Module: OpenSSL


OpenSSL provides SSL, TLS and general purpose cryptography. It wraps the OpenSSL library.


All examples assume you have loaded OpenSSL with:

require 'openssl'

These examples build atop each other. For example the key created in the next is used in throughout these examples.


Creating a Key

This example creates a 2048 bit RSA keypair and writes it to the current directory.

key = OpenSSL::PKey::RSA.new 2048

open 'private_key.pem', 'w' do |io| io.write key.to_pem end
open 'public_key.pem', 'w' do |io| io.write key.public_key.to_pem end
Exporting a Key

Keys saved to disk without encryption are not secure as anyone who gets ahold of the key may use it unless it is encrypted. In order to securely export a key you may export it with a pass phrase.

cipher = OpenSSL::Cipher.new 'aes-256-cbc'
pass_phrase = 'my secure pass phrase goes here'

key_secure = key.export cipher, pass_phrase

open 'private.secure.pem', 'w' do |io|
  io.write key_secure

Cipher.ciphers returns a list of available ciphers.

Loading a Key

A key can also be loaded from a file.

key2 = OpenSSL::PKey.read File.read 'private_key.pem'
key2.public? # => true
key2.private? # => true


key3 = OpenSSL::PKey.read File.read 'public_key.pem'
key3.public? # => true
key3.private? # => false
Loading an Encrypted Key

OpenSSL will prompt you for your pass phrase when loading an encrypted key. If you will not be able to type in the pass phrase you may provide it when loading the key:

key4_pem = File.read 'private.secure.pem'
pass_phrase = 'my secure pass phrase goes here'
key4 = OpenSSL::PKey.read key4_pem, pass_phrase

RSA Encryption

RSA provides encryption and decryption using the public and private keys. You can use a variety of padding methods depending upon the intended use of encrypted data.

Encryption & Decryption

Asymmetric public/private key encryption is slow and victim to attack in cases where it is used without padding or directly to encrypt larger chunks of data. Typical use cases for RSA encryption involve “wrapping” a symmetric key with the public key of the recipient who would “unwrap” that symmetric key again using their private key. The following illustrates a simplified example of such a key transport scheme. It shouldn’t be used in practice, though, standardized protocols should always be preferred.

wrapped_key = key.public_encrypt key

A symmetric key encrypted with the public key can only be decrypted with the corresponding private key of the recipient.

original_key = key.private_decrypt wrapped_key

By default PKCS#1 padding will be used, but it is also possible to use other forms of padding, see ::OpenSSL::PKey::RSA for further details.


Using “private_encrypt” to encrypt some data with the private key is equivalent to applying a digital signature to the data. A verifying party may validate the signature by comparing the result of decrypting the signature with “public_decrypt” to the original data. However, PKey already has methods “sign” and “verify” that handle digital signatures in a standardized way - “private_encrypt” and “public_decrypt” shouldn’t be used in practice.

To sign a document, a cryptographically secure hash of the document is computed first, which is then signed using the private key.

signature = key.sign 'SHA256', document

To validate the signature, again a hash of the document is computed and the signature is decrypted using the public key. The result is then compared to the hash just computed, if they are equal the signature was valid.

if key.verify 'SHA256', signature, document
  puts 'Valid'
  puts 'Invalid'

PBKDF2 Password-based Encryption

If supported by the underlying OpenSSL version used, Password-based Encryption should use the features of PKCS5. If not supported or if required by legacy applications, the older, less secure methods specified in RFC 2898 are also supported (see below).

PKCS5 supports PBKDF2 as it was specified in PKCS#5 v2.0. It still uses a password, a salt, and additionally a number of iterations that will slow the key derivation process down. The slower this is, the more work it requires being able to brute-force the resulting key.


The strategy is to first instantiate a Cipher for encryption, and then to generate a random IV plus a key derived from the password using PBKDF2. PKCS #5 v2.0 recommends at least 8 bytes for the salt, the number of iterations largely depends on the hardware being used.

cipher = OpenSSL::Cipher.new 'aes-256-cbc'
iv = cipher.random_iv

pwd = 'some hopefully not to easily guessable password'
salt = OpenSSL::Random.random_bytes 16
iter = 20000
key_len = cipher.key_len
digest = OpenSSL::Digest.new('SHA256')

key = OpenSSL::PKCS5.pbkdf2_hmac(pwd, salt, iter, key_len, digest)
cipher.key = key

Now encrypt the data:

encrypted = cipher.update document
encrypted << cipher.final

Use the same steps as before to derive the symmetric AES key, this time setting the Cipher up for decryption.

