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.
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 = OpenSSL::Digest.new('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 = OpenSSL::Digest.new('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 = OpenSSL::Digest.new('SHA256') sha256 << data1 sha256 << data2 sha256 << data3 digest = sha256.digest
Reuse a Digest instance
data1 = File.binread('file1') sha256 = OpenSSL::Digest.new('SHA256') digest1 = sha256.digest(data1) data2 = File.binread('file2') sha256.reset digest2 = sha256.digest(data2)
# File tmp/rubies/ruby-3.2.0/ext/openssl/lib/openssl/digest.rb, line 29
def self.digest(name, data)
super(data, name)
end
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.digest('SHA256', "abc")
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;
}
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_block_length(VALUE self)
{
EVP_MD_CTX *ctx;
GetDigest(self, ctx);
return INT2NUM(EVP_MD_CTX_block_size(ctx));
}
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.new('SHA1') puts digest.block_length # => 64
static VALUE
ossl_digest_size(VALUE self)
{
EVP_MD_CTX *ctx;
GetDigest(self, ctx);
return INT2NUM(EVP_MD_CTX_size(ctx));
}
Returns the output size of the digest, i.e. the length in bytes of the final message digest result.
Example
digest = OpenSSL::Digest.new('SHA1') puts digest.digest_length # => 20
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;
}
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;
}
static VALUE
ossl_digest_name(VALUE self)
{
EVP_MD_CTX *ctx;
GetDigest(self, ctx);
return rb_str_new_cstr(EVP_MD_name(EVP_MD_CTX_get0_md(ctx)));
}
Returns the sn of this Digest algorithm.
Example
digest = OpenSSL::Digest.new('SHA512') puts digest.name # => SHA512
static VALUE
ossl_digest_reset(VALUE self)
{
EVP_MD_CTX *ctx;
GetDigest(self, ctx);
if (EVP_DigestInit_ex(ctx, EVP_MD_CTX_get0_md(ctx), NULL) != 1) {
ossl_raise(eDigestError, "Digest initialization failed.");
}
return self;
}
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.
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;
}
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.new('SHA256') digest.update('First input') digest << 'Second input' # equivalent to digest.update('Second input') result = digest.digest