# class OpenSSL::Digest

`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 function.

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 database.

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.

Among the supported message digest algorithms are:

- SHA, SHA1, SHA224, SHA256, SHA384 and SHA512

- MD2, MD4, MDC2 and MD5

- RIPEMD160

- DSS, DSS1 (Pseudo algorithms to be used for DSA signatures. DSS is equal to SHA and DSS1 is equal to SHA1)

For each of these algorithms, there is a sub-class of `Digest` that can be instantiated as simply as e.g.

    digest = OpenSSL::Digest::SHA1.new

### Mapping between `Digest` class and sn/ln¶ ↑

The sn (short names) and ln (long names) are defined in <openssl/object.h> and <openssl/obj_mac.h>. They are textual representations of ASN.1 OBJECT IDENTIFIERs. Each supported digest algorithm has an OBJECT IDENTIFIER associated to it and those again have short/long names assigned to them. E.g. the OBJECT IDENTIFIER for SHA-1 is 1.3.14.3.2.26 and its sn is “SHA1” and its ln is “sha1”.

#### MD2¶ ↑

- sn: MD2

- ln: md2

#### MD4¶ ↑

- sn: MD4

- ln: md4

#### MD5¶ ↑

- sn: MD5

- ln: md5

#### SHA¶ ↑

- sn: SHA

- ln: SHA

#### SHA-1¶ ↑

- sn: SHA1

- ln: sha1

#### SHA-224¶ ↑

- sn: SHA224

- ln: sha224

#### SHA-256¶ ↑

- sn: SHA256

- ln: sha256

#### SHA-384¶ ↑

- sn: SHA384

- ln: sha384

#### SHA-512¶ ↑

- sn: SHA512

- ln: sha512

“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.read('document')
    sha256 = OpenSSL::Digest::SHA256.new
    digest = sha256.digest(data)

### Hashing several pieces of data at once¶ ↑

    data1 = File.read('file1')
    data2 = File.read('file2')
    data3 = File.read('file3')
    sha256 = OpenSSL::Digest::SHA256.new
    sha256 << data1
    sha256 << data2
    sha256 << data3
    digest = sha256.digest

### Reuse a `Digest` instance¶ ↑

    data1 = File.read('file1')
    sha256 = OpenSSL::Digest::SHA256.new
    digest1 = sha256.digest(data1)

    data2 = File.read('file2')
    sha256.reset
    digest2 = sha256.digest(data2)

### Public Class Methods

digest(name, data) click to toggle source

Return the hash value computed with name `Digest`. name is either the long name or short name of a supported digest algorithm.

### Examples¶ ↑

    OpenSSL::Digest.digest("SHA256", "abc")

which is equivalent to:

    OpenSSL::Digest::SHA256.digest("abc")

Calls superclass method

    # File ext/openssl/lib/openssl/digest.rb, line 34
    def self.digest(name, data)
      super(data, name)
    end

new(string [, data]) → Digest click to toggle source

Creates a `Digest` instance based on string, which is either the ln (long name) or sn (short name) of a supported digest algorithm.

If data (a `String`) is given, it is used as the initial input to the `Digest` instance, i.e.

    digest = OpenSSL::Digest.new('sha256', 'digestdata')

is equivalent to

    digest = OpenSSL::Digest.new('sha256')
    digest.update('digestdata')

    static VALUE
    ossl_digest_initialize(int argc, VALUE *argv, VALUE self)
    {
        EVP_MD_CTX *ctx;
        const EVP_MD *md;
        VALUE type, data;

        rb_scan_args(argc, argv, "11", &type, &data);
        md = ossl_evp_get_digestbyname(type);
        if (!NIL_P(data)) StringValue(data);

        TypedData_Get_Struct(self, EVP_MD_CTX, &ossl_digest_type, ctx);
        if (!ctx) {
            RTYPEDDATA_DATA(self) = ctx = EVP_MD_CTX_new();
            if (!ctx)
                ossl_raise(eDigestError, "EVP_MD_CTX_new");
        }

        if (!EVP_DigestInit_ex(ctx, md, NULL))
            ossl_raise(eDigestError, "Digest initialization failed");

        if (!NIL_P(data)) return ossl_digest_update(self, data);
        return self;
    }

### Public Instance Methods

<<(p1)
Alias for: update
block_length → integer click to toggle source

Returns the block length of the digest algorithm, i.e. the length in bytes of an individual block. Most modern algorithms partition a message to be digested into a sequence of fix-sized blocks that are processed consecutively.

