• Achraf Feydi
• Mohamed Saïd Fayache

This project is a demonstration of some cryptography algorithms using Flutter Framework including:

• Coding/Decoding.
• Hashing / Hash cracking.
• Symmetric encryption/decryption.
• Asymmetric encryption/decryption.

Also for fun, we implemented A Chatroom App & Messenger chat app inside the main app in order to demonstrate how Symmetric and Asymmetric encyption works .

The message exchange is performed through Sockets.

The project is composed of two parts.

• The Server: NodeJs App hosted on heroku that will play the role of the bridge between client and our Keyserver.
  • Project Repo: securityProjectServer
• The Client App: A Flutter mobile app that contains all our buisness logic.

“A user interface is like a joke. If you have to explain it, it’s not that good”. — Martin Leblanc

Our User Interface is super userfriendly, but for educational purposes 👨‍🎓 we will explain it.

Now the fun part 😎, Let's discover the app.

By clicking the ⚙️ icon on the top of the screen this screen will open up.

From here you can setup the username that will be used as the identifier of our user.

By clicking the Encoding button on the Main screen you will get this interface:

Encoding supports 3 coding algorithms:

• To Base64
• To Binary
• To Ascii

To illustrate, this is the output of running encoding Hello world in :

• Base64: aGVsbG8gd29ybGQ=
• Binary: 1101000 1100101 1101100 1101100 1101111 100000 1110111 1101111 1110010 1101100 1100100
• Ascii: 104 101 108 108 111 032 119 111 114 108 100

Nothing special, It just reverses the operation of encoding.

In this part you'll see how we can hash text and crack the hashing using a brute-force attack .

You can choose one of the following Algorithms for hashing:

• SHA-1
• SHA-224
• SHA-256
• SHA-384
• SHA-512
• MD5

To illustrate, this is the output of running hashing Hello world in :

• SHA-1 : 2aae6c35c94fcfb415dbe95f408b9ce91ee846ed
• SHA-224 : 2f05477fc24bb4faefd86517156dafdecec45b8ad3cf2522a563582b
• SHA-256 : b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9
• SHA-384 : fdbd8e75a67f29f701a4e040385e2e23986303ea10239211af907fcbb83578b3e417cb71ce646efd0819dd8c088de1bd
• SHA-512 : 309ecc489c12d6eb4cc40f50c902f2b4d0ed77ee511a7c7a9bcd3ca86d4cd86f989dd35bc5ff499670da34255b45b0cfd830e81f605dcf7dc5542e93ae9cd76f
• MD5 : 5eb63bbbe01eeed093cb22bb8f5acdc3

For the brute-force attack we are using a 5 Million words dictionnary downloaded from this link .

Because of the huge computational power required for this operation we had to divide the dictionary onto mini-batchs with 10000 words in each of them.

Also we executed the code using the compute function to do all the work on a different "Thread" to avoid skipping frames and give a feed back on the operation's progress since it takes a long time.

Device : Xiaomi Note 7

• Qualcomm SDM660 Snapdragon 660 (14 nm)
• 8gb RAM

Test on the 5 milions words : 59.8525 minutes

Tests per second : 1392 test/s

This is the screen you'll see when you open go to Symmetric Encryption from the main screen.

For symmetric encryption we need, in addition to the text, a key that both side of the encrypted communication knows.

Any one with the key can encrypt and decrypt messages.

You can choose one of the following Algorithms for Encryption:

• AES CBC
• AES CFB-64
• AES CTR
• AES ECB
• AES OFB-64/GCTR
• AES OFB-64
• AES SIC

To illustrate, this is the output of running encryption Hello world using VincentRijmen as the Key :

• AES CBC : eLVXrWTx1oBdRqy9PAbcAw==
• AES CFB-64 : lWc7T08TGmJzU3nP6hS8nQ==
• AES CTR : lWc7T08TGmLYvH25MZR7IQ==
• AES ECB : eLVXrWTx1oBdRqy9PAbcAw==
• AES OFB-64/GCTR : GGkCZJK5inYtcyzxpCVEug==
• AES OFB-64 : lWc7T08TGmL8Y1Ip3KiWEg==
• AES SIC : lWc7T08TGmLYvH25MZR7IQ==

Nothing special about it, It just reverses the operation of encryption using the same key.

The Server is hosted online, you can download and try the app with your friends 👨‍💻

To enter a chatroom the user should specify the roomName and the key used for symmetric encryption.

Without the key, the user won't be able to see other people messages.

