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core/crypto/chacha20: Explicitly allow the one final block

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Only affects the incremental use case where the internal counter is
exactly the maximum.
This commit is contained in:
Yawning Angel
2026-08-05 18:04:06 +09:00
parent 3bdb32949b
commit 97b38169de
1 changed files with 9 additions and 3 deletions
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12
core/crypto/_chacha20/chacha20.odin
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@@ -1,5 +1,6 @@
package _chacha20
import "base:intrinsics"
import "core:crypto"
import "core:encoding/endian"
import "core:math/bits"
@@ -108,11 +109,16 @@ check_counter_limit :: proc(ctx: ^Context, nr_blocks: int) {
ctr_ok: bool
if ctx._is_ietf_flavor {
ctr_ok = u64(ctx._s[12]) + u64(nr_blocks) <= MAX_CTR_IETF
if intrinsics.unlikely(ctx._s[12] == MAX_CTR_IETF && nr_blocks > 1) {
// Allow the final block.
ctr_ok = false
} else {
ctr_ok = u64(ctx._s[12]) + u64(nr_blocks) <= MAX_CTR_IETF
}
} else {
ctr := (u64(ctx._s[13]) << 32) | u64(ctx._s[12])
_, carry := bits.add_u64(ctr, u64(nr_blocks), 0)
ctr_ok = carry == 0
new_ctr, carry := bits.add_u64(ctr, u64(nr_blocks), 0)
ctr_ok = carry == 0 || new_ctr != 0 // Allow the final block.
}
ensure(ctr_ok, "crypto/chacha20: maximum (X)ChaCha20 keystream per IV reached")