kem-gost3410-hkdf {
{field . {map}}
{field a {str} =gost3410-hkdf}
{field cek {bin} >0} {# wrapped CEK}
{field ukm {bin} len=16} {# additional keying material}
{field pub {bin} >0} {# sender's ephemeral public key}
{field to {with fpr} optional} {# recipient's public key}
{field from {with fpr} optional} {# sender's public key}
}