14.4. "sha" — SHA-1 message digest algorithm
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2.5 版後已棄用: Use the "hashlib" module instead.

This module implements the interface to NIST’s secure hash  algorithm,
known as SHA-1.  SHA-1 is an improved version of the original SHA hash
algorithm.  It is used in the same way as the "md5" module: use
"new()" to create an sha object, then feed this object with arbitrary
strings using the "update()" method, and at any point you can ask it
for the *digest* of the concatenation of the strings fed to it so far.
SHA-1 digests are 160 bits instead of MD5’s 128 bits.

sha.new([string])

   Return a new sha object.  If *string* is present, the method call
   "update(string)" is made.

The following values are provided as constants in the module and as
attributes of the sha objects returned by "new()":

sha.blocksize

   Size of the blocks fed into the hash function; this is always "1".
   This size is used to allow an arbitrary string to be hashed.

sha.digest_size

   The size of the resulting digest in bytes.  This is always "20".

An sha object has the same methods as md5 objects:

sha.update(arg)

   Update the sha object with the string *arg*.  Repeated calls are
   equivalent to a single call with the concatenation of all the
   arguments: "m.update(a); m.update(b)" is equivalent to
   "m.update(a+b)".

sha.digest()

   Return the digest of the strings passed to the "update()" method so
   far. This is a 20-byte string which may contain non-ASCII
   characters, including null bytes.

sha.hexdigest()

   Like "digest()" except the digest is returned as a string of length
   40, containing only hexadecimal digits.  This may  be used to
   exchange the value safely in email or other non-binary
   environments.

sha.copy()

   Return a copy (「clone」) of the sha object.  This can be used to
   efficiently compute the digests of strings that share a common
   initial substring.

也參考:

  Secure Hash Standard
     The Secure Hash Algorithm is defined by NIST document FIPS PUB
     180-2: Secure Hash Standard, published in August 2002.

  Cryptographic Toolkit (Secure Hashing)
     Links from NIST to various information on secure hashing.
