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| author |  Jason Gross <jgross@mit.edu> | 2019-01-14 19:05:15 -0500 |
|---|---|---|
| committer |  Jason Gross <jasongross9@gmail.com> | 2019-01-15 14:00:52 -0500 |
| commit | c61d5be86e3efb978883fc60687af42192aacaff (patch) | |
| tree | ea7da7858e1561490b8795d8e71b21819fca4319 /curve25519_32.c | |
| parent | 8faf6852f5bb36f5c663386f7dfbd0ae258445f9 (diff) | |
Don't cast signed to unsigned before shifting
Unfortunately, signed->unsigned casts do not commute with shifts. We
take care to only extend the range when it needs extending, now. This
was previously causing issues with subborrow.
We should really get proofs about casts in C semantics at some point
soon.
Fixes #489
Diffstat (limited to 'curve25519_32.c')
| -rw-r--r-- | curve25519_32.c | 6 |
1 files changed, 3 insertions, 3 deletions
diff --git a/curve25519_32.c b/curve25519_32.c index 5e6ee6486..b173a7498 100644 --- a/curve25519_32.c +++ b/curve25519_32.c @@ -39,7 +39,7 @@ static void fiat_25519_addcarryx_u26(uint32_t* out1, fiat_25519_uint1* out2, fia */ static void fiat_25519_subborrowx_u26(uint32_t* out1, fiat_25519_uint1* out2, fiat_25519_uint1 arg1, uint32_t arg2, uint32_t arg3) { int32_t x1 = ((int32_t)(arg2 - arg1) - (int32_t)arg3); - fiat_25519_int1 x2 = (fiat_25519_int1)((uint32_t)x1 >> 26); + fiat_25519_int1 x2 = (fiat_25519_int1)((int64_t)x1 >> 26); uint32_t x3 = (x1 & UINT32_C(0x3ffffff)); *out1 = x3; *out2 = (fiat_25519_uint1)(0x0 - x2); @@ -73,7 +73,7 @@ static void fiat_25519_addcarryx_u25(uint32_t* out1, fiat_25519_uint1* out2, fia */ static void fiat_25519_subborrowx_u25(uint32_t* out1, fiat_25519_uint1* out2, fiat_25519_uint1 arg1, uint32_t arg2, uint32_t arg3) { int32_t x1 = ((int32_t)(arg2 - arg1) - (int32_t)arg3); - fiat_25519_int1 x2 = (fiat_25519_int1)((uint32_t)x1 >> 25); + fiat_25519_int1 x2 = (fiat_25519_int1)((int64_t)x1 >> 25); uint32_t x3 = (x1 & UINT32_C(0x1ffffff)); *out1 = x3; *out2 = (fiat_25519_uint1)(0x0 - x2); @@ -748,7 +748,7 @@ static void fiat_25519_to_bytes(uint8_t out1[32], const uint32_t arg1[10]) { uint8_t x81 = (uint8_t)(x78 & UINT8_C(0xff)); uint8_t x82 = (uint8_t)(x80 >> 8); uint8_t x83 = (uint8_t)(x80 & UINT8_C(0xff)); - fiat_25519_uint1 x84 = (fiat_25519_uint1)(x82 >> 8); + fiat_25519_uint1 x84 = (fiat_25519_uint1)((int64_t)x82 >> 8); uint8_t x85 = (uint8_t)(x82 & UINT8_C(0xff)); uint32_t x86 = (x84 + x32); uint32_t x87 = (x86 >> 8); |