diff --git a/ch04.asciidoc b/ch04.asciidoc
index 87806d9d..3270b116 100644
--- a/ch04.asciidoc
+++ b/ch04.asciidoc
@@ -441,10 +441,9 @@ geometric operation on the curve.
 
 [TIP]
 ====
-((("OpenSSL cryptographic library")))Most bitcoin implementations use
-the http://bit.ly/1ql7bn8[OpenSSL cryptographic library] to do the
-elliptic curve math. For example, to derive the public key, the function
-+EC_POINT_mul()+ is used.((("", startref="KAover04")))
+((("OpenSSL cryptographic library")))Many Bitcoin implementations use
+the https://github.com/bitcoin-core/secp256k1[libsecp256k1 crytographic
+library] to do the elliptic curve math.
 ====
 
 [[ecc_illustrated]]