contrib: improve optimize-pngs.py

- Check that image contents match pre- and post- crushing.
- Also remove use of external tool to compute sha256 in favor of hashlib.
- contrib: remove all use of shell=True in strip_pngs.py
  Using `shell=True` can be a security hazard. See e.g.
  https://docs.python.org/2/library/subprocess.html#subprocess.check_output

contrib/devtools/optimize-pngs.py

@@ -3,12 +3,28 @@
 import os
 import sys
 import subprocess
+import hashlib
+from PIL import Image
+
+def file_hash(filename):
+    '''Return hash of raw file contents'''
+    with open(filename, 'rb') as f:
+        return hashlib.sha256(f.read()).hexdigest()
+
+def content_hash(filename):
+    '''Return hash of RGBA contents of image'''
+    i = Image.open(filename)
+    i = i.convert('RGBA')
+    data = i.tostring()
+    return hashlib.sha256(data).hexdigest()
 
 #optimize png, remove various color profiles, remove ancillary chunks (alla) and text chunks (text)
 #pngcrush -brute -ow -rem gAMA -rem cHRM -rem iCCP -rem sRGB -rem alla -rem text
 
+pngcrush = 'pngcrush'
+git = 'git'
 folders = ["src/qt/res/movies", "src/qt/res/icons", "src/qt/res/images"]
-basePath = subprocess.check_output("git rev-parse --show-toplevel", shell=True).rstrip('\n')
+basePath = subprocess.check_output([git, 'rev-parse', '--show-toplevel']).rstrip('\n')
 totalSaveBytes = 0
 
 outputArray = []
@@ -19,30 +35,38 @@ for folder in folders:
         if extension.lower() == '.png':
             print("optimizing "+file+"..."),
             file_path = os.path.join(absFolder, file)
-            fileMetaMap = {'file' : file, 'osize': os.path.getsize(file_path), 'sha256Old' : subprocess.check_output("openssl dgst -sha256 "+file_path, shell=True).rstrip('\n')};
+            fileMetaMap = {'file' : file, 'osize': os.path.getsize(file_path), 'sha256Old' : file_hash(file_path)};
+            fileMetaMap['contentHashPre'] = content_hash(file_path)
         
             pngCrushOutput = ""
             try:
-                pngCrushOutput = subprocess.check_output("pngcrush -brute -ow -rem gAMA -rem cHRM -rem iCCP -rem sRGB -rem alla -rem text "+file_path+" >/dev/null 2>&1", shell=True).rstrip('\n')
+                pngCrushOutput = subprocess.check_output(
+                        [pngcrush, "-brute", "-ow", "-rem", "gAMA", "-rem", "cHRM", "-rem", "iCCP", "-rem", "sRGB", "-rem", "alla", "-rem", "text", file_path],
+                        stderr=subprocess.STDOUT).rstrip('\n')
             except:
                 print "pngcrush is not installed, aborting..."
                 sys.exit(0)
         
             #verify
-            if "Not a PNG file" in subprocess.check_output("pngcrush -n -v "+file_path+" >/dev/null 2>&1", shell=True):
+            if "Not a PNG file" in subprocess.check_output([pngcrush, "-n", "-v", file_path], stderr=subprocess.STDOUT):
                 print "PNG file "+file+" is corrupted after crushing, check out pngcursh version"
                 sys.exit(1)
             
+            fileMetaMap['sha256New'] = file_hash(file_path)
+            fileMetaMap['contentHashPost'] = content_hash(file_path)
+
+            if fileMetaMap['contentHashPre'] != fileMetaMap['contentHashPost']:
+                print "Image contents of PNG file "+file+" before and after crushing don't match"
+                sys.exit(1)
 
-            fileMetaMap['sha256New'] = subprocess.check_output("openssl dgst -sha256 "+file_path, shell=True).rstrip('\n')
             fileMetaMap['psize'] = os.path.getsize(file_path)
             outputArray.append(fileMetaMap)
             print("done\n"),
 
 print "summary:\n+++++++++++++++++"
 for fileDict in outputArray:
-    oldHash = fileDict['sha256Old'].split("= ")[1]
+    oldHash = fileDict['sha256Old']
-    newHash = fileDict['sha256New'].split("= ")[1]
+    newHash = fileDict['sha256New']
     totalSaveBytes += fileDict['osize'] - fileDict['psize']
     print fileDict['file']+"\n  size diff from: "+str(fileDict['osize'])+" to: "+str(fileDict['psize'])+"\n  old sha256: "+oldHash+"\n  new sha256: "+newHash+"\n"