[crypto] Use constant-time big integer multiplication

Big integer multiplication currently performs immediate carry
propagation from each step of the long multiplication, relying on the
fact that the overall result has a known maximum value to minimise the
number of carries performed without ever needing to explicitly check
against the result buffer size.

This is not a constant-time algorithm, since the number of carries
performed will be a function of the input values.  We could make it
constant-time by always continuing to propagate the carry until
reaching the end of the result buffer, but this would introduce a
large number of redundant zero carries.

Require callers of bigint_multiply() to provide a temporary carry
storage buffer, of the same size as the result buffer.  This allows
the carry-out from the accumulation of each double-element product to
be accumulated in the temporary carry space, and then added in via a
single call to bigint_add() after the multiplication is complete.

Since the structure of big integer multiplication is identical across
all current CPU architectures, provide a single shared implementation
of bigint_multiply().  The architecture-specific operation then
becomes the multiplication of two big integer elements and the
accumulation of the double-element product.

Note that any intermediate carry arising from accumulating the lower
half of the double-element product may be added to the upper half of
the double-element product without risk of overflow, since the result
of multiplying two n-bit integers can never have all n bits set in its
upper half.  This simplifies the carry calculations for architectures
such as RISC-V and LoongArch64 that do not have a carry flag.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 29 insertions, 5 deletions

diff --git a/src/arch/arm32/include/bits/bigint.h b/src/arch/arm32/include/bits/bigint.h
index e4b511da7..0a368a0c0 100644
--- a/src/arch/arm32/include/bits/bigint.h
+++ b/src/arch/arm32/include/bits/bigint.h
@@ -309,10 +309,34 @@ bigint_done_raw ( const uint32_t *value0, unsigned int size __unused,
 		*(--out_byte) = *(value_byte++);
 }
 
-extern void bigint_multiply_raw ( const uint32_t *multiplicand0,
-				  unsigned int multiplicand_size,
-				  const uint32_t *multiplier0,
-				  unsigned int multiplier_size,
-				  uint32_t *value0 );
+/**
+ * Multiply big integer elements
+ *
+ * @v multiplicand	Multiplicand element
+ * @v multiplier	Multiplier element
+ * @v result		Result element pair
+ * @v carry		Carry element
+ */
+static inline __attribute__ (( always_inline )) void
+bigint_multiply_one ( const uint32_t multiplicand, const uint32_t multiplier,
+		      uint32_t *result, uint32_t *carry ) {
+	uint32_t discard_low;
+	uint32_t discard_high;
+
+	__asm__ __volatile__ ( /* Perform multiplication */
+			       "umull %0, %1, %5, %6\n\t"
+			       /* Accumulate result */
+			       "adds %2, %0\n\t"
+			       "adcs %3, %1\n\t"
+			       /* Accumulate carry (cannot overflow) */
+			       "adc %4, #0\n\t"
+			       : "=r" ( discard_low ),
+				 "=r" ( discard_high ),
+				 "+r" ( result[0] ),
+				 "+r" ( result[1] ),
+				 "+r" ( *carry )
+			       : "r" ( multiplicand ),
+				 "r" ( multiplier ) );
+}
 
 #endif /* _BITS_BIGINT_H */