Comment improvements for x86_64 ecc_secp256r1_redc

diff --git a/x86_64/ecc-secp256r1-redc.asm b/x86_64/ecc-secp256r1-redc.asm
index a03059ae4d900a3fc5516f069d72cbd05727bd6c..91f5522589ba7bd9836f471eba34582ca0f48ad2 100644 (file)
--- a/x86_64/ecc-secp256r1-redc.asm
+++ b/x86_64/ecc-secp256r1-redc.asm
@@ -47,7 +47,7 @@ define(`F1', `%r13')
 define(`F2', `%rbx')
 define(`F3', `%rbp')
 
-C FOLD(x), sets (F3,F2,F1,F0)  <-- (x << 224) - (x << 128) - (x<<32)
+C FOLD(x), sets (F3,F2,F1,F0)  <-- (x << 160) - (x << 128) - (x<<32)
 define(`FOLD', `
        mov     $1, F2
        mov     $1, F3
@@ -104,8 +104,10 @@ PROLOGUE(_nettle_ecc_secp256r1_redc)
        adc     U6, U2
        adc     56(XP), U3
 
-       C If carry, we need to add in
-       C 2^256 - p = <0xfffffffe, 0xff..ff, 0xffffffff00000000, 1>
+       C Sum, including carry, is < 2^{256} + p.
+       C If carry, we need to add in 2^{256} mod p = 2^{256} - p
+       C     = <0xfffffffe, 0xff..ff, 0xffffffff00000000, 1>
+       C and this addition can not overflow.
        sbb     F2, F2
        mov     F2, F0
        mov     F2, F1