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Python bindings to the Ed25519 public-key signature system

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README.md

Python Bindings to the Ed25519 Digital Signature System

Build Status

This package provides python bindings to a C implementation of the Ed25519 public-key signature system 1. The C code is copied from the SUPERCOP benchmark suite 2, using the portable "ref" implementation (not the high-performance assembly code), and is very similar to the copy in the NaCl library 3. The C code is in the public domain 4. This python binding is released under the MIT license (see LICENSE in this distribution).

With this library, you can quickly (2ms) create signing+verifying keypairs, derive a verifying key from a signing key, sign messages, and verify the signatures. The keys and signatures are very short, making them easy to handle and incorporate into other protocols. All known attacks take at least 2^128 operations, providing the same security level as AES-128, NIST P-256, and RSA-3072.

Dependencies

This library includes a copy of all the C code necessary. You will need Python 2.x (2.5 or later, not 3.x) and a C compiler. The tests are run automatically against python2.5, python2.6, and python2.7 .

Speed and Key Sizes

Signing key seeds are merely 32 bytes of random data, so generating a signing key is trivial. Deriving a public verifying key takes more time, as do the actual signing and verifying operations.

On my 2010-era Mac laptop (2.8GHz Core2Duo), deriving a verifying key takes 1.9ms, signing takes 1.9ms, and verification takes 6.3ms. The high-performance assembly code in SUPERCOP (amd64-51-30k and amd64-64-24k) is up to 100x faster than the portable reference version, and the python overhead appears to be minimal (1-2us), so future releases may run even faster.

Ed25519 private signing keys are 32 bytes long (this seed is expanded to 64 bytes when necessary). The public verifying keys are also 32 bytes long. Signatures are 64 bytes long. All operations provide a 128-bit security level.

Testing

The Ed25519 web site includes a (spectacularly slow) pure-python implementation for educational purposes. That code includes a set of known-answer-tests. Those tests are included in this distribution, and takes about 17 seconds to execute. The distribution also includes unit tests of the object-oriented SigningKey / VerifyingKey layer. Run test.py to execute these tests.

Security

The Ed25519 algorithm and C implementation are carefully designed to prevent timing attacks. The Python wrapper might not preserve this property. Until it has been audited for this purpose, do not allow attackers to measure how long it takes you to generate a keypair or sign a message. Key generation depends upon a strong source of random numbers. Do not use it on a system where os.urandom() is weak.

Unlike typical DSA/ECDSA algorithms, signing does not require a source of entropy. Ed25519 signatures are deterministic: using the same key to sign the same data any number of times will result in the same signature each time.

Compilation

To build and install the library, run the normal setup.py command:

python setup.py build
sudo python setup.py install

You can run the (fast) test suite, the (slower) known-answer-tests, and the speed-benchmarks through setup.py commands too:

python setup.py test
python setup.py test_kat
python setup.py speed

Prefixes and Encodings

The basic keypair/sign/verify operations work on binary bytestrings: signing keys are created with a 32-byte seed or a 64-byte expanded form, verifying keys are serialized as 32-byte binary strings, and signatures are 64-byte binary strings.

All methods that generate or accept bytestrings take a prefix= argument, which is simply prepended to the output or stripped from the input. This can be used for a cheap version check: if you use e.g. prefix="pubkey0-" when handling verifying keys, and later update your application to use a different kind of key (and update to "pubkey1-"), then older receivers will throw a clean error when faced with a key format that they cannot handle.

These methods also accept an encoding= argument, which makes them return an ASCII string instead of a binary bytestring. This makes it convenient to display verifying keys or signatures to cut-and-paste or encode into JSON messages. Be careful when encouraging users to cut-and-paste signing keys, since you might enable them to accidentally reveal those keys: in general, it should require slightly more attention to handle signing keys than verifying keys.

encoding= can be set to one of "base64", "base32", "base16", or "hex" (an alias for "base16"). The strings are stripped of trailing "=" markers and lowercased (for base32/base16).

Usage

The first step is to create a signing key and store it. The safest way to generate a key is with the create_keypair() function, which uses 32 bytes of random data from os.urandom() (although you can provide an alternative entropy source with the entropy= argument):

import ed25519
signing_key, verifying_key = ed25519.create_keypair()
open("my-secret-key","wb").write(signing_key.to_bytes())
vkey_hex = verifying_key.to_ascii(encoding="hex")
print "the public key is", vkey_hex

The private signing key string produced by to_bytes() is 64 bytes long, and includes a copy of the public key (to avoid the 1.9ms needed to recalculate it later). If you want to store less data (and recompute the public key later), you can store just the 32 byte seed instead:

open("my-secret-seed","wb").write(signing_key.to_seed())

The signing key is an instance of the ed25519.SigningKey class. To reconstruct this instance from a serialized form, the constructor accepts the output of either .to_bytes() or .to_seed():

keydata = open("my-secret-key","rb").read()
signing_key = ed25519.SigningKey(keydata)

seed = open("my-secret-seed","rb").read()
signing_key2 = ed25519.SigningKey(seed)
assert signing_key == signing_key2

Special-purpose applications may want to derive keypairs from existing secrets; any 32-byte uniformly-distributed random string can be provided as a seed:

import os, hashlib
master = os.urandom(87)
seed = hashlib.sha256(master).digest()
signing_key = ed25519.SigningKey(seed)

Once you have the SigningKey instance, use its .sign() method to sign a message. The signature is 64 bytes, but can be generated in printable form with the encoding= argument:

sig = signing_key.sign("hello world", encoding="base64")
print "sig is:", sig

On the verifying side, the receiver first needs to construct a ed25519.VerifyingKey instance from the serialized string, then use its .verify() method on the signature and message:

vkey_hex = "1246b84985e1ab5f83f4ec2bdf271114666fd3d9e24d12981a3c861b9ed523c6"
verifying_key = ed25519.VerifyingKey(vkey_hex, encoding="hex")
try:
  verifying_key.verify(sig, "hello world", encoding="base64")
  print "signature is good"
except ed25519.BadSignatureError:
  print "signature is bad!"

If you happen to have the SigningKey but not the corresponding VerifyingKey, you can derive it with .get_verifying_key(). This allows the sending side to hold just 32 bytes of data and derive everything else from that:

keydata = open("my-secret-seed","rb").read()
signing_key = ed25519.SigningKey(keydata)
verifying_key = signing_key.get_verifying_key()

There is also a basic command-line keygen/sign/verify tool in bin/edsig .

API Summary

The complete API is summarized here:

sk,vk = ed25519.create_keypair(entropy=os.urandom)
vk = sk.get_verifying_key()

signature = sk.sign(message, prefix=, encoding=)
vk.verify(signature, message, prefix=, encoding=)

seed = sk.to_seed(prefix=)
sk = SigningKey(seed, prefix=)
bytes = sk.to_bytes(prefix=)
sk = SigningKey(bytes, prefix=)
ascii = sk.to_ascii(prefix=, encoding=)  # encodes seed
sk = SigningKey(ascii, prefix=, encoding=)

bytes = vk.to_bytes(prefix=)
vk = VerifyingKey(bytes, prefix=)
ascii = vk.to_ascii(prefix=, encoding=)
vk = VerifyingKey(ascii, prefix=, encoding=)
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