require 'openssl'
OpenSSL
OpenSSL provides SSL, TLS and general purpose cryptography. It wraps the OpenSSL library.
Examples
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.
Keys
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-128-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
end
OpenSSL::Cipher.ciphers returns a list of available ciphers.
Loading a Key
A key can also be loaded from a file.
key2 = OpenSSL::PKey::RSA.new File.read 'private_key.pem'
key2.public? # => true
key2.private? # => true
or
key3 = OpenSSL::PKey::RSA.new 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::RSA.new 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 PKey::RSA for further details.
Signatures
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, OpenSSL::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.
digest = OpenSSL::Digest::SHA256.new
signature = key.sign digest, 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.
digest = OpenSSL::Digest::SHA256.new
if key.verify digest, signature, document
puts 'Valid'
else
puts 'Invalid'
end
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.
Encryption
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-128-CBC'
cipher.encrypt
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::SHA256.new
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
Decryption
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-128-CBC'
cipher.decrypt
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::SHA256.new
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'
Encryption
First set up the cipher for encryption
encryptor = OpenSSL::Cipher.new 'AES-128-CBC'
encryptor.encrypt
encryptor.pkcs5_keyivgen pass_phrase, salt
Then pass the data you want to encrypt through
encrypted = encryptor.update 'top secret document'
encrypted << encryptor.final
Decryption
Use a new Cipher instance set up for decryption
decryptor = OpenSSL::Cipher.new 'AES-128-CBC'
decryptor.decrypt
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 \
extension_factory.create_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 OpenSSL::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::SHA1.new
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-128-CBC'
open 'ca_key.pem', 'w', 0400 do |io|
io.write ca_key.export(cipher, pass_phrase)
end
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 \
extension_factory.create_extension(
'keyUsage', 'cRLSign,keyCertSign', true)
Root CA certificates are self-signed.
ca_cert.sign ca_key, OpenSSL::Digest::SHA1.new
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
end
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::SHA1.new
A CSR is saved to disk and sent to the CA for signing.
open 'csr.pem', 'w' do |io|
io.write csr.to_pem
end
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 \
extension_factory.create_extension(
'keyUsage', 'keyEncipherment,dataEncipherment,digitalSignature')
csr_cert.add_extension \
extension_factory.create_extension('subjectKeyIdentifier', 'hash')
csr_cert.sign ca_key, OpenSSL::Digest::SHA1.new
open 'csr_cert.pem', 'w' do |io|
io.write csr_cert.to_pem
end
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 = connection.gets
response = "I got #{data.dump}"
puts response
connection.puts "I got #{data.dump}"
connection.close
end
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.connect
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.connect
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.