Per RFC 7049 2.1 Major Types, CBOR supports the following data types:

• Major type 0: an unsigned integer
• Major type 1: a negative integer
• Major type 2: a byte string
• Major type 3: a text string
• Major type 4: an array of data items
• Major type 5: a map of pairs of data items
• Major type 6: optional semantic tagging of other major types
• Major type 7: floating-point numbers and simple data types that need no content, as well as the "break" stop code

By default, ByteArrays are encoded/decoded as major type 4. When major type 2 is desired, then the @ByteString annotation can be used.

For example:

@Serializable
data class Data(
    @ByteString
    val a: ByteArray, // CBOR Major type 2

    val b: ByteArray  // CBOR Major type 4
)