VBScript

Decrypt a JWE to a String

See more JSON Web Encryption (JWE) Examples

Demonstrates Jwe.Decrypt, which decrypts a recipient of a loaded JWE and returns the plaintext, decoded using the given charset. Before decrypting, the example inspects the JWE protected header and, under application policy, accepts only permitted algorithms and selects an acceptable recipient.

Background. A received JWE should never be trusted to specify safe algorithms. The example reads the alg and enc values from the protected header (via GetProtectedHeader) and enforces an allowlist, rejecting anything not permitted. It then selects the recipient the application holds a key for — using FindRecipient to locate a recipient by a per-recipient header value such as kid, falling back to index 0 for a single-recipient compact JWE — and only then supplies the key and decrypts.

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VBScript

Dim fso, outFile
Set fso = CreateObject("Scripting.FileSystemObject")
'Create a Unicode (utf-16) output text file.
Set outFile = fso.CreateTextFile("output.txt", True, True)

success = 0

set jwe = CreateObject("Chilkat.Jwe")

'  Load the JWE compact serialization to be decrypted.
jweCompact = "eyJhbGciOiJBMjU2S1ciLCJlbmMiOiJBMjU2R0NNIn0.<...>.<iv>.<ciphertext>.<tag>"
success = jwe.LoadJwe(jweCompact)
If (success = 0) Then
    outFile.WriteLine(jwe.LastErrorText)
    WScript.Quit
End If

'  Application policy: before decrypting, inspect the JWE header and accept only algorithms the
'  application trusts.  Never assume the algorithms in a received JWE are safe -- a sender could
'  specify a weak or unexpected algorithm.
set protHeader = CreateObject("Chilkat.JsonObject")
success = jwe.GetProtectedHeader(protHeader)
If (success = 0) Then
    outFile.WriteLine(jwe.LastErrorText)
    WScript.Quit
End If

'  Read the alg (key-management) and enc (content-encryption) header values into StringBuilder
'  objects so they can be compared.
set sbAlg = CreateObject("Chilkat.StringBuilder")
success = protHeader.StringOfSb("alg",sbAlg)
If (success = 0) Then
    outFile.WriteLine(protHeader.LastErrorText)
    WScript.Quit
End If

set sbEnc = CreateObject("Chilkat.StringBuilder")
success = protHeader.StringOfSb("enc",sbEnc)
If (success = 0) Then
    outFile.WriteLine(protHeader.LastErrorText)
    WScript.Quit
End If

outFile.WriteLine("JWE algorithms: alg=" & sbAlg.GetAsString() & " enc=" & sbEnc.GetAsString())

'  Enforce an allowlist of acceptable algorithms.  Reject anything not permitted by policy.  String
'  values are compared using StringBuilder.ContentsEqual.
bCaseSensitive = 1
If (sbAlg.ContentsEqual("A256KW",bCaseSensitive) <> 1) Then
    outFile.WriteLine("Rejecting JWE: key-management algorithm is not permitted by policy.")
    WScript.Quit
End If

If (sbEnc.ContentsEqual("A256GCM",bCaseSensitive) <> 1) Then
    outFile.WriteLine("Rejecting JWE: content-encryption algorithm is not permitted by policy.")
    WScript.Quit
End If

'  Select an acceptable recipient.  For a multi-recipient JWE, FindRecipient locates the recipient
'  this application holds a key for by a per-recipient header value such as "kid".  A compact JWE
'  has a single recipient at index 0, so FindRecipient returns -1; fall back to index 0 in that case.
recipientIndex = jwe.FindRecipient("kid","recipient-key-1",bCaseSensitive)
If (recipientIndex < 0) Then
    recipientIndex = 0
End If

'  Provide the key for the selected recipient.  In production, obtain the key from a secure source
'  rather than hard-coding it.
base64Key = "YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXowMTIzNDU="
success = jwe.SetWrappingKey(recipientIndex,base64Key,"base64")
If (success = 0) Then
    outFile.WriteLine(jwe.LastErrorText)
    WScript.Quit
End If

'  Decrypt the selected recipient and return the plaintext, decoded using the given charset.
content = jwe.Decrypt(recipientIndex,"utf-8")
If (jwe.LastMethodSuccess = 0) Then
    outFile.WriteLine(jwe.LastErrorText)
    WScript.Quit
End If

outFile.WriteLine(content)

outFile.Close