SoftHSM v2 lets you restrict or expand the set of PKCS#11 mechanisms that are advertised by C_GetMechanismList and accepted by C_GetMechanismInfo. This is controlled by the slots.mechanisms option in softhsm2.conf.
Configuration option
# Enable all mechanisms (default)
slots.mechanisms = ALL
# Enable only specific mechanisms
slots.mechanisms = CKM_RSA_PKCS,CKM_RSA_PKCS_KEY_PAIR_GEN,CKM_SHA256_RSA_PKCS
# Disable specific mechanisms, keep all others
slots.mechanisms = -CKM_DES_KEY_GEN,CKM_DES_ECB,CKM_DES_CBC
A comma-separated list of PKCS#11 mechanism names. Controls which mechanisms are reported by C_GetMechanismList and permitted for use.Syntax:| Value | Effect |
|---|
ALL | All supported mechanisms are enabled (default) |
CKM_FOO,CKM_BAR,... | Only the listed mechanisms are enabled (allowlist) |
-CKM_FOO,CKM_BAR,... | All mechanisms except the listed ones are enabled (denylist) |
ALLUnknown mechanism names are silently ignored. This option applies to all slots. slots.mechanisms takes precedence over the CKA_ALLOWED_MECHANISMS attribute set on individual key objects.
Allowlist vs. denylist
Allowlist — specify exactly which mechanisms are available:
# Only RSA operations
slots.mechanisms = CKM_RSA_PKCS_KEY_PAIR_GEN,CKM_RSA_PKCS,CKM_RSA_X_509,CKM_RSA_PKCS_OAEP,CKM_SHA256_RSA_PKCS
-):
# Disable legacy DES, keep everything else
slots.mechanisms = -CKM_DES_KEY_GEN,CKM_DES2_KEY_GEN,CKM_DES3_KEY_GEN,CKM_DES_ECB,CKM_DES_CBC
When hardening a deployment, an explicit allowlist is safer than a denylist because newly added mechanisms in future versions are excluded automatically.
Supported mechanisms
The following mechanism names are recognised by SoftHSM v2. Names not in this list are silently ignored.
Digests
| Mechanism name | Description |
|---|
CKM_MD5 | MD5 digest |
CKM_SHA_1 | SHA-1 digest |
CKM_SHA224 | SHA-224 digest |
CKM_SHA256 | SHA-256 digest |
CKM_SHA384 | SHA-384 digest |
CKM_SHA512 | SHA-512 digest |
HMAC
| Mechanism name | Description |
|---|
CKM_MD5_HMAC | HMAC with MD5 |
CKM_SHA_1_HMAC | HMAC with SHA-1 |
CKM_SHA224_HMAC | HMAC with SHA-224 |
CKM_SHA256_HMAC | HMAC with SHA-256 |
CKM_SHA384_HMAC | HMAC with SHA-384 |
CKM_SHA512_HMAC | HMAC with SHA-512 |
RSA
| Mechanism name | Description |
|---|
CKM_RSA_PKCS_KEY_PAIR_GEN | RSA key pair generation |
CKM_RSA_PKCS | RSA PKCS#1 v1.5 sign/encrypt |
CKM_RSA_X_509 | Raw RSA (no padding) |
CKM_MD5_RSA_PKCS | RSA PKCS#1 v1.5 with MD5 |
CKM_SHA1_RSA_PKCS | RSA PKCS#1 v1.5 with SHA-1 |
CKM_SHA224_RSA_PKCS | RSA PKCS#1 v1.5 with SHA-224 |
CKM_SHA256_RSA_PKCS | RSA PKCS#1 v1.5 with SHA-256 |
CKM_SHA384_RSA_PKCS | RSA PKCS#1 v1.5 with SHA-384 |
CKM_SHA512_RSA_PKCS | RSA PKCS#1 v1.5 with SHA-512 |
CKM_RSA_PKCS_OAEP | RSA OAEP encryption |
CKM_RSA_AES_KEY_WRAP | RSA-AES key wrap |
CKM_RSA_PKCS_PSS | RSA PSS (raw, no hash) |
CKM_SHA1_RSA_PKCS_PSS | RSA PSS with SHA-1 |
CKM_SHA224_RSA_PKCS_PSS | RSA PSS with SHA-224 |
CKM_SHA256_RSA_PKCS_PSS | RSA PSS with SHA-256 |
CKM_SHA384_RSA_PKCS_PSS | RSA PSS with SHA-384 |
CKM_SHA512_RSA_PKCS_PSS | RSA PSS with SHA-512 |
Generic secret
| Mechanism name | Description |
|---|
CKM_GENERIC_SECRET_KEY_GEN | Generic secret key generation |
DES
| Mechanism name | Description |
|---|
CKM_DES_KEY_GEN | DES key generation |
CKM_DES2_KEY_GEN | DES2 (2-key Triple-DES) key generation |
CKM_DES3_KEY_GEN | DES3 (3-key Triple-DES) key generation |
CKM_DES_ECB | DES ECB |
CKM_DES_CBC | DES CBC |
CKM_DES_CBC_PAD | DES CBC with PKCS padding |
CKM_DES_ECB_ENCRYPT_DATA | DES ECB encrypt data (key derivation) |
CKM_DES_CBC_ENCRYPT_DATA | DES CBC encrypt data (key derivation) |
CKM_DES3_ECB | Triple-DES ECB |
CKM_DES3_CBC | Triple-DES CBC |
CKM_DES3_CBC_PAD | Triple-DES CBC with PKCS padding |
CKM_DES3_ECB_ENCRYPT_DATA | Triple-DES ECB encrypt data (key derivation) |
CKM_DES3_CBC_ENCRYPT_DATA | Triple-DES CBC encrypt data (key derivation) |
CKM_DES3_CMAC | Triple-DES CMAC |
DES and Triple-DES are considered legacy algorithms. Consider disabling all CKM_DES* mechanisms unless they are required for compatibility with existing tokens or applications.
