Google Authenticator: Difference between revisions
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offset := last nybble of hash |
offset := last nybble of hash |
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truncatedHash := hash[offset..offset+4] <span style="color:green;">//4 bytes starting at the offset</span> |
truncatedHash := hash[offset..offset+4] <span style="color:green;">//4 bytes starting at the offset</span> |
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of truncatedHash <span style="color:green;">//remove the most significat bit </span> |
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code := truncatedHash '''mod''' 1000000 |
code := truncatedHash '''mod''' 1000000 |
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'''pad''' code with 0 until length of code is 6 |
'''pad''' code with 0 until length of code is 6 |
Revision as of 13:28, 26 January 2012
Google Authenticator is a software based two-factor authentication token developed by Google. The Authenticator provides a six digit number users must provide in addition to their username and password to log in to Google services
Implementation
Google provides iOS, Blackberry and Android versions of Authenticator. Several third party implementations are available.
- Windows Phone 7: Authenticator
- Windows Mobile: Google Authenticator for Windows Mobile
- Java CLI: Authenticator.jar
- Java GUI: JAuth
- J2ME: gauthj2me lwuitgauthj2me Mobile-OTP
- PalmOS: gauthj2me
- webOS: GAuth
- Windows: gauth4win
Technical description
Google generates a 80-bit secret for each user. This is provided as a 16 character base32 string or as a QR code. The client creates a HMAC-SHA1 using this secret key, with the message being the number of 30 second periods having elapsed since the Unix epoch. A portion of the HMAC is extracted and converted to a 6 digit code.
Pseudocode
function GoogleAuthenticatorCode(string secret)
key := base32decode(secret)
message := current Unix time ÷ 30
hash := HMAC-SHA1(key, message)
offset := last nybble of hash
truncatedHash := hash[offset..offset+4] //4 bytes starting at the offset
Set the first bit of truncatedHash to zero //remove the most significat bit
code := truncatedHash mod 1000000
pad code with 0 until length of code is 6
return code
Technology
Google Authenticator uses the time-based One-time Password Algorithm as defined in RFC 6238.