Footprint

Footprint

The crypto floor is the same wolfcrypt objects in wolfNano and wolfSSL, so it is not the differentiator. The TLS layer is. Run the comparison with:

sh bench/footprint.sh

TLS-layer comparison (host clang, -Os)

	__TEXT bytes	source lines
wolfNano slim shell (full TLS 1.3 PSK+ECDHE client)	8,724	1,351
wolfSSL TLS layer (tls13.c + tls.c only)	52,318	(subset)
wolfSSL tls13.c+tls.c+internal.c+ssl.c	n/a	96,433

The complete wolfNano TLS 1.3 client (handshake driver, ClientHello/ServerHello, key schedule, transcript, record protection, key share, wire codec) is roughly 6x smaller in compiled .text than just tls13.c + tls.c, and it omits internal.c and ssl.c entirely (the WOLFSSL object model), which is the bulk of the wolfSSL TLS-layer size.

Crypto floor on Cortex-M33 (cross-compiled, static)

Code size is static, so the on-target crypto floor can be measured on the host by cross-compiling for Thumb2 and running size (no device needed):

make floor-thumb2     # cross-compile the floor for Cortex-M33
arm-none-eabi-size -t build/thumb2/*.o

The default floor (all curves + ASN + both SP backends, pre---gc-sections) sums to ~250 KB .text as an upper bound. The shippable number is much smaller: roughly 40% is optional adders (X.509/ASN ~50 KB, Ed25519 group ops ~49 KB) a minimal PSK/ECDHE-P256 build does not link, and --gc-sections drops the unreferenced remainder. A minimal TLS 1.3 floor (ECDHE-P256 + AES-GCM + SHA-256 + HKDF + DRBG) lands far lower.

Whole TLS 1.3 client vs MbedTLS / wolfSSL (Cortex-M33, measured)

Whole TLS 1.3 client linked from source for Cortex-M33 (X25519, AES-128-GCM, SHA-256), arm-none-eabi-gcc -Os -flto -ffunction-sections -fdata-sections -Wl,--gc-sections + nano specs (ArmGNU 14.2), with wolfNano and mbedTLS 3.6 both hard-minimized to the identical scope. .text bytes:

Client	wolfNano	mbedTLS (hard-min)	full wolfSSL	smaller by
PSK + ECDHE, X25519	18000	42100	-	57%
PSK + ECDHE, P-256	25840	50848	-	49%
cert / X.509, P-256	62297	101232	150913	38%

The wolfNano column builds from the public configs/ starter templates (user_settings_minimal.h, user_settings_psk_p256.h, user_settings_cert.h), so the numbers reproduce from the same user_settings.h a deployment ships.

mbedTLS is given its smallest config too (MBEDTLS_ECP_FIXED_POINT_OPTIM 0, ECP_WINDOW_SIZE 2) so the comparison is not inflated in wolfNano's favor.

Both sides are hard-minimized to SHA-256 only (verified: zero SHA-384/512/3, MD5, SHA-1, DES, ChaCha, CBC/CTR, RSA, ECDSA symbols in either PSK binary). SHA-256 itself is mandatory in TLS 1.3 (HKDF key schedule, transcript hash, Finished MAC, PSK binder) and present on both. The remaining wolfNano vs mbedTLS gap is architectural: wolfNano uses specialized fe_* X25519 field arithmetic and direct wc_* calls; mbedTLS routes X25519 through its general ECP + bignum and the mandatory PSA dispatch layer, and links full AES tables. P-256's SP math is heavier than Curve25519, so both sides grow for P-256. The P-256 PSK client is interop-verified live against OpenSSL and wolfSSL.

The honest framing:

• Hard-minimized both sides (the fair number): 34% (PSK) / 40% (cert) smaller. Getting mbedTLS this small required a custom minimal PSA_WANT_* crypto config (MBEDTLS_PSA_CRYPTO_CONFIG) and stripping restartable-ECP, SHA-384/512, and the non-GCM AES modes, because mbedTLS 3.6's PSA layer pulls in RSA, SHA-1/3, Camellia, DES, ChaCha by default (~80 KB stock PSK).
• Reproduce: sh bench/footprint-clients.sh. Exact configs: bench/min/mbedtls_config_psk_hardmin.h + bench/min/mbedtls_crypto_config_psk.h (mbedTLS), configs/user_settings_*.h (wolfNano).
• Both harness clients use opaque (volatile) I/O stubs. A constant-returning stub lets LTO prove the handshake aborts after ClientHello and dead-strip the rest, understating the footprint; the opaque stub forces the whole reachable handshake to stay. Verified: making the mbedTLS bio opaque too moved its PSK number by <30 bytes, so neither side is being dead-stripped.
• Raw crypto primitives are ~parity (mbedTLS's compact bignum/ECP is its design strength); wolfNano's win is the TLS layer plus whole-stack assembly.
• Full wolfSSL with X.509 is ~147 KB, which is the reason a slim shell exists.
• Classical numbers. Enabling PQC or asm adds flash; this is the lean classical floor.

Which devices fit

Build	flash + RAM budget	device classes
PSK (17 KB)	~17 KB flash, ~8-16 KB RAM	Cortex-M0+/M3/M4 from ~32 KB flash: LoRaWAN/NB-IoT/Matter sensors, wearables (STM32L0/L4, nRF52)
cert (60 KB)	~60 KB flash, ~24-40 KB RAM	Cortex-M4/M33 from ~128 KB flash: cloud-IoT endpoints, gateways (STM32L4/U5/H5, nRF53, ESP32)

The footprint edge lands hardest on small parts: at ~17 KB the X25519 PSK client fits where even a hard-minimized mbedTLS (41 KB) cannot, and a stock mbedTLS (~80 KB) is out of the question.

Notes

• The crypto-floor and TLS-layer numbers earlier are host (x86_64) for relative comparison; the whole-client table above is on-target Cortex-M33.
• The comparison is deliberately the TLS layer only; the shared crypto floor is excluded because it is byte-identical in both.
• This measures the current shell (key schedule, transcript, record). It grows as the handshake state machine lands, but stays a small fraction of the wolfSSL TLS layer because the WOLFSSL/internal.c machinery is never pulled in.