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}
}