exp-tai {{field . {tai} prec=s utc}}
expiration {{field . {list} {of exp-tai} len=2}}

pub-sig-tbs {
    {field . {map}}
    {field sid {with fpr}}
    {field cid {hexlet}}
    {field exp {with expiration}}
    {field nonce {bin} >0 optional}
    {field when {tai} utc prec=ms optional}
}