# 15.3 `sha` -- SHA-1 message digest algorithm

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.

`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.