/// <summary>
/// Performs AES decryption in GCM chaning mode over cipher text
/// </summary>
/// <param name="key">aes key</param>
/// <param name="iv">initialization vector</param>
/// <param name="aad">additional authn data</param>
/// <param name="plainText">plain text message to be encrypted</param>
/// <returns>decrypted plain text messages</returns>
/// <exception cref="CryptographicException">if decryption failed by any reason</exception>
public static byte[] Decrypt(byte[] key, byte[] iv, byte[] aad, byte[] cipherText, byte[] authTag)
{
using var gcm = new System.Security.Cryptography.AesGcm(key);
var plaintext = new byte[cipherText.Length];
gcm.Decrypt(nonce: iv, ciphertext: cipherText, tag: authTag, plaintext: plaintext, associatedData: aad);
return(plaintext);
}
Decrypt(byte[] key, byte[] infoCipher)
{
if (key == null || key.Length != 32)
{
throw new ArgumentException("AesGcm key must be 32 bytes long.");
}
// Authentication tag.
byte[] tag = new byte[16];
// IV.
byte[] nonce = new byte[12];
int cipher_text_len;
byte[] cipher_text;
try
{
// Length of the cipher_text of interest inside the AES-GCM payload.
cipher_text_len = infoCipher.Length - tag.Length - nonce.Length;
cipher_text = new byte[cipher_text_len];
// Extract each entity of AES-GCM payload.
Buffer.BlockCopy(infoCipher, infoCipher.Length - tag.Length, tag, 0, tag.Length);
Buffer.BlockCopy(infoCipher, 0, nonce, 0, nonce.Length);
Buffer.BlockCopy(infoCipher, nonce.Length, cipher_text, 0, cipher_text_len);
}
catch (Exception e)
{
Debug.WriteLine(e.Message);
return(null);
}
//Debug.WriteLine(
// $"tag({Convert.ToBase64String(tag)}), "
// + $"nonce({Convert.ToBase64String(nonce)}), "
// + $"cipher({Convert.ToBase64String(cipher_text)})"
//);
byte[] plain_text = new byte[cipher_text_len];
using (var c = new SysCrypto.AesGcm(key))
{
try
{
c.Decrypt(nonce, cipher_text, tag, plain_text, null);
return(plain_text);
}
catch (Exception e)
{
Debug.WriteLine(e.Message);
return(null);
}
}
}
internal static byte[] Decrypt(byte[] secretKeyBytes, JweObject jweObject)
{
#if NETSTANDARD2_1
byte[] plaintext;
using (var aes = new System.Security.Cryptography.AesGcm(secretKeyBytes))
{
byte[] nonce = Base64Utils.URLDecode(jweObject.Iv);
byte[] aad = Encoding.ASCII.GetBytes(jweObject.RawHeader);
byte[] authTag = Base64Utils.URLDecode(jweObject.AuthTag);
byte[] ciphertext = Base64Utils.URLDecode(jweObject.CipherText);
plaintext = new byte[ciphertext.Length];
aes.Decrypt(nonce, ciphertext, authTag, plaintext, aad);
}
return(plaintext);
#else
throw new EncryptionException("AES/GCM/NoPadding is unsupported on .NET Standard < 2.1");
#endif
}
public void Decrypt(ReadOnlySpan <byte> nonce, ReadOnlySpan <byte> ciphertext, ReadOnlySpan <byte> tag, Span <byte> plaintext)
{
_aesGcm.Decrypt(nonce, ciphertext, tag, plaintext);
}