HMAC Implementation - Problem

Implement HMAC (Hash-based Message Authentication Code) using a custom hash function. HMAC is a cryptographic authentication technique that combines a secret key with a message to produce a hash-based authentication code.

Your task is to:

Implement a simple custom hash function
Use this hash function to compute HMAC according to the standard algorithm
Return the final HMAC value as a hexadecimal string

The HMAC algorithm works as follows:

If the key is longer than the hash block size, hash it to reduce its size
If the key is shorter than the block size, pad it with zeros
Create inner and outer padding by XORing the key with specific constants
Compute the inner hash: hash(inner_pad + message)
Compute the outer hash: hash(outer_pad + inner_hash)

Custom Hash Function: Use a simple polynomial rolling hash with prime number 31 and modulo 2^32.

Constants:

Block size: 64 bytes
Inner pad byte: 0x36
Outer pad byte: 0x5C

Input & Output

Example 1 — Basic HMAC

$ Input: key = "secret", message = "hello"

› Output: "8b9a4c7d"

💡 Note: Using key 'secret' and message 'hello', the HMAC algorithm creates inner and outer padded keys, computes two hash rounds, and returns the final authentication code

Example 2 — Short Key

$ Input: key = "key", message = "data"

› Output: "f2e1d8c5"

💡 Note: Short key 'key' gets padded with zeros to 64 bytes, then HMAC process generates the authentication code for message 'data'

Example 3 — Long Key

$ Input: key = "verylongkeythatexceedstheblocksizeandneedstobehashedfirst", message = "test"

› Output: "a7c9b1e4"

💡 Note: Long key gets hashed first to reduce its size, then padded and processed through standard HMAC algorithm

Constraints

1 ≤ key.length ≤ 1000
1 ≤ message.length ≤ 1000
Key and message contain only ASCII characters

Visualization

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Asked in

G Google 25 a Amazon 18 M Microsoft 22 f Meta 15

The key insight is implementing proper HMAC using double hashing with inner and outer padding. The optimal approach follows RFC standards: prepare the key (hash if too long, pad if too short), create inner/outer padded keys using XOR with constants 0x36/0x5C, then compute nested hashes. Time: O(n), Space: O(1)

Common Approaches

✓ Direct Hash Concatenation

⏱️ Time: O(n) Space: O(1)

Concatenate the key and message directly, then apply the custom hash function. This is the most naive approach to creating a message authentication code.

Standard HMAC Implementation

⏱️ Time: O(n) Space: O(1)

Follow the official HMAC algorithm using inner and outer padding with XOR operations. This provides cryptographic security and resistance to various attacks.

Direct Hash Concatenation — Algorithm Steps

Concatenate key + message
Apply custom hash function
Return hexadecimal result

Visualization

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Step-by-Step Walkthrough

Concatenate

Combine key + message into single string

Hash

Apply polynomial rolling hash function

Result

Convert to hexadecimal format

Code -

solution.c — C

#include <stdio.h>
#include <string.h>
#include <stdint.h>

uint32_t customHash(const char* data) {
    uint32_t hashVal = 0;
    for (int i = 0; data[i]; i++) {
        hashVal = (hashVal * 31 + (unsigned char)data[i]) % (1UL << 32);
    }
    return hashVal;
}

void solution(char* key, char* message, char* result) {
    char combined[1000];
    strcpy(combined, key);
    strcat(combined, message);
    uint32_t hashResult = customHash(combined);
    sprintf(result, "%08x", hashResult);
}

int main() {
    char key[500], message[500], result[20];
    fgets(key, sizeof(key), stdin);
    key[strcspn(key, "\n")] = 0;
    fgets(message, sizeof(message), stdin);
    message[strcspn(message, "\n")] = 0;
    
    solution(key, message, result);
    printf("%s\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single pass through the combined key and message

✓ Linear Growth

Space Complexity

O(1)

Only stores hash state and result

✓ Linear Space

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HMAC Implementation - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Direct Hash Concatenation — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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