Hi,
here's code for extracting hash from signature :
// core aspect of PDF signatures :
// signatures are made by a special / partial hash
MessageDigest messageDigest = MessageDigest.getInstance(
getDocumentHashAlgorithm() );
byte read_buf[] = new byte[ hashBufferSize ];
int n;
InputStream inp = sap.getRangeStream();
while ( ( n = inp.read( read_buf ) ) > 0) {
messageDigest.update( read_buf, 0, n );
}
byte pdf_hash[] = messageDigest.digest();
Replacing the existing signature content is easy as I created it upfront with
well known content, big enough to hold the hex-coded signature. Once the
signature is ready, it's a simple find'n'replace method on the PDF file.
Greetings
Andreas
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