btcec: Ensure reduction when > P in all cases.

As noted in issue btcsuite#706, the existing code had an issue where the
normalized result was > P when both the first and second words of the
field representation being normalized were BOTH greater than or equal to
the first and second words of P.  Although this condition is rare in
practice, it needs to be handled properly.

This resolves the issue by comparing the low words in the final
reduction step against the normalized low order prime bits to ensure the
final subtraction occurs correctly any time they're > P.  This approach
retains the constant time property as well.

btcec/field.go

@@ -1,5 +1,5 @@
-// Copyright (c) 2013-2014 The btcsuite developers
-// Copyright (c) 2013-2014 Dave Collins
+// Copyright (c) 2013-2016 The btcsuite developers
+// Copyright (c) 2013-2016 Dave Collins
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
@@ -94,14 +94,16 @@ const (
 	fieldMSBMask = (1 << fieldMSBBits) - 1
 
 	// fieldPrimeWordZero is word zero of the secp256k1 prime in the
-	// internal field representation.  It is used during modular reduction
-	// and negation.
+	// internal field representation.  It is used during negation.
 	fieldPrimeWordZero = 0x3fffc2f
 
 	// fieldPrimeWordOne is word one of the secp256k1 prime in the
-	// internal field representation.  It is used during modular reduction
-	// and negation.
+	// internal field representation.  It is used during negation.
 	fieldPrimeWordOne = 0x3ffffbf
+
+	// primeLowBits is the lower 2*fieldBase bits of the secp256k1 prime in
+	// its standard normalized form.  It is used during modular reduction.
+	primeLowBits = 0xffffefffffc2f
 )
 
 // fieldVal implements optimized fixed-precision arithmetic over the
@@ -331,12 +333,8 @@ func (f *fieldVal) Normalize() *fieldVal {
 	// zero even though it won't change the value to ensure constant time
 	// between the branches.
 	var mask int32
-	if t0 < fieldPrimeWordZero {
-		mask |= -1
-	} else {
-		mask |= 0
-	}
-	if t1 < fieldPrimeWordOne {
+	lowBits := uint64(t1)<<fieldBase | uint64(t0)
+	if lowBits < primeLowBits {
 		mask |= -1
 	} else {
 		mask |= 0
@@ -381,8 +379,9 @@ func (f *fieldVal) Normalize() *fieldVal {
 	} else {
 		mask |= 0
 	}
-	t0 = t0 - uint32(^mask&fieldPrimeWordZero)
-	t1 = t1 - uint32(^mask&fieldPrimeWordOne)
+	lowBits -= ^uint64(mask) & primeLowBits
+	t0 = uint32(lowBits & fieldBaseMask)
+	t1 = uint32((lowBits >> fieldBase) & fieldBaseMask)
 	t2 = t2 & uint32(mask)
 	t3 = t3 & uint32(mask)
 	t4 = t4 & uint32(mask)