From pence@tetra.gsfc.nasa.gov Thu Jan 5 16:11:08 1995 Path: solitaire.cv.nrao.edu!mail-to-news-gateway From: William Pence Newsgroups: sci.astro.fits Subject: Re: FITS checksum proposal Date: 4 Jan 1995 18:04:02 -0500 Organization: National Radio Astronomy Observatory Lines: 43 Sender: daemon@solitaire.cv.nrao.edu Message-ID: <199501042302.SAA08132@tetra.gsfc.nasa.gov> There have been no comments on the FITS CHECKSUM proposal posted here on 20 December by Rob Seaman, so before it fades away, it may be worth posting a reminder about this proposal to groups that maintain archives of FITS data, or that distribute FITS files to users, and urge them to consider adding the CHECKSUM and DATASUM keywords to their FITS files. These 2 keywords allow the recipient of the FITS file to verify the integrity of the file and ensure (within some reasonable probability) that the copied file is identical to the original. It would be easy to modify existing FITS reading programs to check for these keywords and verify the checksum values. If the computed checksums are not consistent with the keyword values then the user should be warned that the file has either been modified since the checksums were inserted, or that the file is corrupted in some way. It is not difficult to calculate the checksum from first principles, but for added convenience, the latest version of the FITSIO subroutine package (V3.7) contains 2 new subroutines for (a) computing and inserting the CHECKSUM and DATASUM keywords into any FITS HDU, and (b) verifying that the computed checksum in a FITS HDU is consistent with the checksum keywords, if they exist. The 2 new subroutines are: call ftpcks( unit, status) (Put ChecKSum) which computes and writes the 2 keywords into the opened FITS file, and call ftvcks( unit, dataok, hduok, status) (Verify ChecKSum) which looks for the 2 keywords in the opened FITS file, and if they exist, verifies the checksum value. The dataok and hduok output parameters indicate whether the checksum for the data unit alone, and the entire HDU, respectively, are correct or not. Finally, the latest version of the VERIFITS program that checks any FITS file for conformance with the FITS Standard will also verify the checksum if these keywords exist. The new versions of FITSIO and VERIFITS that support the checksum keywords will be made available via anonymous ftp on legacy.gsfc.nasa.gov in the /software/fitsio and /software/fitsio/verifits directories by the end of this week. -Bill Pence