AES
| Mechanism name | Description |
|---|
CKM_AES_KEY_GEN | AES key generation |
CKM_AES_ECB | AES ECB |
CKM_AES_CBC | AES CBC |
CKM_AES_CBC_PAD | AES CBC with PKCS padding |
CKM_AES_CTR | AES CTR |
CKM_AES_GCM | AES GCM |
CKM_AES_KEY_WRAP | AES key wrap (RFC 3394) |
CKM_AES_KEY_WRAP_PAD | AES key wrap with padding (RFC 5649) |
CKM_AES_ECB_ENCRYPT_DATA | AES ECB encrypt data (key derivation) |
CKM_AES_CBC_ENCRYPT_DATA | AES CBC encrypt data (key derivation) |
CKM_AES_CMAC | AES CMAC |
DSA
| Mechanism name | Description |
|---|
CKM_DSA_PARAMETER_GEN | DSA domain parameter generation |
CKM_DSA_KEY_PAIR_GEN | DSA key pair generation |
CKM_DSA | DSA (raw, no hash) |
CKM_DSA_SHA1 | DSA with SHA-1 |
CKM_DSA_SHA224 | DSA with SHA-224 |
CKM_DSA_SHA256 | DSA with SHA-256 |
CKM_DSA_SHA384 | DSA with SHA-384 |
CKM_DSA_SHA512 | DSA with SHA-512 |
Diffie-Hellman
| Mechanism name | Description |
|---|
CKM_DH_PKCS_KEY_PAIR_GEN | DH key pair generation |
CKM_DH_PKCS_PARAMETER_GEN | DH domain parameter generation |
CKM_DH_PKCS_DERIVE | DH key derivation |
EC / ECDSA / ECDH
| Mechanism name | Description |
|---|
CKM_EC_KEY_PAIR_GEN | EC key pair generation |
CKM_ECDSA | ECDSA (raw, no hash) |
CKM_ECDSA_SHA1 | ECDSA with SHA-1 |
CKM_ECDSA_SHA224 | ECDSA with SHA-224 |
CKM_ECDSA_SHA256 | ECDSA with SHA-256 |
CKM_ECDSA_SHA384 | ECDSA with SHA-384 |
CKM_ECDSA_SHA512 | ECDSA with SHA-512 |
CKM_ECDH1_DERIVE | ECDH key derivation |
EdDSA
| Mechanism name | Description |
|---|
CKM_EC_EDWARDS_KEY_PAIR_GEN | Edwards curve key pair generation (Ed25519/Ed448) |
CKM_EDDSA | EdDSA signature |
GOST R 34
| Mechanism name | Description |
|---|
CKM_GOSTR3411 | GOST R 34.11 digest |
CKM_GOSTR3411_HMAC | GOST R 34.11 HMAC |
CKM_GOSTR3410_KEY_PAIR_GEN | GOST R 34.10 key pair generation |
CKM_GOSTR3410 | GOST R 34.10 signature |
CKM_GOSTR3410_WITH_GOSTR3411 | GOST R 34.10 signature with GOST digest |
ML-DSA (CRYSTALS-Dilithium)
| Mechanism name | Description |
|---|
CKM_ML_DSA_KEY_PAIR_GEN | ML-DSA key pair generation |
CKM_ML_DSA | ML-DSA signature |
Key derivation utilities
| Mechanism name | Description |
|---|
CKM_CONCATENATE_DATA_AND_BASE | Concatenate data with base key |
CKM_CONCATENATE_BASE_AND_DATA | Concatenate base key with data |
CKM_CONCATENATE_BASE_AND_KEY | Concatenate two keys |
Examples
Allowlist: RSA and AES only
slots.mechanisms = CKM_RSA_PKCS_KEY_PAIR_GEN,CKM_RSA_PKCS,CKM_RSA_PKCS_OAEP,\
CKM_SHA256_RSA_PKCS,CKM_SHA384_RSA_PKCS,CKM_SHA512_RSA_PKCS,\
CKM_SHA256_RSA_PKCS_PSS,CKM_SHA384_RSA_PKCS_PSS,CKM_SHA512_RSA_PKCS_PSS,\
CKM_AES_KEY_GEN,CKM_AES_CBC,CKM_AES_CBC_PAD,CKM_AES_GCM,CKM_AES_KEY_WRAP
Denylist: disable all legacy DES mechanisms
slots.mechanisms = -CKM_DES_KEY_GEN,CKM_DES2_KEY_GEN,CKM_DES3_KEY_GEN,\
CKM_DES_ECB,CKM_DES_CBC,CKM_DES_CBC_PAD,\
CKM_DES_ECB_ENCRYPT_DATA,CKM_DES_CBC_ENCRYPT_DATA,\
CKM_DES3_ECB,CKM_DES3_CBC,CKM_DES3_CBC_PAD,\
CKM_DES3_ECB_ENCRYPT_DATA,CKM_DES3_CBC_ENCRYPT_DATA,CKM_DES3_CMAC
Line continuation with \ is not supported by the SoftHSM config parser. The examples above use it for readability only — in a real configuration file, write all mechanism names on a single line.