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VbAsyncSocket

Simple and thin WinSock API wrappers for VB6 loosly based on the original CAsyncSocket wrapper in MFC.

Description

Base class cAsyncSocket wraps OS non-blocking sockets that can be used to implement various network components in VB6 -- clients and servers -- and supports both async and blocking network communications.

Additionally there is a source-compatible cTlsSocket class for transparent TLS transport layer encryption with several crypto backend implementations:

  1. mdTlsThunks is a pure VB6 with ASM thunks implementation for TLS 1.3 and (legacy) TLS 1.2 client-side and server-side support with no dependency on external libraries (like openssl)

  2. mdTlsNative is a native client-side and server-side TLS support using OS provided SSPI/Schannel library for all available protocol versions.

  3. mdTlsSodium is a stripped down compact backend with dependency on libsodium for crypto primitives (no ASM thunking used) with a total compiled size of 64KB.

The VB6 with thunks backend implementation auto-detects AES-NI and PCLMULQDQ instruction set availability on client machine and switches to performance optimized implementation of AES-GCM which is even faster that OS native SSPI/Schannel implementation of this cipher suit. The VB6 with thunks backend and native backend support legacy OSes up to NT 4.0 while libsodium DLL is compiled with XP support only.

Usage

Start by including src\cAsyncSocket.cls in your project to have a convenient wrapper of most WinSock API functions.

Optionally you can add src\cTlsSocket.cls and src\mdTlsThunks.bas pair of source files to your project for TLS secured connections using VB6 with thunks backend or add src\cTlsSocket.cls and src\mdTlsNative.bas pair of source files for an alternative backend using native OS provided SSPI/Schannel library.

Sample SMTP with STARTTLS

Here is a working sample with error checking omitted for brevity for accessing smtp.gmail.com over port 587.

At first the communication goes over unencrypted plain-text socket, then later it is switched to TLS secured one before issuing the final QUIT command.

With New cTlsSocket
    .SyncConnect "smtp.gmail.com", 587, UseTls:=False
    Debug.Print .SyncReceiveText();
    .SyncSendText "HELO 127.0.0.1" & vbCrLf
    Debug.Print .SyncReceiveText();
    .SyncSendText "STARTTLS" & vbCrLf
    Debug.Print .SyncReceiveText();
    .SyncStartTls "smtp.gmail.com"
    Debug.Print "TLS handshake complete: " & .RemoteHostName
    .SyncSendText "QUIT" & vbCrLf
    Debug.Print .SyncReceiveText();
End With

Which produces debug output in Immediate Window similar to this:

220 smtp.gmail.com ESMTP c69sm2955334lfg.23 - gsmtp
250 smtp.gmail.com at your service
220 2.0.0 Ready to start TLS
1428790.043 [INFO] Using TLS_AES_128_GCM_SHA256 from smtp.gmail.com [mdTlsThunks.pvTlsParseHandshakeServerHello]
1428790.057 [INFO] Valid ECDSA_SECP256R1_SHA256 signature [mdTlsThunks.pvTlsSignatureVerify]
TLS handshake complete: smtp.gmail.com
221 2.0.0 closing connection c69sm2955334lfg.23 - gsmtp

Implemented Cipher Suites

This list includes cipher suites as implemented in the ASM thunks backend while the native backend list depends on the OS version and SSPI/Schannel settings.

Cipher Suite First In Selection String Notes
TLS_AES_128_GCM_SHA256 TLS 1.3 EECDH+AESGCM AEAD
TLS_AES_256_GCM_SHA384 TLS 1.3 EECDH+AESGCM AEAD
TLS_CHACHA20_POLY1305_SHA256 TLS 1.3 EECDH+AESGCM AEAD
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 TLS 1.2 EECDH+AESGCM AEAD
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 TLS 1.2 EECDH+AESGCM AEAD
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 TLS 1.2 EECDH+AESGCM AEAD
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 TLS 1.2 EECDH+AESGCM AEAD
TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 TLS 1.2 EECDH+CHACHA20 AEAD
TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256 TLS 1.2 EECDH+CHACHA20 AEAD
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 TLS 1.2 EECDH+AES+SHA256 Exotic
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 TLS 1.2 EECDH+AES+SHA256 Exotic
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 TLS 1.2 EECDH+AES+SHA384 Exotic
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 TLS 1.2 EECDH+AES+SHA384 Exotic
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA TLSv1 EECDH+AES+SHA1 HMAC-SHA1
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA TLSv1 EECDH+AES+SHA1 HMAC-SHA1
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA TLSv1 EECDH+AES+SHA1 HMAC-SHA1
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA TLSv1 EECDH+AES+SHA1 HMAC-SHA1
TLS_RSA_WITH_AES_128_GCM_SHA256 TLS 1.2 RSA+AESGCM No FS
TLS_RSA_WITH_AES_256_GCM_SHA384 TLS 1.2 RSA+AESGCM No FS
TLS_RSA_WITH_AES_128_CBC_SHA256 TLS 1.2 RSA+AES+SHA256 No FS, Exotic
TLS_RSA_WITH_AES_256_CBC_SHA256 TLS 1.2 RSA+AES+SHA256 No FS, Exotic
TLS_RSA_WITH_AES_128_CBC_SHA SSLv3 RSA+AES+SHA1 No FS, HMAC-SHA1
TLS_RSA_WITH_AES_256_CBC_SHA SSLv3 RSA+AES+SHA1 No FS, HMAC-SHA1

Note that "exotic" cipher suites are included behind a conditional compilation flag only (off by default).

ToDo

  • Allow client to assign client certificate for connection
  • Provide UI for end-user to choose suitable certificates from Personal certificate store
  • Add wrappers for http and ftp protocols
  • Add WinSock control replacement
  • Add more samples (incl. vbcurl.exe utility)
  • Refactor subclassing thunk to use msg queue not to re-enter IDE in debug mode