I use Aegis as my 2fa. Today on new token creation I observed that there’s hash function set to SHA-1, later checked all my tokens and the result is same type of encryption used for all. So I have edited all my tokens to SHA-256 as a result my totp doesn’t authenticate. Do I have to rescan my tokens for updating to SHA-256 or it doesn’t work like that?
Security: SHA-1 < SHA-256 < SHA-512
Speed: SHA-1 > SHA-256 > SHA-512
My doubts are: Why can’t we use SHA-256? Is it because TOTP requires less time so faster one(SHA-1) is chosen? Can we use SHA-256 for TOTPs?
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First of all, let’s recap how TOTP works: your authenticator app stores “TOTP password” in an accessible way (some apps store it plaintext, Aegis stores them in an encrypted DB. They can also be stored on Yubikey.). To generate one time code, the app takes the password, concatenates it with current time rounded to 30 seconds and compute a hash from this string.
The resulting hash is used to create a 6 digit code, that you’ll enter on login.
To check the code, server repeats the same procedure and compare your result with the just generated one.
As you can see, for this system to work you and the server must use the same hash function.
Conclusion: 1)Hash function in TOTP has nothing to do with password storage, it is used only for TOTP-code generation. 2) Cryptographic security of the hash function is not much a concern here: TOTP code expose an extremely low amount of information about TOTP password, so to reliably recover the password from TOTP-codes you’ll have to intercept millions of them.
The entropy of otp codes is low compared to the seed’s. I’d never though about what that meant for reversion. Good point !