PsaExportPublicKey

Export a public key or the public part of a key pair in binary format. Opcode: 7 (0x0007)

Parameters

Name	Type	Description
`key_name`	String	Name of the public key to export

Name	Type	Description
`data`	Vector of bytes	Buffer containing the key data

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

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

For RSA public keys, with key type RsaPublicKey, the DER encoding of the representation defined by Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certifiate and Certificate Revocation List (CRL) Profile RFC 3279 §2.3.1 as RSAPublicKey ([1]).
For elliptic curve public keys, with key type EccPublicKey, the format depends on the curve family:
- For Weierstrass curve families sectXX, secpXX, FRP and Brainpool, the uncompressed representation of an elliptic curve point as an octet string defined in SEC 1: Elliptic Curve Cryptography SEC1 §2.3.3. If m is 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 Montgomery curve family, the scalar value of the ‘public key’ in little-endian order as defined by Elliptic Curves for Security RFC 7748 §6. This is a ceiling(m/8)-byte string where m is the key size in bits.
  - This is 32 bytes for Curve25519, computed as X25519(private_key, 9).
  - This is 56 bytes for Curve448, computed as X448(private_key, 5).
For Diffie-Hellman key exchange public keys, with key types DhPublicKey, 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.

[1]: The RSAPublicKey representation is:

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