B4X

Verify an S/MIME Signed Message

Demonstrates the Chilkat Mime.Verify method, which verifies a CMS/PKCS #7 signed MIME entity and removes the signature wrapper. It takes no arguments and supports both detached (multipart/signed) and opaque (signed-data) messages.

Note: The file paths are relative to the application's current working directory. Absolute paths may also be used. Supply the paths appropriate to your own environment.

Background: Verification confirms two things: that the content was signed by the holder of the signer's certificate, and that it has not been altered since. On success Chilkat strips the signature layer so the object holds the original content, ready to read — effectively "open and check" in one step. A false return means the signature is invalid or the content was tampered with, which is exactly the case your code must not treat as trusted.

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Dim success As Boolean = False

Dim mime As ChilkatMime
mime.Initialize
success = mime.LoadMimeFile("qa_data/signed.eml")
If success = False Then
    Log(mime.LastErrorText)
    Return
End If


'  Verify the signature and remove the signature wrapper, restoring the underlying content.  Both
'  detached (multipart/signed) and opaque (CMS signed-data) messages are supported.
success = mime.Verify
If success <> True Then
    Log("Signature verification failed.")
    Log(mime.LastErrorText)
    Return
End If


Log("Signature verified. Number of signers: " & mime.NumSignerCerts)

'  The object now holds the original content with the signature removed.
Dim body As String = mime.GetBodyDecoded
If mime.LastMethodSuccess = False Then
    Log(mime.LastErrorText)
    Return
End If

Log(body)