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README.md

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+hlextend
+========
+
+Pure Python Hash Length Extension module.
+
+Currently supports SHA1, SHA256 and SHA512, more algorithms will
+be added in the future.
+
+
+Create a hash by calling one of the named constuctor functions:
+sha1(), sha256(), and sha512(), or by calling new(algorithm).
+
+The hash objects have the following methods:
+
+	hash(message):      
+
+	    Feeds data into the hash function using the normal interface.
+
+	extend(appendData, knownData, secretLength, startHash, raw=False):
+
+	    Performs a hash length extension attack.  Returns the string to
+	    use when appending data.
+
+	hexdigest():        
+
+	    Returns a hexlified version of the hash output.
+
+
+Assume you have a hash generated from an unknown secret value concatenated with
+a known value, and you want to be able to produce a valid hash after appending 
+additional data to the known value.
+
+If the hash algorithm used is one of the vulnerable functions implemented in
+this module, is is possible to achieve this without knowing the secret value
+as long as you know (or can guess, perhaps by brute force) the length of that
+secret value.  This is called a hash length extension attack. 
+
+
+Given an existing sha1 hash value '52e98441017043eee154a6d1af98c5e0efab055c',
+known data of 'hello', an unknown secret of length 10 and data you wish
+to append of 'file', you would do the following to perform the attack:
+
+>>> import hlextend
+>>> sha = hlextend.new('sha1')
+>>> print sha.extend('file', 'hello', 10, '52e98441017043eee154a6d1af98c5e0efab055c')
+'hello\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
+\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
+\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00xfile'
+>>> print sha.hexdigest()
+c60fa7de0860d4048a3bfb36b70299a95e6587c9
+
+The unknown secret (of length 10), that when hashed appended with 'hello' produces
+a SHA1 hash of '52e98441017043eee154a6d1af98c5e0efab055c', will then produce 
+a SHA1 hash of 'c60fa7de0860d4048a3bfb36b70299a95e6587c9' when appended with the output 
+from the extend function above.
+
+If you are not sure of the exact length of the secret value, simply try the above
+multiple times specifying different values for the length to brute force.