cipher = OpenSSL::Cipher.new 'aes-256-cbc'
cipher.iv = iv # the one generated with #random_iv

pwd = 'some hopefully not to easily guessable password'
salt = ... # the one generated above
iter = 20000
key_len = cipher.key_len
digest = OpenSSL::Digest.new('SHA256')

key = OpenSSL::PKCS5.pbkdf2_hmac(pwd, salt, iter, key_len, digest)
cipher.key = key

Now decrypt the data:

decrypted = cipher.update encrypted
decrypted << cipher.final

PKCS #5 Password-based Encryption

PKCS #5 is a password-based encryption standard documented at RFC2898. It allows a short password or passphrase to be used to create a secure encryption key. If possible, PBKDF2 as described above should be used if the circumstances allow it.

PKCS #5 uses a Cipher, a pass phrase and a salt to generate an encryption key.

pass_phrase = 'my secure pass phrase goes here'
salt = '8 octets'

First set up the cipher for encryption

encryptor = OpenSSL::Cipher.new 'aes-256-cbc'
encryptor.pkcs5_keyivgen pass_phrase, salt

Then pass the data you want to encrypt through

encrypted = encryptor.update 'top secret document'
encrypted << encryptor.final

Use a new Cipher instance set up for decryption

decryptor = OpenSSL::Cipher.new 'aes-256-cbc'
decryptor.pkcs5_keyivgen pass_phrase, salt

Then pass the data you want to decrypt through

plain = decryptor.update encrypted
plain << decryptor.final

X509 Certificates

Creating a Certificate

This example creates a self-signed certificate using an RSA key and a SHA1 signature.

key = OpenSSL::PKey::RSA.new 2048
name = OpenSSL::X509::Name.parse '/CN=nobody/DC=example'

cert = OpenSSL::X509::Certificate.new
cert.version = 2
cert.serial = 0
cert.not_before = Time.now
cert.not_after = Time.now + 3600

cert.public_key = key.public_key
cert.subject = name
Certificate Extensions

You can add extensions to the certificate with OpenSSL::SSL::ExtensionFactory to indicate the purpose of the certificate.

extension_factory = OpenSSL::X509::ExtensionFactory.new nil, cert

cert.add_extension \
  extension_factory.create_extension('basicConstraints', 'CA:FALSE', true)

cert.add_extension \
    'keyUsage', 'keyEncipherment,dataEncipherment,digitalSignature')

cert.add_extension \
  extension_factory.create_extension('subjectKeyIdentifier', 'hash')

The list of supported extensions (and in some cases their possible values) can be derived from the “objects.h” file in the OpenSSL source code.

Signing a Certificate

To sign a certificate set the issuer and use X509::Certificate#sign with a digest algorithm. This creates a self-signed cert because we’re using the same name and key to sign the certificate as was used to create the certificate.

cert.issuer = name
cert.sign key, OpenSSL::Digest.new('SHA1')

open 'certificate.pem', 'w' do |io| io.write cert.to_pem end
Loading a Certificate

Like a key, a cert can also be loaded from a file.

cert2 = OpenSSL::X509::Certificate.new File.read 'certificate.pem'
Verifying a Certificate

Certificate#verify will return true when a certificate was signed with the given public key.

raise 'certificate can not be verified' unless cert2.verify key

Certificate Authority

A certificate authority (CA) is a trusted third party that allows you to verify the ownership of unknown certificates. The CA issues key signatures that indicate it trusts the user of that key. A user encountering the key can verify the signature by using the CA’s public key.

CA Key

CA keys are valuable, so we encrypt and save it to disk and make sure it is not readable by other users.

ca_key = OpenSSL::PKey::RSA.new 2048
pass_phrase = 'my secure pass phrase goes here'

cipher = OpenSSL::Cipher.new 'aes-256-cbc'

open 'ca_key.pem', 'w', 0400 do |io|
  io.write ca_key.export(cipher, pass_phrase)
CA Certificate

A CA certificate is created the same way we created a certificate above, but with different extensions.

ca_name = OpenSSL::X509::Name.parse '/CN=ca/DC=example'

ca_cert = OpenSSL::X509::Certificate.new
ca_cert.serial = 0
ca_cert.version = 2
ca_cert.not_before = Time.now
ca_cert.not_after = Time.now + 86400

ca_cert.public_key = ca_key.public_key
ca_cert.subject = ca_name
ca_cert.issuer = ca_name

extension_factory = OpenSSL::X509::ExtensionFactory.new
extension_factory.subject_certificate = ca_cert
extension_factory.issuer_certificate = ca_cert

ca_cert.add_extension \
  extension_factory.create_extension('subjectKeyIdentifier', 'hash')

