🐛 [BUG] - Conformance - sa_setARSNW Does Not Enforce the SA's Authentication-Capability Precondition

Description

Bug Report: sa_setARSNW Does Not Enforce the SA's Authentication-Capability Precondition

Summary

Field	Value
Product	NASA CryptoLib (SDLS Protocol Implementation)
Version	1.4.2 & dev branch - the details are specific to v1.4.2
Component	src/sa/internal/sa_interface_inmemory.template.c — sa_setARSNW
Issue	sa_setARSNW does not evaluate whether the target SA is authentication-capable before writing its anti-replay window field
Bug Type	Conformance
Severity	Medium
Impact	The anti-replay sequence-number window can be set on an SA that isn't configured for authentication at all (e.g. a plaintext or encryption-only SA) — a value with no protocol meaning on that SA is silently written and reported as accepted

---

Description

An SA's est/ast service-type flags determine whether authentication is part of that SA's
security service. The anti-replay window (arsnw) only has meaning for an SA that provides
authentication — it's the tolerance window used when validating an incoming authenticated
frame's sequence number.

sa_setARSNW reads the target SPI, bounds-checks it against NUM_SA, and — if in range —
unconditionally writes sa[spi].arsnw from the wire. It never inspects sa[spi].est/ast
before doing so. Any in-bounds SPI accepts an ARSNW update regardless of whether that SA
actually uses authentication.

Affected Code

sa_setARSNW (src/sa/internal/sa_interface_inmemory.template.c:1730-1775)

static int32_t sa_setARSNW(TC_t *tc_frame)
{
    ...
    if (spi < NUM_SA)
    {
        if (sa[spi].arsnw_len > (ARSN_SIZE)) { sa[spi].arsnw_len = ARSN_SIZE; }
        sa[spi].arsnw = (((uint8_t)sdls_frame.tlv_pdu.data[2]));   // no est/ast check anywhere
    }
    else { ... }

    return CRYPTO_LIB_SUCCESS;
}

There is no est/ast (or any other service-type) condition anywhere in this function's body —
verified by reading the function in full.

Reproduction Sequence (from the fuzzing logs)

Two-frame sequence: create a non-authentication-capable SA, then set its ARSNW.

#	Direction	Hex	Meaning
1	ground → flight	002c10a6000000001104d0003f0c001001020c00000000000000000000000001010080000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000002ab0	TC frame, SA Create, SPI 63, est=0/ast=0 (no auth service)
2	ground → flight	002c100e00000000150010003f9fcb	TC frame, sg=0x1, pid=0x5 (SA SetARSNW), SPI 63

Debug replay of frame 2:

SA setARSNW
spi = 63
ARSN set to: 0
Status code: 0

No diagnostic at all is printed for the auth-capability precondition — unlike its sa_setARSN
sibling (which does evaluate and log this same precondition, just doesn't propagate the
failure), sa_setARSNW never attempts the check in the first place. The oracle model correctly
rejects this exact same two-frame sequence with SetARSNW(): SA does not have authentication service, cannot set ARSNW.

Discovered using the StratoFuzz protocol fuzzing framework.

Branch Name

dev

Reproduction steps

## Reproduction Sequence (from the fuzzing logs)

Two-frame sequence: create a non-authentication-capable SA, then set its ARSNW.

| # | Direction | Hex | Meaning |
|---|-----------|-----|---------|
| 1 | ground → flight | `002c10a6000000001104d0003f0c001001020c00000000000000000000000001010080000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000002ab0` | TC frame, SA Create, SPI 63, `est=0/ast=0` (no auth service) |
| 2 | ground → flight | `002c100e00000000150010003f9fcb` | TC frame, `sg=0x1, pid=0x5` (SA SetARSNW), SPI 63 |

Debug replay of frame 2:


SA setARSNW
spi = 63
ARSN set to: 0
Status code: 0


No diagnostic at all is printed for the auth-capability precondition — unlike its `sa_setARSN`
sibling (which does evaluate and log this same precondition, just doesn't propagate the
failure), `sa_setARSNW` never attempts the check in the first place. The oracle model correctly
rejects this exact same two-frame sequence with `SetARSNW(): SA does not have authentication
service, cannot set ARSNW`.

Screenshots

Logs

OS

Linux

Start in src/sa/internal/sa_interface_inmemory.template.c, at sa_setARSNW around lines 1730-1775, and compare its checks with the sa_setARSN sibling. Replay the two-frame sequence in the issue against an SA with est=0/ast=0. Done means the non-authentication-capable SA does not accept the ARSNW update and the behavior is covered by a regression test.