### Example¶ ↑

    digest = OpenSSL::Digest::SHA1.new
    puts digest.block_length # => 64

    static VALUE
    ossl_digest_block_length(VALUE self)
    {
        EVP_MD_CTX *ctx;

        GetDigest(self, ctx);

        return INT2NUM(EVP_MD_CTX_block_size(ctx));
    }

digest_length → integer click to toggle source

Returns the output size of the digest, i.e. the length in bytes of the final message digest result.

### Example¶ ↑

    digest = OpenSSL::Digest::SHA1.new
    puts digest.digest_length # => 20

    static VALUE
    ossl_digest_size(VALUE self)
    {
        EVP_MD_CTX *ctx;

        GetDigest(self, ctx);

        return INT2NUM(EVP_MD_CTX_size(ctx));
    }

initialize_copy(p1) click to toggle source

    static VALUE
    ossl_digest_copy(VALUE self, VALUE other)
    {
        EVP_MD_CTX *ctx1, *ctx2;

        rb_check_frozen(self);
        if (self == other) return self;

        TypedData_Get_Struct(self, EVP_MD_CTX, &ossl_digest_type, ctx1);
        if (!ctx1) {
            RTYPEDDATA_DATA(self) = ctx1 = EVP_MD_CTX_new();
            if (!ctx1)
                ossl_raise(eDigestError, "EVP_MD_CTX_new");
        }
        GetDigest(other, ctx2);

        if (!EVP_MD_CTX_copy(ctx1, ctx2)) {
            ossl_raise(eDigestError, NULL);
        }
        return self;
    }

name → string click to toggle source

Returns the sn of this `Digest` algorithm.

### Example¶ ↑

    digest = OpenSSL::Digest::SHA512.new
    puts digest.name # => SHA512

    static VALUE
    ossl_digest_name(VALUE self)
    {
        EVP_MD_CTX *ctx;

        GetDigest(self, ctx);

        return rb_str_new2(EVP_MD_name(EVP_MD_CTX_md(ctx)));
    }

reset → self click to toggle source

Resets the `Digest` in the sense that any `Digest#update` that has been performed is abandoned and the `Digest` is set to its initial state again.

    static VALUE
    ossl_digest_reset(VALUE self)
    {
        EVP_MD_CTX *ctx;

        GetDigest(self, ctx);
        if (EVP_DigestInit_ex(ctx, EVP_MD_CTX_md(ctx), NULL) != 1) {
            ossl_raise(eDigestError, "Digest initialization failed.");
        }

        return self;
    }

update(string) → aString click to toggle source

Not every message digest can be computed in one single pass. If a message digest is to be computed from several subsequent sources, then each may be passed individually to the `Digest` instance.

### Example¶ ↑

    digest = OpenSSL::Digest::SHA256.new
    digest.update('First input')
    digest << 'Second input' # equivalent to digest.update('Second input')
    result = digest.digest

    VALUE
    ossl_digest_update(VALUE self, VALUE data)
    {
        EVP_MD_CTX *ctx;

        StringValue(data);
        GetDigest(self, ctx);

        if (!EVP_DigestUpdate(ctx, RSTRING_PTR(data), RSTRING_LEN(data)))
            ossl_raise(eDigestError, "EVP_DigestUpdate");

        return self;
    }

Also aliased as: <<

### Private Instance Methods

finish → aString click to toggle source

    static VALUE
    ossl_digest_finish(int argc, VALUE *argv, VALUE self)
    {
        EVP_MD_CTX *ctx;
        VALUE str;
        int out_len;

        GetDigest(self, ctx);
        rb_scan_args(argc, argv, "01", &str);
        out_len = EVP_MD_CTX_size(ctx);

        if (NIL_P(str)) {
            str = rb_str_new(NULL, out_len);
        } else {
            StringValue(str);
            rb_str_resize(str, out_len);
        }

        if (!EVP_DigestFinal_ex(ctx, (unsigned char *)RSTRING_PTR(str), NULL))
            ossl_raise(eDigestError, "EVP_DigestFinal_ex");

        return str;
    }