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Stay compatible with the authenticator apps #22

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Merged
merged 1 commit into from
Feb 15, 2025
Merged

Stay compatible with the authenticator apps #22

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33 changes: 23 additions & 10 deletions index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -55,17 +55,30 @@ async function generateHOTP(
['sign']
)
const signature = await crypto.subtle.sign('HMAC', key, byteCounter)
const hashBytes = new Uint8Array(signature)
const hashBytes = new Uint8Array(signature)
// offset is always the last 4 bits of the signature; 0-15
const offset = hashBytes[hashBytes.length-1] & 0xf

// Use more bytes for longer OTPs
const bytesNeeded = Math.ceil((digits * Math.log2(charSet.length)) / 8)
const offset = hashBytes[hashBytes.length - 1] & 0xf

// Convert bytes to BigInt for larger numbers
let hotpVal = 0n
for (let i = 0; i < Math.min(bytesNeeded, hashBytes.length - offset); i++) {
hotpVal = (hotpVal << 8n) | BigInt(hashBytes[offset + i])
}
let hotpVal = 0n
if (digits === 6) {
// stay compatible with the authenticator apps and only use the bottom 32 bits of BigInt
hotpVal = 0n |
BigInt((hashBytes[offset] & 0x7f)) << 24n |
BigInt((hashBytes[offset + 1])) << 16n |
BigInt((hashBytes[offset + 2])) << 8n |
BigInt(hashBytes[offset + 3])
} else {
// otherwise create a 64bit value from the hashBytes
hotpVal = 0n |
BigInt((hashBytes[offset] & 0x7f)) << 56n |
BigInt((hashBytes[offset + 1])) << 48n |
BigInt((hashBytes[offset + 2])) << 40n |
BigInt((hashBytes[offset + 3])) << 32n |
BigInt((hashBytes[offset + 4])) << 24n |
BigInt((hashBytes[offset + 5])) << 16n |
BigInt((hashBytes[offset + 6])) << 8n |
BigInt(hashBytes[offset + 7])
}

let hotp = ''
const charSetLength = BigInt(charSet.length)
Expand Down