More than 2 persons can join the room. Actually anyone with the key can.

You can see in the screen the cool UI of the Chatroom.

By Clicking any chat item you will be redirected to a page with all the details about that message including :

• The Symmetric Encryption Algorithm
• The key
• The Encrypted Message that is transmitted through internet.
• The Decrypted message that was decrypted locally in your device using the key .

We don't spy on your messages, we are not WhatsApp 😭 , be like us 😎 .

This is the screen you'll see when you click Asymmetric encryption on the main screen.

The key manager allows the user to generate key pairs (public key, private key) and upload the public key to the keyServer.

• Algorithm: RSA
• Key size: 2048 bit

This Interface allows the user to encrypt a message using a public key.

• Algorithm: RSA
• Hash: SHA256
• Padding scheme: OAEP (Optimal asymmetric encryption padding)

This Interface allows the user to decrypt a message using a private key.

This Interface allows the user to sign a message using a private key.

• Algorithm: RSA
• Hash: SHA256
• Encoding methods: PSS (Provably Secure Encoding Method for Digital Signatures)

This Interface allows the user to verify the signature of a message using the combination of public key + message.

The Server is hosted online, you can download and try the app with your friends 👨‍💻

Now to the coolest part.

First, let's understand how things work then we will see the UI.

Let's say Bob want to send a message to Alice , this is what going to happen:

1. Bob download Alice's public key from our KeyServer.
2. Bob encrypts the message using Alice public key.
3. Bob signs the message using his private key
4. Bob send the signature and encrypted message to Alice
5. Alice download Bob's public key from our keyServer.
6. Alice decrypt the message using her private key.
7. Alice verify Bob's signature using his public key.

Life is not always a bed of roses, we learned it the hard way that RSA actually has a maximum size of message to encrypt.

To calculate it we can use this table:

Since we are using RSA 2048 with SHA-256 then we only have 190 bytes as maximum size.

And if we take in concediration that OAEP padding takes 42 bytes, then the remaining is 148 bytes 🤔.

To solve the issue we had to make our message's size a multiple of 148 by adding extra spaces on the top right. then we divide the message in blocks of 148 bytes, encrypt each of them and send the concatenated result. 😎

We also send the size of an encrypted block so the receiver can reverse the operation and get the message back.

This is an example of a transmetter message:

{
    "receiverChatID": "Achraf",
    "senderChatID": "Said",
    "content": {
        "message": "LP1hplB9IczCgoWBYu3yWCP9K7+uZF3RM0iKwSU6L33SVfS1eMDqW6WYWnQs5SdCsiFLriOIQU0y8pgouUlrODIR32SGSmgQwMaZJLbHCUvRMub3kBnmTb7iygdtHq6kzEWBydAsZ4iIjQ8jdg3MeJk/pHLaeDRJzM/dv8eg+QXZgcQbsJRk5KfFtnwMkvyzq1lYZA5Q2f+8rsfnbCzJ4OEsqfXPpsB/MXwrf3m1sxvuM/TeXZvPVeLvJ546pcnKKusDYRoqiGlEM/pfmB2ESle+VC6VyMXCk603JSwhTSPXZehVqFnGgNWLnJ20K+VTtV212DGHRjE3jsrNImGsJw==",
        "blockSize":344,
 "signature":"SaQYhD+U6gXPz1Go+otOPiGRPkaiwQZuI4bcDEiuwTvVgivcbF6uE2v37oaI5tQC9HeusEIou0INGLSLxxO+J91gQ5tKuW3tiUi9L6JNwJ4UHb8j0Ucfjvdon41xqt2duVnq5p+/67SlvSKBOR/3CIOdFKMuF9XZhCkwsDfj+PkNBCBGfx1kpQTlLEFYWWmf2X8TeTvxn6Kch3tHORft5gVhpAxBCCPLEN2+5NWe9zJYqhiRJA28Sy15J2wiSpqKNn2MM2HY3biez38lt2fbnJGfaKJ0fz+NRoxI2czB2TBbQilZRtJ9bW0ngQSmp/DZMnHLy4Xjcq0X61qG/2p06g=="}
}

By clicking the ➕ button on the bottom of the screen, the user gets the list of users who published their public keys on the keyServer.

By clicking on a user name, our user will be redirected to the chat screen.

By clicking on a received message the user can see all the details of the communication process.

And back to the messenger screen, the user can switch between different conversations.

We are not saving messages locally, so once the user leaves the app, all data will be removed from the RAM.