Xbase++

How to Tell if a File is .p7m or .p7s

See more Misc Examples

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?

Xbase++

LOCAL nSuccess
LOCAL oBd
LOCAL oSb
LOCAL nCaseSensitive

nSuccess := 0

//  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.

oBd := CreateObject("Chilkat.BinData")

nSuccess := oBd:LoadFile("qa_data/p7m/a.p7m")
IF (nSuccess == 0)
    ? "Failed to load the file."
    oBd:destroy()
    RETURN
ENDIF

oSb := CreateObject("Chilkat.StringBuilder")
oSb:Append(oBd:GetEncodedChunk(0, 2, "hex"))

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

oBd:destroy()
oSb:destroy()