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Parse ASN.1 DER format

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  Parse ASN.1 DER format news@zzo38computer.org.invalid - 2025-10-22 14:38 -0700

#4019 — Parse ASN.1 DER format

I wrote a PostScript code to parse ASN.1 DER format. (This file is
public domain.)

Currently it only implements decoding and not encoding. Also, it only
decodes the framing and does not decode values.

/ASN1class [/Universal /Application /Context /Private] def

/ASN1parse {
  dup read {
    << >> begin
    /Tag exch def
    /File exch def
    /Constructed Tag -5 bitshift 0 ne def
    /Class //ASN1class Tag -6 bitshift get def
    /Tag Tag 16#1F and dup 16#1F eq {
      pop 0 {File read {
        exch 7 bitshift 1 index 16#7F and or
        exch 16#80 lt {exit} if
      } {exit} ifelse} loop
    } if def
    /Length File read pop def
    Length 16#80 ge {
      /Length 0 Length 16#7F and {
        8 bitshift File read {or} if
      } repeat def
    } if
    /Data Constructed {
      /File File Length () /SubFileDecode filter def
      [{File ASN1parse not {exit} if} loop]
    } {
      Length 0 eq {()} {File Length string readstring pop} ifelse
    } ifelse def
    {File Constructed Length} {currentdict exch undef} forall
    currentdict end //true
  } {
    pop //false
  } ifelse
} bind def

-- 
Don't laugh at the moon when it is day time in France.

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