Start with pydkim 0.3.

dkimsign.py

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+#!/usr/local/bin/python2.5
+
+# This software is provided 'as-is', without any express or implied
+# warranty.  In no event will the author be held liable for any damages
+# arising from the use of this software.
+# 
+# Permission is granted to anyone to use this software for any purpose,
+# including commercial applications, and to alter it and redistribute it
+# freely, subject to the following restrictions:
+# 
+# 1. The origin of this software must not be misrepresented; you must not
+#    claim that you wrote the original software. If you use this software
+#    in a product, an acknowledgment in the product documentation would be
+#    appreciated but is not required.
+# 2. Altered source versions must be plainly marked as such, and must not be
+#    misrepresented as being the original software.
+# 3. This notice may not be removed or altered from any source distribution.
+# 
+# Copyright (c) 2008 Greg Hewgill http://hewgill.com
+
+import sys
+
+import dkim
+
+if len(sys.argv) < 4 or len(sys.argv) > 5:
+    print >>sys.stderr, "Usage: dkimsign.py selector domain privatekeyfile [identity]"
+    sys.exit(1)
+
+selector = sys.argv[1]
+domain = sys.argv[2]
+privatekeyfile = sys.argv[3]
+if len(sys.argv) > 5:
+    identity = sys.argv[4]
+else:
+    identity = None
+
+message = sys.stdin.read()
+try:
+    sig = dkim.sign(message, selector, domain, open(privatekeyfile, "r").read(), identity = identity)
+    sys.stdout.write(sig)
+    sys.stdout.write(message)
+except Exception, e:
+    print >>sys.stderr, e
+    sys.stdout.write(message)