<p>There is no concrete determination that makes one 'y' value negative or not in an EC point. Feel free to make your own convention, like y-values &lt;= than half of p are negative, and &gt; half of p are positive. That's just a convention, though.</p>
<p>Related:</p>
<ul>
<li><a href="https://bitcoin.stackexchange.com/questions/21907/what-does-the-curve-used-in-bitcoin-secp256k1-look-like">What does the curve used in Bitcoin, secp256k1, look like?</a></li>
</ul>
<hr>
<blockquote>
<p>Also, how can you identify which pub key is odd and which is even?</p>
<p>ex: private key 1 x,y is odd , private key 2 x,y is even</p>
</blockquote>
<p>You can't! Not without knowing the private key itself. If you could, you would be part of the way towards solving the <a href="https://en.wikipedia.org/wiki/Discrete_logarithm" rel="noreferrer">discrete log problem</a>.</p>

CC BY-SA 3.0
2015-09-27T04:50:59.143Z
Hi StephenM347, y-values <= than half of p are negative, and > half of p are positive, please can you explain this clear with some example
Prabu r
CC BY-SA 4.0
2020-01-24T02:56:40.523Z
How would one break the discrete log problem by knowing whether the Y is positive, negative, odd, or even? Would it absolutely break the cryptography or just narrow the field search to find a private key?
Pedro
CC BY-SA 4.0
2020-04-18T18:50:03.040Z
@PedroGonçalves Knowing whether the Y coordinate is odd or even is of course not a break of anything, because it's public information. Knowing whether the Y coordinate is "positive or negative" depends on how you define positive or negative (as stated in this answer, you can make up your own convention). IF you define it as "the corresponding private key is even/odd", then indeed knowing whether Y is positive or negative is trivially a security break (you're not supposed to be able to infer any information about the private key).
Pieter Wuille
CC BY-SA 3.0
2015-09-15T15:19:28.850Z
2015-09-15T15:37:33.127Z
2020-06-16T11:10:03.350Z
morsecoder
<p>Let's look at this more formally.</p>
<p>Assume that there is a function <code>lsbPriv(P)</code> such that it returns the LSB of the private key <code>k</code> from the public key <code>P = [k]G</code> where <code>G</code> is the base point.</p>
<p>Now, we can use this function to determine all of the bits of the private key <code>k</code> as follows;</p>
<pre><code>func DlogbyLSB(P)
  for i in range(1..256)
    x = lsbPriv(P)
    secretKey.append(x)
    if x == 0 then    ; test the bit
      P = [2^-1]P     ; i.e. we can divide `k` by `2` since the last bit is `0`
   else 
      P = [1]G        ; i.e. we substructed the bit `1` from the `k` and 
      P = [2^-1]P    ; now it is again divisible by `2`
   return secretKey
</code></pre>
<p>This is almost like the reverse of the <a href="https://en.wikipedia.org/wiki/Elliptic_curve_point_multiplication#Double-and-add" rel="nofollow noreferrer">double-and-add</a> algorithm that computes the scalar multiplication on the Elliptic Curves.</p>
<p>So, with the help of the <code>lsbPriv</code> we can learn every bit of the private key <code>k</code>. In Cryptography we call this <code>lsb bit</code> as <a href="https://en.wikipedia.org/wiki/Hard-core_predicate" rel="nofollow noreferrer">hard core predicate</a>. As we can see, after the 256 calls, the <code>lsbPriv</code> reveals the key. So the <code>lsbpriv</code> is as hard as to recover all bits.</p>
<p>This also implies that if such <code>lsbPriv</code> function exists it is equal to solving the <code>Dlog</code> problem on the curve <code>Secp256k</code>. Currently, we know that is hard so there is no such <code>lsbPriv</code> function.</p>

CC BY-SA 4.0
2021-11-14T15:07:03.450Z
2021-11-14T15:07:03.450Z
user108963
<p>Take a bitcoin public key (x, y) and its additive inverse (x, -y). How do you identify which is the positive point and which is the negative point?</p>

