SessionKeyGenerator

internal final class SessionKeyGenerator

A generator for session keys using specified key derivation mechanisms.

The SessionKeyGenerator class is responsible for deriving session keys based on defined key derivation mechanisms. These session keys are used for various purposes, such as encryption, message authentication and security protocols.

Key Derivation Function

The key derivation function KDF(K, c) takes the shared secret value K and a 32-bit, big-endian integer c as inputs. It produces an octet string keydata as output, which is computed as keydata = H(K || c). The hash function H() used in the key derivation must have a bit-length greater than or equal to the derived key’s bit-length.

Note

The hash value is interpreted as a big-endian byte output.

Using 3DES

To derive 128-bit (112-bit excluding parity bits) 3DES keys, the SHA-1 hash function is used, and the following additional steps must be performed:

  • Use octets 1 to 8 of keydata to form keydataA and octets 9 to 16 to form keydataB. Additional octets are not used.
  • Optionally, adjust the parity bits of keydataA and keydataB to form correct DES keys.

Using AES

To derive 128-bit AES keys, the SHA-1 hash function is used, and the following additional step must be performed:

  • Use octets 1 to 16 of keydata. Additional octets are not used.

For 192-bit and 256-bit AES keys, SHA-256 is used as the hash function.

For 192-bit AES keys, the following additional step must be performed:

  • Use octets 1 to 24 of keydata; additional octets are not used.

KeyDerivationFunctionMode

The SessionKeyGenerator includes an enum called KeyDerivationFunctionMode, which defines different modes for deriving session keys. Each mode corresponds to a specific use case:

  • ENC_MODE: For deriving session keys used in encryption.
  • MAC_MODE: For deriving session keys used in message authentication.

  • PACE_MODE: For deriving session keys used in the PACE protocol.

See also

NFCSecureSession, SecurityConfiguration SecureChannel and SecureMessaging

  • Declaration

    Swift

    internal init(securityConfig: SecurityConfiguration)

  • Derives a session key based on the provided key seed, nonce, and key derivation function mode.

    Throws

    An error if key derivation fails.

    Declaration

    Swift

    internal func deriveKey(keySeed: [UInt8], nonce: [UInt8]? = nil, mode: KeyDerivationFunctionMode) throws -> [UInt8]

    Parameters

    keySeed

    The key seed for key derivation.
    nonce

    An optional nonce value (default is nil).
    mode

    Return Value

    The derived session key.

  • This enum contains all the different modes using to derive session keys.

    The KDF input requires a 32-bit, big-endian integer counter c, and its value is different according to the session key uses. Here, the following uses/modes are defined:

    • ENC_MODE: To derive a session key for encryption.
    • MAC_MODE: To derive a session key for authentication of data.
    • PACE_MODE: To derive a session key for the PACE protocol.
    See more

    Declaration

    Swift

    internal enum KeyDerivationFunctionMode : UInt8