What is the need of xor-ing the key with an outer and inner pad in HMAC?

Abhisek Dash

2/9/23, 9:54 AM

The diagram above shows the standard HMAC algorithm. However consider the algorithm of HMAC without xoring the key with outer or inner pad. In other words let the structure of HMAC remain the same without xoring the key with outer and inner pad. Let the key go in as input alone at both points. Is it possible to commit existential forgery with this modified architecture ?

I understand that the standard HMAC architecture prevents an attacker from computing MACs for extended messages (i.e. message blocks appended to a message for which MAC is known). But in my view this can also be prevented without xoring with inner and outer pad. Please suggest.

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kelalaka

2/9/23, 10:21 AM

I suggest a little [search](https://crypto.stackexchange.com/search?q=HMAC+ipad+) 1) [What do the magic numbers 0x5c and 0x36 in the opad/ipad calc in HMAC do?](https://crypto.stackexchange.com/a/3006/18298) 2) [HMAC ipad and opad choice](https://crypto.stackexchange.com/q/20695/18298) 3) [Keys in HMAC and NMAC](https://crypto.stackexchange.com/q/27051/18298)

Mikero

2/9/23, 3:41 PM

Does this answer your question? [What do the magic numbers 0x5c and 0x36 in the opad/ipad calc in HMAC do?](https://crypto.stackexchange.com/questions/3005/what-do-the-magic-numbers-0x5c-and-0x36-in-the-opad-ipad-calc-in-hmac-do)

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: What is the need of xor-ing the key with an outer and inner pad in HMAC?

TH: ความจำเป็นในการ xor-ing คีย์ด้วยแผ่นรองด้านนอกและด้านในใน HMAC คืออะไร

RO: Care este nevoie de xor-ing cheia cu un pad exterior și interior în HMAC?

RU: Какая необходимость в XOR-соединении ключа с внешней и внутренней площадкой в HMAC?

VI: Nhu cầu xor-ing chìa khóa với một miếng đệm bên ngoài và bên trong trong HMAC là gì?

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