(Visual FoxPro) 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?

Chilkat ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

LOCAL loBd
LOCAL lnSuccess
LOCAL loSb
LOCAL lnCaseSensitive

* 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.

* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.BinData')
loBd = CreateObject('Chilkat.BinData')

lnSuccess = loBd.LoadFile("qa_data/p7m/a.p7m")
IF (lnSuccess = 0) THEN
    ? "Failed to load the file."
    RELEASE loBd
    CANCEL
ENDIF

* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.StringBuilder')
loSb = CreateObject('Chilkat.StringBuilder')
loSb.Append(loBd.GetEncodedChunk(0,2,"hex"))

lnCaseSensitive = 1
IF (loSb.ContentsEqual("3080",lnCaseSensitive) OR loSb.ContentsEqual("3081",lnCaseSensitive) OR loSb.ContentsEqual("3082",lnCaseSensitive) OR loSb.ContentsEqual("3083",lnCaseSensitive)) THEN
    ? "This is a .p7m or .p7s file."
ELSE
    ? "This is not a .p7m or .p7s file."
ENDIF

RELEASE loBd
RELEASE loSb