node.js - HKDF function in Python and NodeJS give different results, why? - Stack Overflow

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While working on implementing encryption/decryption between a Python backend server and a NodeJS frontend, my decryption attempts on the frontend were failing. I noticed that HKDF result generated by cryptography library in Python and by crypto in NodeJS don't output the same results.

Here's the code I used to test this:

Python code (with output):

import base64
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.kdf.hkdf import HKDF

input = b'helloworld'

hkdf = HKDF(
    algorithm=hashes.SHA256(),
    length=48,  # 32 bytes for key + 16 bytes for IV
    salt=b'\x00' * 32,  # Explicit salt
    info=b'',
)
output = hkdf.derive(input)

print(base64.b16encode(output))
# Output: b'E76D8FF8CE3E6FBFA6EBDD3BCE19766940316D2973503BB7B174C3F667EDE0AA65C9A74686D38E5B3FF8411A6E8354A8'

NodeJS code (with output):

import crypto from 'crypto';

const input = Buffer.from("helloworld");
const salt = Buffer.alloc(32, 0); // Explicit salt

const output = crypto.hkdfSync('sha256', salt, input, Buffer.alloc(0), 48);

console.log(Buffer.from(output).toString("hex"));
// Output: 10523f4571d67851f2e3549a6071cef99db6cc88619a30e7d0419b38054ef63873409a0dbf4e5f4e66b693af44c3e393

Why? What am I missing?

While working on implementing encryption/decryption between a Python backend server and a NodeJS frontend, my decryption attempts on the frontend were failing. I noticed that HKDF result generated by cryptography library in Python and by crypto in NodeJS don't output the same results.

Here's the code I used to test this:

Python code (with output):

import base64
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.kdf.hkdf import HKDF

input = b'helloworld'

hkdf = HKDF(
    algorithm=hashes.SHA256(),
    length=48,  # 32 bytes for key + 16 bytes for IV
    salt=b'\x00' * 32,  # Explicit salt
    info=b'',
)
output = hkdf.derive(input)

print(base64.b16encode(output))
# Output: b'E76D8FF8CE3E6FBFA6EBDD3BCE19766940316D2973503BB7B174C3F667EDE0AA65C9A74686D38E5B3FF8411A6E8354A8'

NodeJS code (with output):

import crypto from 'crypto';

const input = Buffer.from("helloworld");
const salt = Buffer.alloc(32, 0); // Explicit salt

const output = crypto.hkdfSync('sha256', salt, input, Buffer.alloc(0), 48);

console.log(Buffer.from(output).toString("hex"));
// Output: 10523f4571d67851f2e3549a6071cef99db6cc88619a30e7d0419b38054ef63873409a0dbf4e5f4e66b693af44c3e393

Why? What am I missing?

• python
• node.js
• cryptography
• hkdf

Share Improve this question Follow asked Dec 3, 2024 at 11:24 ZeroByterZeroByter 37422 gold badges99 silver badges2222 bronze badges 1

• 2 In the NodeJS code, the arguments ikm and salt are swapped in the hkdfSync() call. – Topaco Commented Dec 3, 2024 at 11:52

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1 Answer 1

Sorted by: Reset to default 0

Turns out I accidentally swapped the ikm and salt arguments in the NodeJS code, here is the corrected code:

import crypto from 'crypto';

const input = Buffer.from("helloworld");
const salt = Buffer.alloc(32, 0); // Explicit salt

const output = crypto.hkdfSync('sha256', input, salt, Buffer.alloc(0), 48); // Correct use of `input` as IKM

console.log(Buffer.from(output).toString("hex"));

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