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        # OpenSSL::Digest < OpenSSL::Digest::Class

(from ruby core)

OpenSSL::Digest allows you to compute message digests (sometimes
interchangeably called "hashes") of arbitrary data that are
cryptographically secure, i.e. a Digest implements a secure one-way

One-way functions offer some useful properties. E.g. given two distinct
inputs the probability that both yield the same output is highly
unlikely. Combined with the fact that every message digest algorithm has
a fixed-length output of just a few bytes, digests are often used to
create unique identifiers for arbitrary data. A common example is the
creation of a unique id for binary documents that are stored in a

Another useful characteristic of one-way functions (and thus the name)
is that given a digest there is no indication about the original data
that produced it, i.e. the only way to identify the original input is to
"brute-force" through every possible combination of inputs.

These characteristics make one-way functions also ideal companions for
public key signature algorithms: instead of signing an entire document,
first a hash of the document is produced with a considerably faster
message digest algorithm and only the few bytes of its output need to be
signed using the slower public key algorithm. To validate the integrity
of a signed document, it suffices to re-compute the hash and verify that
it is equal to that in the signature.

You can get a list of all digest algorithms supported on your system by
running this command in your terminal:

    openssl list -digest-algorithms

Among the OpenSSL 1.1.1 supported message digest algorithms are:
*   SHA224, SHA256, SHA384, SHA512, SHA512-224 and SHA512-256
*   SHA3-224, SHA3-256, SHA3-384 and SHA3-512
*   BLAKE2s256 and BLAKE2b512

Each of these algorithms can be instantiated using the name:

    digest ='SHA256')

"Breaking" a message digest algorithm means defying its one-way function
characteristics, i.e. producing a collision or finding a way to get to
the original data by means that are more efficient than brute-forcing
etc. Most of the supported digest algorithms can be considered broken in
this sense, even the very popular MD5 and SHA1 algorithms. Should
security be your highest concern, then you should probably rely on
SHA224, SHA256, SHA384 or SHA512.

### Hashing a file

    data = File.binread('document')
    sha256 ='SHA256')
    digest = sha256.digest(data)

### Hashing several pieces of data at once

    data1 = File.binread('file1')
    data2 = File.binread('file2')
    data3 = File.binread('file3')
    sha256 ='SHA256')
    sha256 << data1
    sha256 << data2
    sha256 << data3
    digest = sha256.digest

### Reuse a Digest instance

    data1 = File.binread('file1')
    sha256 ='SHA256')
    digest1 = sha256.digest(data1)

    data2 = File.binread('file2')
    digest2 = sha256.digest(data2)
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