Asymmetric Public Key Algorithms
Asymmetric public key algorithms solve the problem of establishing and sharing secret keys to en/decrypt messages. The key in such an algorithm consists of two parts: a public key that may be distributed to others and a private key that needs to remain secret.
Messages encrypted with a public key can only be decrypted by recipients that are in possession of the associated private key. Since public key algorithms are considerably slower than symmetric key algorithms (cf. OpenSSL::Cipher
) they are often used to establish a symmetric key shared between two parties that are in possession of each other’s public key.
Asymmetric algorithms offer a lot of nice features that are used in a lot of different areas. A very common application is the creation and validation of digital signatures. To sign a document, the signatory generally uses a message digest algorithm (cf. OpenSSL::Digest
) to compute a digest of the document that is then encrypted (i.e. signed) using the private key. Anyone in possession of the public key may then verify the signature by computing the message digest of the original document on their own, decrypting the signature using the signatory’s public key and comparing the result to the message digest they previously computed. The signature is valid if and only if the decrypted signature is equal to this message digest.
The PKey
module offers support for three popular public/private key algorithms:

Elliptic Curve Cryptography (
OpenSSL::PKey::EC
)
Each of these implementations is in fact a subclass of the abstract PKey
class which offers the interface for supporting digital signatures in the form of PKey#sign
and PKey#verify
.
DiffieHellman Key Exchange
Finally PKey
also features OpenSSL::PKey::DH
, an implementation of the DiffieHellman key exchange protocol based on discrete logarithms in finite fields, the same basis that DSA
is built on. The DiffieHellman protocol can be used to exchange (symmetric) keys over insecure channels without needing any prior joint knowledge between the participating parties. As the security of DH
demands relatively long “public keys” (i.e. the part that is overtly transmitted between participants) DH
tends to be quite slow. If security or speed is your primary concern, OpenSSL::PKey::EC
offers another implementation of the DiffieHellman protocol.
static VALUE
ossl_pkey_s_generate_key(int argc, VALUE *argv, VALUE self)
{
return pkey_generate(argc, argv, self, 0);
}
Generates a new key (pair).
If a String
is given as the first argument, it generates a new random key for the algorithm specified by the name just as ::generate_parameters
does. If an OpenSSL::PKey::PKey
is given instead, it generates a new random key for the same algorithm as the key, using the parameters the key contains.
See ::generate_parameters
for the details of options and the given block.
Example
pkey_params = OpenSSL::PKey.generate_parameters("DSA", "dsa_paramgen_bits" => 2048) pkey_params.priv_key #=> nil pkey = OpenSSL::PKey.generate_key(pkey_params) pkey.priv_key #=> #<OpenSSL::BN 6277...
static VALUE
ossl_pkey_s_generate_parameters(int argc, VALUE *argv, VALUE self)
{
return pkey_generate(argc, argv, self, 1);
}
Generates new parameters for the algorithm. algo_name is a String
that represents the algorithm. The optional argument options is a Hash
that specifies the options specific to the algorithm. The order of the options can be important.
A block can be passed optionally. The meaning of the arguments passed to the block varies depending on the implementation of the algorithm. The block may be called once or multiple times, or may not even be called.
For the supported options, see the documentation for the ‘openssl genpkey’ utility command.
Example
pkey = OpenSSL::PKey.generate_parameters("DSA", "dsa_paramgen_bits" => 2048) p pkey.p.num_bits #=> 2048
static VALUE
ossl_pkey_new_from_data(int argc, VALUE *argv, VALUE self)
{
EVP_PKEY *pkey;
BIO *bio;
VALUE data, pass;
rb_scan_args(argc, argv, "11", &data, &pass);
bio = ossl_obj2bio(&data);
pkey = ossl_pkey_read_generic(bio, ossl_pem_passwd_value(pass));
BIO_free(bio);
if (!pkey)
ossl_raise(ePKeyError, "Could not parse PKey");
return ossl_pkey_new(pkey);
}
Reads a DER or PEM encoded string from string or io and returns an instance of the appropriate PKey
class.
Parameters

string is a DER or PEMencoded string containing an arbitrary private or public key.

io is an instance of
IO
containing a DER or PEMencoded arbitrary private or public key. 
pwd is an optional password in case string or io is an encrypted PEM resource.