<p><strong>Example</strong></p>

<p><strong>Private key 1 -> (x, y)</strong></p>

<p>x =  0x79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798L</p>

<p>y =  0x483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8L</p>

<p>-y =  0xb7c52588d95c3b9aa25b0403f1eef75702e84bb7597aabe663b82f6f04ef2777L</p>

<p><strong>Private key 2 -> (x, y)</strong></p>

<p>x =  0xc6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5L</p>

<p>y =  0x1ae168fea63dc339a3c58419466ceaeef7f632653266d0e1236431a950cfe52aL</p>

<p>-y =  0xe51e970159c23cc65c3a7be6b99315110809cd9acd992f1edc9bce55af301705L</p>

<p><strong>Private key 3 -> (x, y)</strong></p>

<p>x =  0xf9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9L</p>

<p>y =  0x388f7b0f632de8140fe337e62a37f3566500a99934c2231b6cb9fd7584b8e672L</p>

<p>-y =  0xc77084f09cd217ebf01cc819d5c80ca99aff5666cb3ddce4934602897b4715bdL</p>

<hr>

<p>Also, how can you identify which pub key is odd and which is even?</p>

<p>ex: private key 1 x,y is odd , private key 2 x,y is even</p>

CC BY-SA 3.0
2015-09-15T15:09:01.807Z
I don't understand the question.  x and y here are elements of the finite field F_p in which there is not generally an unambiguous meaning of "odd" and "even" - every number is divisible by 2 with respect to the field multiplication.  There is a similar issue with "positive" and "negative" since F_p is not an ordered field.
Nate Eldredge
CC BY-SA 3.0
2015-09-15T16:00:32.373Z
@NateEldredge, I think Prabu is looking to know whether or not the point was generated from a private key that is odd/even, not whether the resulting coordinates are odd/even.
morsecoder
<bitcoind><public-key><secp256k1>
How to determine if a public key point y is negative or positive, odd or even?
Prabu r
# How to determine if a public key point y is negative or positive, odd or even?

(Asked by: username_0 on 2015-09-15)

Take a bitcoin public key (x, y) and its additive inverse (x, -y). How do you identify which is the positive point and which is the negative point?

**Example**

**Private key 1 -> (x, y)**

x = 0x79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798L

y = 0x483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8L

-y = 0xb7c52588d95c3b9aa25b0403f1eef75702e84bb7597aabe663b82f6f04ef2777L

**Private key 2 -> (x, y)**

x = 0xc6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5L

y = 0x1ae168fea63dc339a3c58419466ceaeef7f632653266d0e1236431a950cfe52aL

-y = 0xe51e970159c23cc65c3a7be6b99315110809cd9acd992f1edc9bce55af301705L

**Private key 3 -> (x, y)**

x = 0xf9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9L

y = 0x388f7b0f632de8140fe337e62a37f3566500a99934c2231b6cb9fd7584b8e672L

-y = 0xc77084f09cd217ebf01cc819d5c80ca99aff5666cb3ddce4934602897b4715bdL

---

Also, how can you identify which pub key is odd and which is even?

ex: private key 1 x,y is odd , private key 2 x,y is even

**Tags:** bitcoind, public-key, secp256k1

> **Comments:**
> **username_1**: I don't understand the question.  x and y here are elements of the finite field F_p in which there is not generally an unambiguous meaning of "odd" and "even" - every number is divisible by 2 with respect to the field multiplication.  There is a similar issue with "positive" and "negative" since F_p is not an ordered field.  
> **username_2**: @NateEldredge, I think Prabu is looking to know whether or not the point was generated from a private key that is odd/even, not whether the resulting coordinates are odd/even.  

---

## 2 answers

### Answer by username_2 on 2015-09-15. Score: 5

There is no concrete determination that makes one 'y' value negative or not in an EC point. Feel free to make your own convention, like y-values <= than half of p are negative, and > half of p are positive. That's just a convention, though.

Related:

* [What does the curve used in Bitcoin, secp256k1, look like?](https://bitcoin.stackexchange.com/questions/21907/what-does-the-curve-used-in-bitcoin-secp256k1-look-like)

---

> Also, how can you identify which pub key is odd and which is even?
>
> ex: private key 1 x,y is odd , private key 2 x,y is even

You can't! Not without knowing the private key itself. If you could, you would be part of the way towards solving the [discrete log problem](https://en.wikipedia.org/wiki/Discrete_logarithm).

> **Comments:**
> **username_0**: Hi StephenM347, y-values <= than half of p are negative, and > half of p are positive, please can you explain this clear with some example  
> **username_3**: How would one break the discrete log problem by knowing whether the Y is positive, negative, odd, or even? Would it absolutely break the cryptography or just narrow the field search to find a private key?  
> **username_4**: @username_3Gonçalves Knowing whether the Y coordinate is odd or even is of course not a break of anything, because it's public information. Knowing whether the Y coordinate is "positive or negative" depends on how you define positive or negative (as stated in this answer, you can make up your own convention). IF you define it as "the corresponding private key is even/odd", then indeed knowing whether Y is positive or negative is trivially a security break (you're not supposed to be able to infer any information about the private key).  

---

### Answer by username_5 on 2021-11-14. Score: 0

Let's look at this more formally.
Assume that there is a function lsbPriv(P) such that it returns the LSB of the private key k from the public key P = [k]G where G is the base point.
Now, we can use this function to determine all of the bits of the private key k as follows;
func DlogbyLSB(P)
  for i in range(1..256)
    x = lsbPriv(P)
    secretKey.append(x)
    if x == 0 then    ; test the bit
      P = [2^-1]P     ; i.e. we can divide `k` by `2` since the last bit is `0`
   else 
      P = [1]G        ; i.e. we substructed the bit `1` from the `k` and 
      P = [2^-1]P    ; now it is again divisible by `2`
   return secretKey

This is almost like the reverse of the double-and-add algorithm that computes the scalar multiplication on the Elliptic Curves.
So, with the help of the lsbPriv we can learn every bit of the private key k. In Cryptography we call this lsb bit as hard core predicate. As we can see, after the 256 calls, the lsbPriv reveals the key. So the lsbpriv is as hard as to recover all bits.
This also implies that if such lsbPriv function exists it is equal to solving the Dlog problem on the curve Secp256k. Currently, we know that is hard so there is no such lsbPriv function.


---

