(Tcl) 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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load ./chilkat.dll

# 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.

set bd [new_CkBinData]

set success [CkBinData_LoadFile $bd "qa_data/p7m/a.p7m"]
if {$success == 0} then {
    puts "Failed to load the file."
    delete_CkBinData $bd
    exit
}

set sb [new_CkStringBuilder]

CkStringBuilder_Append $sb [CkBinData_getEncodedChunk $bd 0 2 "hex"]

set caseSensitive 1
if {expr [CkStringBuilder_ContentsEqual $sb "3080" $caseSensitive]  ||  [expr [CkStringBuilder_ContentsEqual $sb "3081" $caseSensitive]  ||  [expr [CkStringBuilder_ContentsEqual $sb "3082" $caseSensitive]  ||  [CkStringBuilder_ContentsEqual $sb "3083" $caseSensitive]]]} then {
    puts "This is a .p7m or .p7s file."
} else {
    puts "This is not a .p7m or .p7s file."
}


delete_CkBinData $bd
delete_CkStringBuilder $sb