(PureBasic) How to Tell if a File is .p7m or .p7s

This example explains how to detect if a file is a .p7m or .p7s. It provides as solution to the following problem:

Sometimes people submit P7S/P7M with wrong extensions. Can I check if file is P7S or P7M with Chilkat?

Also see What is a P7M or P7S File?

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IncludeFile "CkBinData.pb"
IncludeFile "CkStringBuilder.pb"

Procedure ChilkatExample()

    ; A .p7m or .p7s file is binary and contains PKCS7.
    ; PKCS7 is ASN.1, and always begins with a SEQUENCE tag (a byte equal to 0x30)
    ; followed by an encoded length.
    ; If the file is larger than 128 bytes (and it SHOULD be), the encoded length will begin with either 0x80, 0x81, 0x82, or 0x83.
    ; It can also begin with 0x84, but only for very large files (where the length is too large for 3 bytes).
    ; See How ASN.1 Encodes Lengths

    ; Therefore, if we have a file that might be .p7m or .p7s, we can check by
    ; examining the 1st 2 bytes of the file.

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

    success.i = CkBinData::ckLoadFile(bd,"qa_data/p7m/a.p7m")
    If success = 0
        Debug "Failed to load the file."
        CkBinData::ckDispose(bd)
        ProcedureReturn
    EndIf

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

    CkStringBuilder::ckAppend(sb,CkBinData::ckGetEncodedChunk(bd,0,2,"hex"))

    caseSensitive.i = 1
    If CkStringBuilder::ckContentsEqual(sb,"3080",caseSensitive) OR CkStringBuilder::ckContentsEqual(sb,"3081",caseSensitive) OR CkStringBuilder::ckContentsEqual(sb,"3082",caseSensitive) OR CkStringBuilder::ckContentsEqual(sb,"3083",caseSensitive)
        Debug "This is a .p7m or .p7s file."
    Else
        Debug "This is not a .p7m or .p7s file."
    EndIf



    CkBinData::ckDispose(bd)
    CkStringBuilder::ckDispose(sb)


    ProcedureReturn
EndProcedure