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[libpng16] Recognize known sRGB ICC profiles while reading; prefer writing the

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iCCP profile over writing the sRGB chunk, controlled by the
    PNG_sRGB_PROFILE_CHECKS option.
This commit is contained in:
John Bowler
2012-03-28 23:36:12 -05:00
committed by Glenn Randers-Pehrson
parent 23a30f8583
commit 921648a997
10 changed files with 347 additions and 160 deletions
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42
scripts/pnglibconf.dfa
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@@ -478,7 +478,46 @@ option WRITE_TEXT requires WRITE_ANCILLARY_CHUNKS enables TEXT
option GAMMA disabled
option COLORSPACE enables GAMMA disabled
# When an ICC profile is read, or png_set, it will be checked for a match
# against known sRGB profiles if the sRGB handling is enabled. This
# setting controls how much work is done during the check:
#
# 0: Just validate the profile MD5 signature if present, otherwise use
# the checks in option 1.
#
# 1: Additionally check the length, intent and adler32 checksum of the
# actual data. If enabled this will reject known profiles that have
# had the rendering intent in the header changed as well as other edits
# done without updating the checksum. See the discussion below.
#
# 2: Additionally checksum all the data using the ethernet CRC32 algorithm.
# This makes it more difficult to fake profiles and makes it less likely
# to get a false positive on profiles with no signature, but is probably
# just a waste of time since all currently approved ICC sRGB profiles have
# a secure MD5 signature.
#
# The rendering intent. An ICC profile stores an intended rendering intent,
# but does not include the value in the signature. The intent is documented
# as the intent that should be used when combining two profiles. The sRGB
# profile is intended, however, to be used with any of the four defined intents.
# For this reason the sRGB chunk includes an 'intent' to be used when displaying
# the image (intent is really a property of the image not the profile.)
#
# Unfortunately the iCCP chunk does not. It may therefore be that some
# applications modify the intent in profiles (including sRGB profiles) to work
# round this problem. Selecting an option other than option '0' will cause such
# modified profiles to be rejected.
#
# Security. The use of Adler32 and CRC32 checksums does not help significantly
# with any security issues. It is relatively easy to produce arbitrary profiles
# with the required checksums on current computer systems. Nevertheless
# security does not seem to be an issue because the only consequence of a false
# positive is a false assertion that the profile is an sRGB profile. This might
# be used to hide data from libpng using applications, but it doesn't seem
# possible to damage them.
setting sRGB_PROFILE_CHECKS default 2
# Artificially align memory - the code typically aligns to 8 byte
# boundaries if this is switched on, it's a small waste of space
# but can help (in theory) on some architectures. Only affects
@@ -650,9 +689,6 @@ option CHECK_FOR_INVALID_INDEX enables WRITE_CHECK_FOR_INVALID_INDEX
option READ_CHECK_FOR_INVALID_INDEX requires READ CHECK_FOR_INVALID_INDEX
option WRITE_CHECK_FOR_INVALID_INDEX requires WRITE CHECK_FOR_INVALID_INDEX
# added at libpng-1.6.0
option WARN_IF_iCCP_IS_sRGB requires READ iCCP
# Simplified API options (added at libpng-1.6.0)
# Read:
option SIMPLIFIED_READ requires SEQUENTIAL_READ READ_TRANSFORMS SETJMP