I'm not taking about general collision attack on weak hash function such as MD5 or SHA-1. I'm wondering security of HMAC which is based on weak hash function
If there are key
Hmac(k,m) = hash(k`xor opad) || hash( (k` xor ipad) || m ) )
First Question, What does it mean that Hmac is secure?
Attacker cannot generates valid Hmac of given message m without knowing key
Attacker cannot finds collision that satisfy
Hmac(k',m') = Hmac(k,m) such k',m'
Attacker can recover key
And Second Question.
I'm reading about paper Transcript Collision
Transcript Collision Attacks: Breaking Authentication in TLS,IKE, and SSH
In this paper, They introduce Transcript collision
m2. And can find collision that satisfy
hash(m1 || m2' ) == hash(m1' || m2 ) such m1' and m2'
If adversary uses this Transcript Collision Attacks in weak hash function,
Can he also finds collision that satisfy
Hmac(k',m') = Hmac(k,m) such k',m'??