This extension indicates the CA’s key may be used as a CA.

ca_cert.add_extension \
  extension_factory.create_extension('basicConstraints', 'CA:TRUE', true)

This extension indicates the CA’s key may be used to verify signatures on both certificates and certificate revocations.

ca_cert.add_extension \
    'keyUsage', 'cRLSign,keyCertSign', true)

Root CA certificates are self-signed.

ca_cert.sign ca_key, OpenSSL::Digest.new('SHA1')

The CA certificate is saved to disk so it may be distributed to all the users of the keys this CA will sign.

open 'ca_cert.pem', 'w' do |io|
  io.write ca_cert.to_pem
Certificate Signing Request

The CA signs keys through a Certificate Signing Request (CSR). The CSR contains the information necessary to identify the key.

csr = OpenSSL::X509::Request.new
csr.version = 0
csr.subject = name
csr.public_key = key.public_key
csr.sign key, OpenSSL::Digest.new('SHA1')

A CSR is saved to disk and sent to the CA for signing.

open 'csr.pem', 'w' do |io|
  io.write csr.to_pem
Creating a Certificate from a CSR

Upon receiving a CSR the CA will verify it before signing it. A minimal verification would be to check the CSR’s signature.

csr = OpenSSL::X509::Request.new File.read 'csr.pem'

raise 'CSR can not be verified' unless csr.verify csr.public_key

After verification a certificate is created, marked for various usages, signed with the CA key and returned to the requester.

csr_cert = OpenSSL::X509::Certificate.new
csr_cert.serial = 0
csr_cert.version = 2
csr_cert.not_before = Time.now
csr_cert.not_after = Time.now + 600

csr_cert.subject = csr.subject
csr_cert.public_key = csr.public_key
csr_cert.issuer = ca_cert.subject

extension_factory = OpenSSL::X509::ExtensionFactory.new
extension_factory.subject_certificate = csr_cert
extension_factory.issuer_certificate = ca_cert

csr_cert.add_extension \
  extension_factory.create_extension('basicConstraints', 'CA:FALSE')

csr_cert.add_extension \
    'keyUsage', 'keyEncipherment,dataEncipherment,digitalSignature')

csr_cert.add_extension \
  extension_factory.create_extension('subjectKeyIdentifier', 'hash')

csr_cert.sign ca_key, OpenSSL::Digest.new('SHA1')

open 'csr_cert.pem', 'w' do |io|
  io.write csr_cert.to_pem

SSL and TLS Connections

Using our created key and certificate we can create an SSL or TLS connection. An SSLContext is used to set up an SSL session.

context = OpenSSL::SSL::SSLContext.new
SSL Server

An SSL server requires the certificate and private key to communicate securely with its clients:

context.cert = cert
context.key = key

Then create an SSLServer with a TCP server socket and the context. Use the SSLServer like an ordinary TCP server.

require 'socket'

tcp_server = TCPServer.new 5000
ssl_server = OpenSSL::SSL::SSLServer.new tcp_server, context

loop do
  ssl_connection = ssl_server.accept

  data = ssl_connection.gets

  response = "I got #{data.dump}"
  puts response

  ssl_connection.puts "I got #{data.dump}"
SSL client

An SSL client is created with a TCP socket and the context. SSLSocket#connect must be called to initiate the SSL handshake and start encryption. A key and certificate are not required for the client socket.

Note that SSLSocket#close doesn’t close the underlying socket by default. Set SSLSocket#sync_close to true if you want.

require 'socket'

tcp_socket = TCPSocket.new 'localhost', 5000
ssl_client = OpenSSL::SSL::SSLSocket.new tcp_socket, context
ssl_client.sync_close = true

ssl_client.puts "hello server!"
puts ssl_client.gets

ssl_client.close # shutdown the TLS connection and close tcp_socket
Peer Verification

An unverified SSL connection does not provide much security. For enhanced security the client or server can verify the certificate of its peer.

The client can be modified to verify the server’s certificate against the certificate authority’s certificate:

context.ca_file = 'ca_cert.pem'
context.verify_mode = OpenSSL::SSL::VERIFY_PEER

require 'socket'

tcp_socket = TCPSocket.new 'localhost', 5000
ssl_client = OpenSSL::SSL::SSLSocket.new tcp_socket, context

ssl_client.puts "hello server!"
puts ssl_client.gets

If the server certificate is invalid or context.ca_file is not set when verifying peers an ::OpenSSL::SSL::SSLError will be raised.

