Get elliptic curve public key from private key using Ruby OpenSSL

Question

I'm playing around with Elliptic Curves using the Ruby 2.5.x OpenSSL library. I can easily generate a private and public key pair using

curve = OpenSSL::PKey::EC.new('secp256k1')
curve.generate_key

But given a private key I want to regenerate the public key.

I know OpenSSL can do it because the command line allows you to do it, and also the Ruby Bitcoin project does it. But the Ruby Bitcoin project has its own interface to OpenSSL using FFI rather than the one provided by Ruby.

Does Ruby 2.5.x openssl library not expose enough of the OpenSSL interfaces to be able to generate an elliptic curve public key from a private key, or that it can but it's not documented?

What methods are you trying to run that don't work as expected? — lacostenycoder
– lacostenycoder, Commented Apr 28, 2018 at 13:34
Suppose I save the private key in a file or database. I want to be able to get the public key from the private key. There's no method to do that, but the respondent below explained the required steps and it worked. — John Small
– John Small, Commented Apr 30, 2018 at 9:16

Sachin Sharma · Accepted Answer · 2020-09-23 08:07:18Z

9

In case someone interested to get public key in pem format too :)

example_key = OpenSSL::PKey::EC.new('secp256k1').generate_key
puts example_key.to_pem
pkey = OpenSSL::PKey::EC.new(example_key.public_key.group)
pkey.public_key = example_key.public_key
puts pkey.to_pem

answered Sep 23, 2020 at 8:07

Sachin Sharma

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2 Comments

Fabio Over a year ago

Thank you very much for this snippet. It's extremely useful and hard to find on the net

Felipe Zavan Over a year ago

`public_key=': pkeys are immutable on OpenSSL 3.0 (OpenSSL::PKey::PKeyError) :/

sincepast · Accepted Answer · 2023-02-26 14:12:55Z

5

The answer in https://stackoverflow.com/a/64023608/2603673 does not work with openssl 3.x because the key is now immutable, we need to change to:

key = OpenSSL::PKey::EC.new('secp256k1').generate_key
group = key.public_key.group
point = key.public_key
asn1 = OpenSSL::ASN1::Sequence(
  [
    OpenSSL::ASN1::Sequence([
      OpenSSL::ASN1::ObjectId('id-ecPublicKey'),
      OpenSSL::ASN1::ObjectId(group.curve_name)
    ]),
    OpenSSL::ASN1::BitString(point.to_octet_string(:uncompressed))
  ]
)
pkey = OpenSSL::PKey::EC.new(asn1.to_der)
puts pkey.to_pem

answered Feb 26, 2023 at 14:12

sincepast

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2 Comments

code_monkey_steve Over a year ago

This works for me, except the key needs to be generated with OpenSSL::PKey::EC.generate('prime256v1') to avoid the pkeys are immutable on OpenSSL 3.0 error.

sincepast Over a year ago

@code_monkey_steve if you just want to get the public key in pem format, you can also get it from the private key object using public_to_pem.

matt · Accepted Answer · 2018-04-28 18:03:24Z

3

The Ruby OpenSSL bindings don’t allow you to directly get the public key from a PKey::EC object as far as I can tell, but they do expose enough to do the calculation yourself, which is straightforward.

Given a private key as an OpenSSL:BN object, which for the example we can generate like this:

example_key = OpenSSL::PKey::EC.new('secp256k1').generate_key
private_key = example_key.private_key

We can calculate the public key by multiplying the group base point (i.e. the generator) by the private key:

group = OpenSSL::PKey::EC::Group.new('secp256k1')
public_key = group.generator.mul(private_key)

The public key is an OpenSSL::PKey::EC::Point. You can compare with the original to see that is the same:

puts example_key.public_key == public_key # => true

answered Apr 28, 2018 at 18:03

matt

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2 Comments

John Small Over a year ago

Excellent, that worked. I thought it might be something to do will #mul but wasn't sure.

John Small Over a year ago

Now I have to work out how to DER encode a signature.

waqas ali · Accepted Answer · 2024-09-10 12:03:10Z

In OpenSSL 3.0, keys are immutable, which means you cannot modify an existing OpenSSL::PKey::EC object after it has been created. Instead of trying to set the public key on an existing object, you should construct the public key using ASN.1 encoding.

Here’s how you can refactor the code to work with OpenSSL 3.0:

  private_key = OpenSSL::PKey::EC.generate("secp256k1")
  asn1 = OpenSSL::ASN1::Sequence(
    [
      OpenSSL::ASN1::Sequence(
        [
          OpenSSL::ASN1::ObjectId("id-ecPublicKey"),
          OpenSSL::ASN1::ObjectId(private_key.public_key.group.curve_name)
        ]
      ),
      OpenSSL::ASN1::BitString(private_key.public_key.to_octet_string(:uncompressed))
    ]
  )
  public_key = OpenSSL::PKey::EC.new(asn1.to_der)
  puts public_key.to_pem

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