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Add EC_POINT_add support #261

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Jul 4, 2019
Merged

Add EC_POINT_add support #261

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29 changes: 29 additions & 0 deletions ext/openssl/ossl_pkey_ec.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1562,6 +1562,34 @@ ossl_ec_point_to_octet_string(VALUE self, VALUE conversion_form)
return str;
}

/*
* call-seq:
* point.add(point) => point
*
* Performs elliptic curve point addition.
*/
static VALUE ossl_ec_point_add(VALUE self, VALUE other)
{
EC_POINT *point_self, *point_other, *point_result;
const EC_GROUP *group;
VALUE group_v = rb_attr_get(self, id_i_group);
VALUE result;

GetECPoint(self, point_self);
GetECPoint(other, point_other);
GetECGroup(group_v, group);

result = rb_obj_alloc(cEC_POINT);
ossl_ec_point_initialize(1, &group_v, result);
GetECPoint(result, point_result);

if (EC_POINT_add(group, point_result, point_self, point_other, ossl_bn_ctx) != 1) {
ossl_raise(eEC_POINT, "EC_POINT_add");
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What does this actually mean?

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@jdhollis jdhollis Jul 4, 2019 •
edited
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I gave this some thought earlier today, and I’m not sure more documentation makes sense here.

In ECC, point multiplication acts as a one-way hash, and while it probably isn’t implemented this way under the hood for performance reasons, point addition is a more primitive operation that forms the basis for point multiplication.

Most people won’t typically need this operation unless they’re implementing their own protocol (at which point, they’ll know what it is and what to do with it).

As you can see from the Wikipedia overview, the math is somewhat involved (though this post on Hacker Noon may be easier to follow).

I’m not sure a fuller description of ECC makes sense in context of the API docs.

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Fair enough - I was just specifically wondering how this could fail, and under what situations the exception would be raised.

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Ah. So digging through the OpenSSL source code, it looks like the top level checks involve whether a point is compatible with a particular curve. But given how points are created in Ruby, this would trigger an exception elsewhere. I suspect there are also other low-level checks further in depending on the group type.

I had assumed from looking at the other code in the openssl gem that ossl_raise would just do the right thing in any case.

}

return result;
}

/*
* call-seq:
* point.mul(bn1 [, bn2]) => point
Expand Down Expand Up @@ -1786,6 +1814,7 @@ void Init_ossl_ec(void)
/* all the other methods */

rb_define_method(cEC_POINT, "to_octet_string", ossl_ec_point_to_octet_string, 1);
rb_define_method(cEC_POINT, "add", ossl_ec_point_add, 1);
rb_define_method(cEC_POINT, "mul", ossl_ec_point_mul, -1);

id_i_group = rb_intern("@group");
Expand Down
16 changes: 16 additions & 0 deletions test/test_pkey_ec.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -289,6 +289,22 @@ def test_ec_point
assert_equal true, point.on_curve?
end

def test_ec_point_add
group = OpenSSL::PKey::EC::Group.new(:GFp, 17, 2, 2)
group.point_conversion_form = :uncompressed
gen = OpenSSL::PKey::EC::Point.new(group, B(%w{ 04 05 01 }))
group.set_generator(gen, 19, 1)

point_a = OpenSSL::PKey::EC::Point.new(group, B(%w{ 04 06 03 }))
point_b = OpenSSL::PKey::EC::Point.new(group, B(%w{ 04 10 0D }))

result = point_a.add(point_b)
assert_equal B(%w{ 04 0D 07 }), result.to_octet_string(:uncompressed)

assert_raise(TypeError) { point_a.add(nil) }
assert_raise(ArgumentError) { point_a.add }
end

def test_ec_point_mul
begin
# y^2 = x^3 + 2x + 2 over F_17
Expand Down