Add original revision extraction for signed PDFs

EnvelopeGenerator.Infrastructure.Crypt/Services/PdfDigitalSignatureService.cs

@@ -162,6 +162,29 @@ public sealed class PdfDigitalSignatureService(IOptions<Pkcs11Options> options)
             Signatures: items));
     }
 
+    public Task<byte[]> ExtractOriginalPdfAsync(byte[] pdfBytes, CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(pdfBytes);
+        cancellationToken.ThrowIfCancellationRequested();
+
+        var byteRanges = ParseByteRanges(pdfBytes);
+        if (byteRanges.Count == 0)
+            return Task.FromResult(pdfBytes);
+
+        var revisionLength = byteRanges
+            .Select(GetSignedRevisionLength)
+            .Where(x => x > 0)
+            .DefaultIfEmpty(pdfBytes.LongLength)
+            .Min();
+
+        if (revisionLength > pdfBytes.LongLength)
+            throw new InvalidOperationException("Invalid ByteRange in signed PDF.");
+
+        var originalPdf = new byte[revisionLength];
+        Buffer.BlockCopy(pdfBytes, 0, originalPdf, 0, (int)revisionLength);
+        return Task.FromResult(originalPdf);
+    }
+
     private static PdfSignatureCertificateInfoDto BuildCertificateInfo(X509Certificate2? certificate, DateTimeOffset now)
     {
         if (certificate is null)
@@ -398,4 +421,17 @@ public sealed class PdfDigitalSignatureService(IOptions<Pkcs11Options> options)
 
         return previousEnd == fileLength;
     }
+
+    private static long GetSignedRevisionLength(long[] byteRange)
+    {
+        if (byteRange.Length < 2)
+            return -1;
+
+        var start = byteRange[^2];
+        var length = byteRange[^1];
+        if (start < 0 || length < 0)
+            return -1;
+
+        return start + length;
+    }
 }