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}
}
