AES encryption C# cipher text length

Question

From:

 

https://crypto.stackexchange.com/questions/54017/can-we-calculate-aes-ciphertext-length-based-on-the-length-of-the-plaintext?newreg=1ce930f3fa664ef3a5c17da4d32462b4

I see: output_size = input_size + (16 - (input_size % 16))

 

But this does not seem to correspond with what I see when running tests using C# and the AesCng with PKCS7 padding and CBC CipherMode.

 

Thoughts on how I calculate the length of the encrypted value?

 

At the end of the code, I have the "actual" length (that seen when running the code) -vs- the theoretical (per the above equation) and it shows how they differ.

 

Here's the code for sample program:

Public Shared Function Encrypt(ByVal plainText As String, ByVal aes256BitKeyValue As String, ByVal initialVector As String) As String
If String.IsNullOrEmpty(plainText) Then
Return String.Empty
End If
Dim initialVectorBytes As Byte() = System.Text.Encoding.ASCII.GetBytes(initialVector)
Dim plainTextBytes As Byte() = System.Text.Encoding.UTF8.GetBytes(plainText)
Dim cipherTextBytes As Byte() = Nothing
Using symmetricKey As New System.Security.Cryptography.AesCng()
symmetricKey.Padding = Security.Cryptography.PaddingMode.PKCS7
symmetricKey.Mode = System.Security.Cryptography.CipherMode.CBC
Dim keyBytes As Byte() = StringToByteArray(aes256BitKeyValue)
Try
Using cryptographyTransform As System.Security.Cryptography.ICryptoTransform = symmetricKey.CreateEncryptor(keyBytes, initialVectorBytes)
Using memoryStream As New System.IO.MemoryStream()
Using cryptographyStream As New System.Security.Cryptography.CryptoStream(memoryStream, cryptographyTransform, System.Security.Cryptography.CryptoStreamMode.Write)
cryptographyStream.Write(plainTextBytes, 0, plainTextBytes.Length)
cryptographyStream.FlushFinalBlock()
cipherTextBytes = memoryStream.ToArray()
memoryStream.Close()
cryptographyStream.Close()
End Using
End Using
End Using
Catch ex As Exception
Throw New Exceptions.EncryptionException(String.Format(System.Globalization.CultureInfo.InvariantCulture, Resources.ExceptionMessages.EncryptionFailed1, ex))
Finally
symmetricKey.Clear()
End Try
End Using
Return Convert.ToBase64String(cipherTextBytes)
End Function

namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
string aes256Key = "1234567890123456789012345678901234567890123456789012345678901234";
string initialVector = "66sDeG*3xsS334dE";
string unencrypted20Text = "12345678901234567890";
string unencrypted40Text = "1234567890123456789012345678901234567890";
string unencrypted64Text = "1234567890123456789012345678901234567890123456789012345678901234";
string unencrypted128Text = "12345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678";
string unencrypted256Text = "1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456";
string unencrypted257Text = "12345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567";
string unencrypted512Text = unencrypted256Text + unencrypted256Text;
string unencrypted1024Text = unencrypted512Text + unencrypted512Text;
string unencrypted2048Text = unencrypted1024Text + unencrypted1024Text;
string unencrypted4096Text = unencrypted2048Text + unencrypted2048Text;
string unencrypted8192Text = unencrypted4096Text + unencrypted4096Text;
string encrypted20Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted20Text, aes256Key, initialVector);
string encrypted40Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted40Text, aes256Key, initialVector);
string encrypted64Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted64Text, aes256Key, initialVector);
string encrypted128Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted128Text, aes256Key, initialVector);
string encrypted256Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted256Text, aes256Key, initialVector);
string encrypted257Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted257Text, aes256Key, initialVector);
string encrypted512Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted512Text, aes256Key, initialVector);
string encrypted1024Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted1024Text, aes256Key, initialVector);
string encrypted2048Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted2048Text, aes256Key, initialVector);
string encrypted4096Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted4096Text, aes256Key, initialVector);
string encrypted8192Text = Grb.Core.Cryptography.AesEncryption.Encrypt(unencrypted8192Text, aes256Key, initialVector);
int encrypted20TextLength = encrypted20Text.Length; // actual: 44, expected 20 + (16 - 4) = 32
int encrypted40TextLength = encrypted40Text.Length; // actual: 64, expected 40 + (116 - 8) = 48
int encrypted64TextLength = encrypted64Text.Length; // actual: 108, expected 64 + (16 - 0) = 80
int encrypted128TextLength = encrypted128Text.Length; // actual: 192, expected 128 + (16 - 0) = 144
int encrypted256TextLength = encrypted256Text.Length; // actual: 364
int encrypted257TextLength = encrypted257Text.Length; // actual: 364
int encrypted512TextLength = encrypted512Text.Length; // actual: 704
int encrypted1024TextLength = encrypted1024Text.Length; // actual: 1388
int encrypted2048TextLength = encrypted2048Text.Length; // actual: 2752
int encrypted4096TextLength = encrypted4096Text.Length; // actual: 5484
int encrypted8192TextLength = encrypted8192Text.Length; // actual: 10944
}
}
}