(PureBasic) Decrypt PKCS7 (CMS) EnvelopedData

This example receives a PKCS7 (CMS) EnvelopedData message that was encrypted to me. To decrypt it:

1. The CMS structure contains -
   • Encrypted content (ciphertext)
   • The symmetric key used to encrypt the content, itself encrypted with my public key
   • Information identifying the recipient (me), typically by X.509 certificate
2. Decryption process (what Chilkat does internally)
   • My software uses my digital certificat's private RSA key (which corresponds to the public key used during encryption) to decrypt the encrypted symmetric key.
   • The decrypted symmetric key is then used to decrypt the content-encryption layer, typically encrypted with a symmetric algorithm like AES.
3. Result -
   • I recover the original plaintext data.

Summary:

• The content itself is encrypted with a symmetric key (e.g., AES).
• That symmetric key is encrypted using my RSA public key and included in the EnvelopedData.
• I use my RSA private key to decrypt the symmetric key and then use it to decrypt the actual data.

Only the intended recipient, who possesses the correct private key, can successfully decrypt the EnvelopedData.

Note: This example requires Chilkat v11.0.0 or greater.

Chilkat PureBasic Module Download Chilkat PureBasic Module

IncludeFile "CkCert.pb"
IncludeFile "CkStringBuilder.pb"
IncludeFile "CkCrypt2.pb"

Procedure ChilkatExample()

    ; Here's the PKCS7 (CMS) EnvelopedData we received.
    ; It was created using this example:  Create PKCS7 CMS EnvelopedData

    ; MIICSgYJKoZIhvcNAQcDoIICOzCCAjcCAQAxggHiMIIB3gIBADCBljCBgTELMAkGA1UEBhMCSVQx
    ; EDAOBgNVBAgMB0JlcmdhbW8xGTAXBgNVBAcMEFBvbnRlIFNhbiBQaWV0cm8xFzAVBgNVBAoMDkFj
    ; dGFsaXMgUy5wLkEuMSwwKgYDVQQDDCNBY3RhbGlzIENsaWVudCBBdXRoZW50aWNhdGlvbiBDQSBH
    ; MwIQPCWvkSv8oQ7xRmEHJ6TzEDA8BgkqhkiG9w0BAQcwL6APMA0GCWCGSAFlAwQCAQUAoRwwGgYJ
    ; KoZIhvcNAQEIMA0GCWCGSAFlAwQCAQUABIIBAKqHAPQNSsQoX7B2NH7QyEOWQRsSVs8oCHXmy8f4
    ; MVZD2er3bvYUCIomxpwbLEAl14qjUIMynahooYGgqip7+4FqL301G+BVjZVfEhHWj+VI1dAWnWuL
    ; VHlvc/pbQNBWqV8rKVJsNIsuAZkdj4WSwLVKxYkYX43B8fh/g71XN2DTJu7Z/824v48KBmgpQBOT
    ; 2q7IcDGxNPAFN2p6eavIVGn2LvhEbf/Fszyj+GR5tMcnQP1BOLJ3s3JzUBbvj8hcZrF1Vhl9HnTU
    ; YQx8G/KdW1mR+Wlhl3BWoK0LYKRTbnTx2BXOs0CY1SXOAdhKr01ZYjA+xW4nGzY0lfXS9QZjh9gw
    ; TAYJKoZIhvcNAQcBMB0GCWCGSAFlAwQBKgQQw0xTbfmnt0zjWHo5SaQIp4AgxTVY9E/Ncqy6t+RM
    ; 8y4c3Av62/wB8IpPUEmtM2OeuZo=

    sb.i = CkStringBuilder::ckCreate()
    If sb.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    bCrlf.i = 1
    CkStringBuilder::ckAppendLine(sb,"MIICSgYJKoZIhvcNAQcDoIICOzCCAjcCAQAxggHiMIIB3gIBADCBljCBgTELMAkGA1UEBhMCSVQx",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"EDAOBgNVBAgMB0JlcmdhbW8xGTAXBgNVBAcMEFBvbnRlIFNhbiBQaWV0cm8xFzAVBgNVBAoMDkFj",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"dGFsaXMgUy5wLkEuMSwwKgYDVQQDDCNBY3RhbGlzIENsaWVudCBBdXRoZW50aWNhdGlvbiBDQSBH",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"MwIQPCWvkSv8oQ7xRmEHJ6TzEDA8BgkqhkiG9w0BAQcwL6APMA0GCWCGSAFlAwQCAQUAoRwwGgYJ",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"KoZIhvcNAQEIMA0GCWCGSAFlAwQCAQUABIIBAKqHAPQNSsQoX7B2NH7QyEOWQRsSVs8oCHXmy8f4",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"MVZD2er3bvYUCIomxpwbLEAl14qjUIMynahooYGgqip7+4FqL301G+BVjZVfEhHWj+VI1dAWnWuL",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"VHlvc/pbQNBWqV8rKVJsNIsuAZkdj4WSwLVKxYkYX43B8fh/g71XN2DTJu7Z/824v48KBmgpQBOT",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"2q7IcDGxNPAFN2p6eavIVGn2LvhEbf/Fszyj+GR5tMcnQP1BOLJ3s3JzUBbvj8hcZrF1Vhl9HnTU",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"YQx8G/KdW1mR+Wlhl3BWoK0LYKRTbnTx2BXOs0CY1SXOAdhKr01ZYjA+xW4nGzY0lfXS9QZjh9gw",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"TAYJKoZIhvcNAQcBMB0GCWCGSAFlAwQBKgQQw0xTbfmnt0zjWHo5SaQIp4AgxTVY9E/Ncqy6t+RM",bCrlf)
    CkStringBuilder::ckAppendLine(sb,"8y4c3Av62/wB8IpPUEmtM2OeuZo=",bCrlf)

    crypt.i = CkCrypt2::ckCreate()
    If crypt.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    CkCrypt2::setCkCryptAlgorithm(crypt, "pki")

    ; The CMS message contains information about the digital certificate that was used to encrypt.
    ; Assuming the certificate is installed in a Windows certificate store, or macOS keychain,
    ; Chilkat will automatically locate and use the certificate + private key for decryption.

    CkCrypt2::setCkEncodingMode(crypt, "base64")
    CkCrypt2::setCkCharset(crypt, "utf-8")

    decryptedText.s = CkCrypt2::ckDecryptStringENC(crypt,CkStringBuilder::ckGetAsString(sb))
    If CkCrypt2::ckLastMethodSuccess(crypt) = 0
        Debug CkCrypt2::ckLastErrorText(crypt)
        CkStringBuilder::ckDispose(sb)
        CkCrypt2::ckDispose(crypt)
        ProcedureReturn
    EndIf

    Debug decryptedText

    ; We can get the digital certificate used for decryption by calling LastDecryptCert
    cert.i = CkCert::ckCreate()
    If cert.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success.i = CkCrypt2::ckLastDecryptCert(crypt,cert)
    If CkCrypt2::ckLastMethodSuccess(crypt) = 0
        Debug CkCrypt2::ckLastErrorText(crypt)
        CkStringBuilder::ckDispose(sb)
        CkCrypt2::ckDispose(crypt)
        CkCert::ckDispose(cert)
        ProcedureReturn
    EndIf

    Debug "Cert used to decrypt: " + CkCert::ckSubjectCN(cert)


    CkStringBuilder::ckDispose(sb)
    CkCrypt2::ckDispose(crypt)
    CkCert::ckDispose(cert)


    ProcedureReturn
EndProcedure