encode($key->sign('zzz'));

Signature verification with C#:

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

class Test
{
    static void Main()
    {
        var key = @"<RSAKeyValue>
  <Modulus>qPfgaTEWEP3S9w0tgsicURfo+nLW09/0KfOPinhYZ4ouzU+3xC4pSlEp8Ut9FgL0AgqNslNaK34Kq+NZjO9DAQ==</Modulus>
  <Exponent>AQAB</Exponent>
</RSAKeyValue>";

        var signature = "MUE536c4UJSAmycs7V6qFaLMATrKMQA8TYj5xX1+fwHINz3/BafgaRt0ycoD5IxTxaclLWavrGSza4xSBHraEw==";
        var data = "zzz";

        var signatureBytes = Convert.FromBase64String(signature);
        var dataBytes = Encoding.ASCII.GetBytes(data);

        var rsa = RSA.Create();
        rsa.FromXmlString(key);

        var result = rsa.VerifyData(dataBytes, signatureBytes, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);

        Console.WriteLine(result ? "valid" : "invalid");
    }
}

Using RSAPKCS1SignatureDeformatter is not recommended because that class requires you hash the data and then compare that to the hash in the signature. ie. it's more steps and more opportunities to screw up.