Key import and export#

psa_import_key#

psa_status_t psa_import_key (const psa_key_attributes_t * attributes, const uint8_t * data, size_t data_length, mbedtls_svc_key_id_t * key)

Import a key in binary format.

This function supports any output from psa_export_key(). Refer to the documentation of psa_export_public_key() for the format of public keys and to the documentation of psa_export_key() for the format for other key types.

The key data determines the key size. The attributes may optionally specify a key size; in this case it must match the size determined from the key data. A key size of 0 in attributes indicates that the key size is solely determined by the key data.

Implementations must reject an attempt to import a key of size 0.

This specification supports a single format for each key type. Implementations may support other formats as long as the standard format is supported. Implementations that support other formats should ensure that the formats are clearly unambiguous so as to minimize the risk that an invalid input is accidentally interpreted according to a different format.

psa_export_key#

psa_status_t psa_export_key (mbedtls_svc_key_id_t key, uint8_t * data, size_t data_size, size_t * data_length)

Export a key in binary format.

The output of this function can be passed to psa_import_key() to create an equivalent object.

If the implementation of psa_import_key() supports other formats beyond the format specified here, the output from psa_export_key() must use the representation specified here, not the original representation.

For standard key types, the output format is as follows:

• For symmetric keys (including MAC keys), the format is the raw bytes of the key.

• For DES, the key data consists of 8 bytes. The parity bits must be correct.

• For Triple-DES, the format is the concatenation of the two or three DES keys.

• For RSA key pairs (PSA_KEY_TYPE_RSA_KEY_PAIR), the format is the non-encrypted DER encoding of the representation defined by PKCS#1 (RFC 8017) as RSAPrivateKey, version 0.

  RSAPrivateKey ::= SEQUENCE {
      version             INTEGER,  -- must be 0
      modulus             INTEGER,  -- n
      publicExponent      INTEGER,  -- e
      privateExponent     INTEGER,  -- d
      prime1              INTEGER,  -- p
      prime2              INTEGER,  -- q
      exponent1           INTEGER,  -- d mod (p-1)
      exponent2           INTEGER,  -- d mod (q-1)
      coefficient         INTEGER,  -- (inverse of q) mod p
  }
  

• For elliptic curve key pairs (key types for which PSA_KEY_TYPE_IS_ECC_KEY_PAIR is true), the format is a representation of the private value as a ceiling(m/8)-byte string where m is the bit size associated with the curve, i.e. the bit size of the order of the curve's coordinate field. This byte string is in little-endian order for Montgomery curves (curve types PSA_ECC_FAMILY_CURVEXXX), and in big-endian order for Weierstrass curves (curve types PSA_ECC_FAMILY_SECTXXX, PSA_ECC_FAMILY_SECPXXX and PSA_ECC_FAMILY_BRAINPOOL_PXXX). For Weierstrass curves, this is the content of the privateKey field of the ECPrivateKey format defined by RFC 5915. For Montgomery curves, the format is defined by RFC 7748, and output is masked according to §5. For twisted Edwards curves, the private key is as defined by RFC 8032 (a 32-byte string for Edwards25519, a 57-byte string for Edwards448).

• For Diffie-Hellman key exchange key pairs (key types for which PSA_KEY_TYPE_IS_DH_KEY_PAIR is true), the format is the representation of the private key x as a big-endian byte string. The length of the byte string is the private key size in bytes (leading zeroes are not stripped).

• For public keys (key types for which PSA_KEY_TYPE_IS_PUBLIC_KEY is true), the format is the same as for psa_export_public_key().

The policy on the key must have the usage flag PSA_KEY_USAGE_EXPORT set.

psa_export_public_key#

psa_status_t psa_export_public_key (mbedtls_svc_key_id_t key, uint8_t * data, size_t data_size, size_t * data_length)

Export a public key or the public part of a key pair in binary format.

The output of this function can be passed to psa_import_key() to create an object that is equivalent to the public key.

This specification supports a single format for each key type. Implementations may support other formats as long as the standard format is supported. Implementations that support other formats should ensure that the formats are clearly unambiguous so as to minimize the risk that an invalid input is accidentally interpreted according to a different format.

For standard key types, the output format is as follows:

• For RSA public keys (PSA_KEY_TYPE_RSA_PUBLIC_KEY), the DER encoding of the representation defined by RFC 3279 §2.3.1 as RSAPublicKey.

  RSAPublicKey ::= SEQUENCE {
     modulus            INTEGER,    -- n
     publicExponent     INTEGER  }  -- e
  

• For elliptic curve keys on a twisted Edwards curve (key types for which PSA_KEY_TYPE_IS_ECC_PUBLIC_KEY is true and PSA_KEY_TYPE_ECC_GET_FAMILY returns PSA_ECC_FAMILY_TWISTED_EDWARDS), the public key is as defined by RFC 8032 (a 32-byte string for Edwards25519, a 57-byte string for Edwards448).

• For other elliptic curve public keys (key types for which PSA_KEY_TYPE_IS_ECC_PUBLIC_KEY is true), the format is the uncompressed representation defined by SEC1 §2.3.3 as the content of an ECPoint. Let m be the bit size associated with the curve, i.e. the bit size of q for a curve over F_q. The representation consists of:

  • The byte 0x04;

  • x_P as a ceiling(m/8)-byte string, big-endian;

  • y_P as a ceiling(m/8)-byte string, big-endian.

• For Diffie-Hellman key exchange public keys (key types for which PSA_KEY_TYPE_IS_DH_PUBLIC_KEY is true), the format is the representation of the public key y = g^x mod p as a big-endian byte string. The length of the byte string is the length of the base prime p in bytes.

Exporting a public key object or the public part of a key pair is always permitted, regardless of the key's usage flags.