Encoding:
    SILC_ASN1_OCTET_STRING(str, str_len)
    SILC_ASN1_OCTET_STRING_T(opts, tag, str, str_len)

    Decoding:
    SILC_ASN1_OCTET_STRING(&str, &str_len)
    SILC_ASN1_OCTET_STRING_T(opts, tag, &str, &str_len)

    Macro used to encode or decode octet string.  The string type is
    unsigned char and string length SilcUInt32.  Octet string is
    considered to be 8-bit unsigned binary data.

    The `opts' is SilcAsn1Options.  The `tag' is a tag number.