SM4 vs AES: Parameter Differences Compared

Two 128-bit block ciphers

SM4 and AES both operate on 128-bit blocks. Past that, they diverge. The confusion usually comes from assuming “block cipher” means “interchangeable” — it does not. The parameters below change how you implement and validate each one.

Side-by-side parameters

Parameter	SM4	AES
Block size	128 bits	128 bits
Key size	128 bits only	128 / 192 / 256 bits
Rounds	32 (unbalanced Feistel)	10 / 12 / 14 (SPN)
Structure	Feistel network	Substitution–permutation network
Standard	GM/T 0002 (China)	FIPS 197 (NIST)
S-box	fixed, 8-bit	fixed, 8-bit (algebraic)
Typical modes	ECB, CBC, CTR, GCM, CCM	ECB, CBC, CTR, GCM, CCM, XTS

Key size is the first fork

AES lets you pick 128, 192, or 256-bit keys. SM4 does not — it is always 128 bits. If a spec or auditor demands AES-256, SM4 cannot satisfy it, and you must use AES.

AES-128  -> 10 rounds, 128-bit key
AES-192  -> 12 rounds, 192-bit key
AES-256  -> 14 rounds, 256-bit key
SM4      -> 32 rounds, 128-bit key  (fixed)

Round structure

AES is a substitution–permutation network: layers of byte substitution, row shifting, column mixing, and key addition. SM4 is an unbalanced Feistel network with 32 rounds and a single non-linear substitution per round. Both are well studied; the practical difference is implementation detail, not security posture.

Mode and padding still matter

Neither cipher defines padding — that comes from the mode. ECB needs padding (don’t use ECB), CBC needs PKCS#7 padding and a random IV, and GCM is an AEAD mode that also gives you integrity, not just confidentiality.

// conceptual: same mode, different key length
aes.encrypt(plaintext, aes256Key, { mode: 'gcm', iv });
sm4.encrypt(plaintext, sm4Key128, { mode: 'gcm', iv });

When to use which

• Use AES when you need 192/256-bit keys, broadest library support, or FIPS/NIST alignment.
• Use SM4 when a Chinese national standard (GM/T 0002) is required, or the other end only speaks SM4.

Try both

Encrypt and decrypt with each to see the parameters side by side:

• SM4 Encrypt / Decrypt on Toolbox
• AES Encrypt / Decrypt on Toolbox

Related tools

• SM4 Encrypt / Decrypt on Toolbox
• AES Encrypt / Decrypt on Toolbox
• AES Encrypt / Decrypt on Toolhub

Next steps

• Password hashing is a different problem from encryption — see bcrypt vs scrypt vs Argon2.
• The text you feed a cipher is often Base64 — see Base64 Images in CSS.