CODEWITHSUNDEEP
vb.netencryptioncryptographydiffie-hellman
Prince Tegaton
Prince Tegaton

Reputation: 322

How can I use my own Key to encrypt data using ECDiffieHellmanCng

My task is to encrypt data using the Elliptic Curve Cryptography in .Net (specific from client), I tried using the sample from Microsoft but it seems to generate its own key everytime.

I need to use my own key for this process, like creating a password "S3curEChaNNel_01?" and converting it to byte then use as a key, but i cant find a way to do this.

Using alice As New ECDiffieHellmanCng()
    Dim abData() As Byte
    Dim Str = txtKey.Text 'custom password
    abData = System.Text.Encoding.Default.GetBytes(Str)
    Str = System.Text.Encoding.Default.GetString(abData)

    Dim bobPublicKey() As Byte
    Dim bobKey() As Byte
    Dim bob As New ECDiffieHellmanCng()

    bob.KeyDerivationFunction = ECDiffieHellmanKeyDerivationFunction.Hash
    bob.HashAlgorithm = CngAlgorithm.Sha256
    bobPublicKey = bob.PublicKey.ToByteArray()
    bob.HmacKey = abData

    bobKey = bob.DeriveKeyMaterial(CngKey.Create(CngAlgorithm.Sha256))
    'at this line i get an exception, "The requested operation is not supported."

    'Dim aliceKey As Byte() = alice.DeriveKeyMaterial(CngKey.Create(CngAlgorithm.Sha256))
    Dim encryptedMessage As Byte() = Nothing
    Dim iv As Byte() = Nothing
    txtOutput.Text = ByteArrayToString(Encrypt(bobKey, txtPlainStr.Text, encryptedMessage, iv))
End Using

Upvotes: 0

Views: 247

Answers (1)

ProXicT
ProXicT

Reputation: 1933

You are mixing two different things together.

When you want your own password, the person you're sending the encrypted data with such password, must know the password in order decrypt it. But you can not send the password, because the MITM could catch it.

That's why you use Diffie - Hellman.
Basically, Diffie - Hellman outputs only a number, but this number can't be used as the encryption key, because it's to weak.

Now here is where KDF (Key Derivation Function) comes in. For example PBKDF2. It takes password and salt as parameters and outputs derived key which is used to encrypt and decrypt the data you want to send.

So you take the exchanged number, pass it as a password to the KDF. As salt you can use Alice's or Bob's IP for example.

The KDF will produce the same strong password on both sides to encrypt and decrypt the data.

Hopefully it makes sense.

Upvotes: 1

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