3. Algorithm and key type encoding¶

These are encodings for a PAKE interface for PSA Certified Crypto API [PSA-CRYPT].

Note

These encodings will be integrated into a future version of [PSA-CRYPT].

3.1. Algorithm encoding¶

A new algorithm category is added for PAKE algorithms. The algorithm category table in [PSA-CRYPT] Appendix B is extended with the information in Table 4.

Table 4 New algorithm identifier categories¶
Algorithm category	CAT	Category details
PAKE	`0x0A`	See PAKE algorithm encoding

3.1.1. PAKE algorithm encoding¶

The algorithm identifier for PAKE algorithms defined in this specification are encoded as shown in Figure 5.

../_images/pake_encoding.svg — Figure 5 PAKE algorithm encoding¶

The defined values for PAKE-TYPE are shown in Table 5.

The permitted values of HASH-TYPE depend on the specific PAKE algorithm.

Table 5 PAKE algorithm sub-type values¶
PAKE algorithm	PAKE-TYPE	Algorithm identifier	Algorithm value
J-PAKE	`0x01`	`PSA_ALG_JPAKE(hash)`	`0x0A0001hh` ^a
SPAKE2+ with HMAC	`0x04`	`PSA_ALG_SPAKE2P_HMAC(hash)`	`0x0A0004hh` ^a
SPAKE2+ with CMAC	`0x05`	`PSA_ALG_SPAKE2P_CMAC(hash)`	`0x0A0005hh` ^a
SPAKE2+ for Matter	`0x06`	`PSA_ALG_SPAKE2P_MATTER`	`0x0A000609`

hh is the HASH-TYPE for the hash algorithm, hash, used to construct the key derivation algorithm.

3.2. Key encoding¶

A new type of asymmetric key is added for the SPAKE2+ algorithms. The Asymmetric key sub-type values table in [PSA-CRYPT] Appendix B is extended with the information in Table 6.

Table 6 New SPAKE2+ asymmetric key sub-type¶
Asymmetric key type	ASYM-TYPE	Details
SPAKE2+	4	See SPAKE2+ key encoding

Rationale

The ASYM-TYPE value 4 is selected as this has the same parity as the ECC sub-type, which have the value 1. The enables the same ECC-FAMILY and P values to be used when encoding a SPAKE2+ key type, as is used in the Elliptic Curve key types.

3.2.1. SPAKE2+ key encoding¶

The key type for SPAKE2+ keys defined in this specification are encoded as shown in Figure 6.

../_images/spake2p_key.svg — Figure 6 SPAKE2+ key encoding¶

PAIR is either 0 for a public key, or 3 for a key pair.

The defined values for ECC-FAMILY and P are shown in Table 7.

Table 7 SPAKE2+ key family values¶
SPAKE2+ group	ECC-FAMILY	P	ECC family ^a	Public key value	Key pair value
SECP R1	0x09	0	`PSA_ECC_FAMILY_SECP_R1`	`0x4412`	`0x7412`
Twisted Edwards	0x21	0	`PSA_ECC_FAMILY_TWISTED_EDWARDS`	`0x4442`	`0x7442`

The key type value is constructed from the Elliptic Curve family using either PSA_KEY_TYPE_SPAKE2P_PUBLIC_KEY(family) or PSA_KEY_TYPE_SPAKE2P_KEY_PAIR(family) as required.