An efficient public key cryptography library for Ruby providing key exchange/agreement.
This gem implements X25519 (a.k.a. Curve25519) Elliptic Curve Diffie-Hellman function as described in RFC7748 as a C extension using the high performance rfc7748_precomputed implementation based on the paper How to (pre-)compute a ladder (with fallback to the ref10 C implementation).
X25519 is one of two notable algorithms implemented atop the Curve25519 elliptic curve. The ed25519 gem is a related project of this one, and implements the Ed25519 signature scheme on the twisted Edwards form of Curve25519.
Is it any good?
What is it useful for?
X25519 is a key exchange/agreement algorithm generally used as a low-level building block in cryptographic protocols.
Can I use X25519 to encrypt things?
Please use RbNaCl::Box or the (experimental) XSTREAM library if you would like to use X25519 for public-key encryption. Otherwise, the X25519 algorithm is not directly useful for encryption without a higher-level encryption protocol built on top of it.
Help and Discussion
Have questions? Want to suggest a feature or change? Join a discussion group:
- Crypto.rb Gitter: web-based chat about Ruby crypto projects including x25519.
- Crypto.rb Google Group: join via web or email (email@example.com)
x25519.rb is supported on and tested against the following platforms:
- MRI 2.2, 2.3, 2.4, 2.5
Add this line to your application's Gemfile:
And then execute:
Or install it yourself as:
$ gem install x25519
The example below shows how to perform a full Diffie-Hellman key exchange:
require "x25519" # Alice generates random scalar (private key) alice_sk = X25519::Scalar.generate # Alice obtains public key for her private key/scalar alice_pk = alice_sk.public_key # Bob generates random scalar (private key) # Ostensibly this would be on a different computer somewhere bob_sk = X25519::Scalar.generate bob_pk = bob_sk.public_key # Alice can perform Diffie-Hellman with Bob's public key alice_secret = alice_sk.diffie_hellman(bob_pk).to_bytes # Bob can perform Diffie-Hellman with Alice's public key bob_secret = bob_sk.diffie_hellman(alice_pk).to_bytes # The resulting secrets should be the same alice_secret == bob_secret # true
X25519::Scalar: private keys
X25519::Scalar class represents secret integers used as X25519 private
keys. These secret integers are multiplied by a well-known base point to
obtain X25519 public keys (
X25519::Scalar.generate(): make a random private key
Generate a random private scalar (using
secret_key = X25519::Scalar.generate
X25519::Scalar.new(bytes): load existing private key
bytes: a 32-byte
Stringvalue containing the private key
secret_key = X25519::Scalar.new(File.read("alice.key"))
X25519::Scalar#public_key(): obtain public key for this scalar
#multiply_base method is an alias of this one.
Performs fixed-base scalar multiplication (i.e. calculates public key)
X25519::MontgomeryU object which represents the public key for this private key/scalar.
secret_key = X25519::Scalar.generate public_key = secret_key.public_key
X25519::Scalar#diffie_hellman(other_public_key): obtain public key for this scalar
#multiply method is an alias of this one.
Performs variable-base scalar multiplication, computing a shared secret between our private scalar and someone else's public key/point.
X25519::MontgomeryUobject containing the public key with which we'd like to compute a shared secret.
X25519::MontgomeryU object which represents the shared secret.
secret_key = X25519::Scalar.generate public_key = X25519::MontgomeryU.new(File.read("bob.pub")) # Returns an X25519::MontgomeryU shared_secret = secret_key.multiply(public_key) # Obtain the shared secret as a serialized byte representation shared_secret_bytes = shared_secret.to_bytes
X25519::Scalar#to_bytes: serialize a scalar as a
String containing a byte representation of this scalar:
secret_key = X25519::Scalar.new(...) File.write("alice.key", secret_key.to_bytes)
X25519::MontgomeryU: public keys and shared secrets
X25519::MontgomeryU class represents a coordinate (specifically a
Montgomery-u coordinate) on the elliptic curve. In the X25519 Diffie-Hellman
function, these serve both as public keys and as shared secrets.
X25519::MontgomeryU.new(bytes): load existing public key
bytes: a 32-byte
Stringvalue containing the public key
public_key = X25519::MontgomeryU.new(File.read("bob.pub"))
X25519::MontgomeryU#to_bytes: serialize a Montgomery-u coordinate as a
String containing a byte representation of a compressed Montgomery-u coordinate:
public_key = X25519::MontgomeryU..new(...) File.write("bob.pub", public_key.to_bytes)
X25519: module-level functionality
X25519.diffie_hellman(secret_key, public_key): shorthand
If you'd like to avoid the object-oriented API, you can use a simplified API which acts entirely on bytestrings.
secret_key: a 32-byte
Stringcontaining a private scalar
public_key: a 32-byte
Stringcontaining a compressed Montgomery-u coordinate
String containing a 32-byte compressed Montgomery-u coordinate
Bug reports and pull requests are welcome on GitHub at https://github.com/crypto-rb/x25519. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.
This gem contains two implementations of X25519: an optimized assembly implementation and a portable C implementation. Implementations are selected based on available CPU features.
rfc7748_precomputed: optimized assembly implementation
- Prime field arithmetic is optimized for the 4th and 6th generation of Intel Core processors (Haswell and Skylake micro-architectures).
- Efficient integer multiplication using MULX instruction.
- Integer additions accelerated with ADCX/ADOX instructions.
- Key generation uses a read-only table of 8 KB for X25519.
ref10: portable C implementation
- Taken from the SUPERCOP cryptographic benchmarking suite (supercop-20171020)
- Portable C code which should compile on any architecture
The X25519 Diffie-Hellman function was originally designed by Dan Bernstein:
The optimized rfc7748_precomputed implementation was designed by:
- Thomaz Oliveira, Computer Science Department, Cinvestav-IPN, Mexico.
- Julio López, University of Campinas, Brazil.
- Hüseyin Hisil, Yasar University, Turkey.
- Armando Faz-Hernández, University of Campinas, Brazil.
- Francisco Rodríguez-Henríquez, Computer Science Department, Cinvestav-IPN, Mexico.
Copyright (c) 2017-2018 Armando Faz, Tony Arcieri
This gem is available as open source under the terms of the BSD-3 Clause License (LICENSE)
Code of Conduct
Everyone interacting in the x25519.rb project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.