core/crypto/_subtle: Refactor out common helpers

1 files changed, 42 insertions, 0 deletions

diff --git a/core/crypto/_subtle/subtle.odin b/core/crypto/_subtle/subtle.odin
new file mode 100644
index 000000000..89328072c
--- /dev/null
+++ b/core/crypto/_subtle/subtle.odin
@@ -0,0 +1,42 @@
+/*
+Various useful bit operations in constant time.
+*/
+package _subtle
+
+import "core:math/bits"
+
+// byte_eq returns 1 iff a == b, 0 otherwise.
+@(optimization_mode="none")
+byte_eq :: proc "contextless" (a, b: byte) -> int {
+	v := a ~ b
+
+	// v == 0 iff a == b.  The subtraction will underflow, setting the
+	// sign bit, which will get returned.
+	return int((u32(v)-1) >> 31)
+}
+
+// u64_eq returns 1 iff a == b, 0 otherwise.
+@(optimization_mode="none")
+u64_eq :: proc "contextless" (a, b: u64) -> u64 {
+	_, borrow := bits.sub_u64(0, a ~ b, 0)
+	return (~borrow) & 1
+}
+
+eq :: proc {
+	byte_eq,
+	u64_eq,
+}
+
+// u64_is_zero returns 1 iff a == 0, 0 otherwise.
+@(optimization_mode="none")
+u64_is_zero :: proc "contextless" (a: u64) -> u64 {
+	_, borrow := bits.sub_u64(a, 1, 0)
+	return borrow
+}
+
+// u64_is_non_zero returns 1 iff a != 0, 0 otherwise.
+@(optimization_mode="none")
+u64_is_non_zero :: proc "contextless" (a: u64) -> u64 {
+	is_zero := u64_is_zero(a)
+	return (~is_zero) & 1
+}