B4J Tutorial [B4x] RSA sign & verify messages (extra: with corresponding php code)

This little tutorial is about signing and verifying data you send/receive between apps (= all "programs", even websites). You can use it in B4A without a change (I don't know how B4i works, but I assume there are similar libs).

You should know how RSA works. If not, take a Google search and you will find tons of documentation and of course in this forum (examples, explanations, etc.).

What is signing/verifying good for?

When you exchange data between two instances - let's say an app and a server - you encrypt it. In RSA you encrypt with the public key (it is really public) and decrypt with the private key (kept very secret). With this only the owner of the private one can decrypt it. Nice one.

Problem here is that anyone who has the PubKey can encrypt and send it. So how do you know who it was? Yes, you can force the sender to add some data like an api-key oder user & pw but there is always a safer way.

Hint: Encryption has nothing to do with signing/verifying. I did not know that first which confused me at the beginning.

As the public and private key depend on each other, you can use this keys to sign/verify a message which is sent (see the www to find out how this works).


The sender signs the message he sends with his private key. With this he creates a "hash" value (the signature) which the can be verifyed by the receiver using the SENDER's public key. With this you can guarantee two things:

1. The message comes from the owner of the private key (= exact ONE sender)
2. The message hasn't be changed on it's way (only the owner of the private key can generate the "hash")

As #1 says, you can only be sure that the message is from ONE unique sender (because the private key is unique), you need a central authority which this sender identifies hisself and leaves some information for the public. You know that from safe websites (click on the lock next to the URL of B4x). Here a central authority holds a SSL cerificate which contains the public key and signature(s).

When you receive the message (remember: you know his public key) you can use RSA methods to check the message against the signature. Vice versa if you send a message back, you sign it with your private key and the receiver can check if it was from you.

The example uses 3 KeyPairGenerators:

1st: Your "own" public and private keys
2nd: The "server" one just to simulate the other keys (of course it is done on the server's side)
3rd: ONE helper KPGs (here I load the keys from each side: Own or Server to sign/verify)

Hint #2:

When you sign the data, sign the RAW and UNENCRYPTED data as any good encryption method never creates the same data when you encrypt data more than one time. AFTER the signing encrypt everything (not before!). Same with verifying: FIRST decrypt the data THEN verify it.

Used libs: Encryption and ByteConverter

'Non-UI application (console / server application)
#Region Project Attributes
    #MergeLibraries: True
#End Region

Sub Process_Globals
    Public BC As ByteConverter
    Public RSAServerPubKeyString As String
    Public RSAServerPubKeyBytes(0) As Byte
    Public RSAServerPrivKeyBytes(0) As Byte
    Public RSAServerPrivKeyString As String
    Public RSAOwnPubKeyString As String
    Public RSAOwnPubKeyBytes(0) As Byte
    Public RSAOwnPrivKeyBytes(0) As Byte
    Public RSAOwnPrivKeyString, RSAOwnPrivKey, RSAOwnPubKey64StringExport As String
    Public RSAServerKPG As KeyPairGenerator
    Public RSAOwnKPG As KeyPairGenerator
    Public RSASignatureKPG As KeyPairGenerator
    Public RSAOwnC, RSAServerC As Cipher
    Private const RSAKeySize As Int = 4096
End Sub

Sub AppStart (Args() As String)
    'First we sign our OWN message with our OWN private key
    Log("Signing OWN message...")
    Dim mess As String = "This is the message we want to sign and verify..."
    Dim SignatureAsBytes() As Byte=SignMessage(mess.GetBytes("UTF8"),RSAOwnPrivKeyBytes)
    Log ("OWN signature for '" & mess & "' is: " &BC.HexFromBytes(SignatureAsBytes))
    'Now we verify if the signature for the message is ok.
    ' This is be done with the public key. So everyone who has the public key can do this (which is meant to be so)
    Log("Verifying OWN message...")
    'Then the receiver (let's say "the server") of our message sends a reply which he has signed with his own private key
    Log("Signing server message...")
    Dim mess As String = "This is the reply of e.g. the server..."
    Dim SignatureAsBytes() As Byte=SignMessage(mess.GetBytes("UTF8"),RSAServerPrivKeyBytes)
    Log ("Server signature for '" & mess & "' is: " &BC.HexFromBytes(SignatureAsBytes))
    'Now we verify if the signature for the message is ok.
    ' This is be done with the public key. So everyone who has the public key can do this (which is meant to be so)
    Log("Verifying SERVER message...")
End Sub

Sub SignMessage (MessB() As Byte, PrivateKey() As Byte) As Byte()
    Dim Sign As Signature
    Return Sign.Sign
End Sub

Sub CheckSignature (MessB() As Byte, SigB() As Byte, PublicKey() As Byte) As Boolean
    Dim SignVerify As Signature
    Dim SignatureIsOK As Boolean
    If SignatureIsOK Then
        Log("Signature is OK")
        Log("Wrong Signature")
    End If
    Return SignatureIsOK
End Sub

Sub CreateRSAKeys
    Log("Generating RSA keys. Please be patient as this takes some time...")
    'Generate "own" keys
    RSAOwnKPG.Initialize("RSA", RSAKeySize)
    'Generate the keys the other partitiant uses
    RSAServerKPG.Initialize("RSA", RSAKeySize)
    RSASignatureKPG.Initialize("RSA", RSAKeySize)
    Log("RSA keys generated...")
End Sub

'Return true to allow the default exceptions handler to handle the uncaught exception.
Sub Application_Error (Error As Exception, StackTrace As String) As Boolean
    Return True
End Sub

PHP Code:

See here:


Important: Use the algorythms below to match the B4x code and see what the methods need (raw data or like pem string).

Note: I only added the two calls for php as the usage needs a lot more like converting/creating/storing keys. See my examples here in the forum.

        $result = openssl_verify($clientpubkeyB64, $RAWsignature, $clientpubkeyASPEMFORMAT, OPENSSL_ALGO_SHA256);
        openssl_sign($data, $signature, file_get_contents('inc/PrivateKey.pem'), "sha256WithRSAEncryption");