From 39500ed5ed1904c5b1a6182b98913bbedee0b412 Mon Sep 17 00:00:00 2001
From: Conner Fromknecht <conner@lightning.engineering>
Date: Tue, 14 May 2019 22:46:54 -0700
Subject: [PATCH] btcec/pubkey: remove redundant checks from compressed pubkey
 parsing

As of https://github.com/btcsuite/btcd/pull/1193, decompressPoint now
validates that the point is on the curve. The x and y cooordinates are
also implicitly <= P, since the modular reduction is applied to both
before the method returns. The checks are moved so that they are still
applied when parsing an uncompressed pubkey, as the checks are not
redundant in that path.
---
 btcec/pubkey.go | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/btcec/pubkey.go b/btcec/pubkey.go
index cf498075..a6a492e7 100644
--- a/btcec/pubkey.go
+++ b/btcec/pubkey.go
@@ -102,6 +102,17 @@ func ParsePubKey(pubKeyStr []byte, curve *KoblitzCurve) (key *PublicKey, err err
 		if format == pubkeyHybrid && ybit != isOdd(pubkey.Y) {
 			return nil, fmt.Errorf("ybit doesn't match oddness")
 		}
+
+		if pubkey.X.Cmp(pubkey.Curve.Params().P) >= 0 {
+			return nil, fmt.Errorf("pubkey X parameter is >= to P")
+		}
+		if pubkey.Y.Cmp(pubkey.Curve.Params().P) >= 0 {
+			return nil, fmt.Errorf("pubkey Y parameter is >= to P")
+		}
+		if !pubkey.Curve.IsOnCurve(pubkey.X, pubkey.Y) {
+			return nil, fmt.Errorf("pubkey isn't on secp256k1 curve")
+		}
+
 	case PubKeyBytesLenCompressed:
 		// format is 0x2 | solution, <X coordinate>
 		// solution determines which solution of the curve we use.
@@ -115,20 +126,12 @@ func ParsePubKey(pubKeyStr []byte, curve *KoblitzCurve) (key *PublicKey, err err
 		if err != nil {
 			return nil, err
 		}
+
 	default: // wrong!
 		return nil, fmt.Errorf("invalid pub key length %d",
 			len(pubKeyStr))
 	}
 
-	if pubkey.X.Cmp(pubkey.Curve.Params().P) >= 0 {
-		return nil, fmt.Errorf("pubkey X parameter is >= to P")
-	}
-	if pubkey.Y.Cmp(pubkey.Curve.Params().P) >= 0 {
-		return nil, fmt.Errorf("pubkey Y parameter is >= to P")
-	}
-	if !pubkey.Curve.IsOnCurve(pubkey.X, pubkey.Y) {
-		return nil, fmt.Errorf("pubkey isn't on secp256k1 curve")
-	}
 	return &pubkey, nil
 }