# 15.3 `sha` -- SHA message digest algorithm

This module implements the interface to NIST's secure hash algorithm, known as SHA. 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 digests are 160 bits instead of MD5's 128 bits.

`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()`:

`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.

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

An sha object has the same methods as md5 objects:

`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)`.

`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.

`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.

`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.

See Also:

Secure Hash Standard
  The Secure Hash Algorithm is defined by NIST document FIPS PUB 180-1: Secure Hash Standard, published in April of 1995. It is available online as plain text (at least one diagram was omitted) and as PDF at http://csrc.nist.gov/publications/fips/fips180-1/fip180-1.pdf.

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