Constant Summary

Class Attribute Summary

Class Method Summary

Class Attribute Details

.debugBoolean (rw, mod_func)

[ GitHub ]

# File 'ext/openssl/ossl.c', line 392

static VALUE
ossl_debug_get(VALUE self)
    return dOSSL;

.debug=(boolean) ⇒ Boolean (rw, mod_func)

Turns on or off debug mode. With debug mode, all errors added to the OpenSSL error queue will be printed to stderr.

[ GitHub ]

# File 'ext/openssl/ossl.c', line 405

static VALUE
ossl_debug_set(VALUE self, VALUE val)
    dOSSL = RTEST(val) ? Qtrue : Qfalse;

    return val;

.fips_modeBoolean (rw, mod_func)

[ GitHub ]

# File 'ext/openssl/ossl.c', line 417

static VALUE
ossl_fips_mode_get(VALUE self)

    VALUE enabled;
    enabled = FIPS_mode() ? Qtrue : Qfalse;
    return enabled;
    return Qfalse;

.fips_mode=(boolean) ⇒ Boolean (rw, mod_func)

Turns FIPS mode on or off. Turning on FIPS mode will obviously only have an effect for FIPS-capable installations of the OpenSSL library. Trying to do so otherwise will result in an error.


OpenSSL.fips_mode = true   # turn FIPS mode on
OpenSSL.fips_mode = false  # and off again
[ GitHub ]

# File 'ext/openssl/ossl.c', line 442

static VALUE
ossl_fips_mode_set(VALUE self, VALUE enabled)

    if (RTEST(enabled)) {
	int mode = FIPS_mode();
	if(!mode && !FIPS_mode_set(1)) /* turning on twice leads to an error */
	    ossl_raise(eOSSLError, "Turning on FIPS mode failed");
    } else {
	if(!FIPS_mode_set(0)) /* turning off twice is OK */
	    ossl_raise(eOSSLError, "Turning off FIPS mode failed");
    return enabled;
    if (RTEST(enabled))
	ossl_raise(eOSSLError, "This version of OpenSSL does not support FIPS mode");
    return enabled;

Class Method Details

Digest(name) (mod_func)

Returns a ::OpenSSL::Digest subclass by name

require 'openssl'

# => OpenSSL::Digest::MD5

# => NameError: wrong constant name Foo
[ GitHub ]

# File 'ext/openssl/lib/openssl/digest.rb', line 67

def Digest(name)

.errors (mod_func)

[ GitHub ]

.fixed_length_secure_compare(string, string) ⇒ Boolean

Constant time memory comparison for fixed length strings, such as results of ::OpenSSL::HMAC calculations.

Returns true if the strings are identical, false if they are of the same length but not identical. If the length is different, ArgumentError is raised.

[ GitHub ]

# File 'ext/openssl/ossl.c', line 634

static VALUE
ossl_crypto_fixed_length_secure_compare(VALUE dummy, VALUE str1, VALUE str2)
    const unsigned char *p1 = (const unsigned char *)StringValuePtr(str1);
    const unsigned char *p2 = (const unsigned char *)StringValuePtr(str2);
    long len1 = RSTRING_LEN(str1);
    long len2 = RSTRING_LEN(str2);

    if (len1 != len2) {
        ossl_raise(rb_eArgError, "inputs must be of equal length");

    switch (CRYPTO_memcmp(p1, p2, len1)) {
        case 0:	return Qtrue;
        default: return Qfalse;

.mem_check_startnil (mod_func)

Calls CRYPTO_mem_ctrl(CRYPTO_MEM_CHECK_ON). Starts tracking memory allocations. See also .print_mem_leaks.

This is available only when built with a capable OpenSSL and –enable-debug configure option.

[ GitHub ]

# File 'ext/openssl/ossl.c', line 477

static VALUE
mem_check_start(VALUE self)
	return Qnil;

.secure_compare(string, string) ⇒ Boolean

Constant time memory comparison. Inputs are hashed using SHA-256 to mask the length of the secret. Returns true if the strings are identical, false otherwise.

[ GitHub ]

# File 'ext/openssl/lib/openssl.rb', line 32

def self.secure_compare(a, b)
  hashed_a = OpenSSL::Digest.digest('SHA256', a)
  hashed_b = OpenSSL::Digest.digest('SHA256', b)
  OpenSSL.fixed_length_secure_compare(hashed_a, hashed_b) && a == b