avoid undefined behavior when shifting into sign bit

Shifting a signed int64_t into the sign bit is undefined behavior in C.

  /dev/portable/crypto/curve25519/curve25519.c:3900:18: runtime error:
  left shift of negative value -222076011
  SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /dev/portable

To avoid this, import int64_lshift21() from BoringSSL, a helper function
that casts the input to uint64_t before shifting and back to int64_t afterward.
This ensures defined behavior when shifting left by 21 bits, avoiding
undefined behavior in expressions like `carry << 21`.

This change addresses potential runtime issues detected by sanitizers
when shifting signed values with high bits set.

ok tb beck

0 files changed, 0 insertions, 0 deletions