Malicious SS7 activity and why SMS should never by used for 2FA

John Adams jna at
Sun Apr 18 19:01:56 UTC 2021

On top of this most TOTP and HOTP systems have additional security checks
like blocking reuse of codes, rate-limiting of guesses, and in some cases
acceptance of earlier codes (in TOTP) if the clock skews too far that make
them much stronger options which decreases security but is certainly more
of a convenience factor.


On Sun, Apr 18, 2021 at 6:06 AM Mel Beckman <mel at> wrote:

> As far as I know, authenticators on cell phone apps don’t require the
> Internet. For example, the Google Authenticator mobile app doesn't require
> any Internet or cellular connection. The authenticated system generates a
> secret key - a unique 16 or 32 character alphanumeric code. This key is
> scanned by GA or can be entered manually and as a result, both the
> authenticated system and GA know the same secret key, and can compute the
> time-based 2nd factor OTP just as hardware tokens do.
> There are two algorithms: HOTP and TOTP. The main difference is in OTP
> expiration time: with HOTP, the OTP is valid until it hasn’t been used;
> TOTP times out after some specified interval - usually 30 or 60 seconds.
> For TOTP, the system time must be synced, otherwise the generated OTPs will
> be wrong. But you can get accurate enough clock time without the Internet,
> either manually using some radio source such as WWV, or by GPS or cellular
> system synchronization.
>  -mel
> > On Apr 18, 2021, at 5:46 AM, Mark Tinka <mark at> wrote:
> >
> > 
> >
> >> On 4/18/21 05:18, Mel Beckman wrote:
> >>
> >> No, every SMS 2FA should be prohibited by regulatory certifications.
> The telcos had years to secure SMS. They did nothing. The plethora of
> well-secured commercial 2FA authentication tokens, many of them free,
> should be a mandatory replacement for 2FA in every security governance
> regime, such as PCI, financial account access, government web portals, etc.
> >
> > While I agree that SMS is insecure at the moment, I think there still
> needs to be a mechanism that does not rely on the presence of an Internet
> connection. One may not be able to have access to the Internet for a number
> of reasons (traveling, coverage, outage, device, money, e.t.c.), and a
> fallback needs to be available to authenticate.
> >
> > I know some companies have been pushing for voice authentication for
> their services through a phone call, in lieu of SMS or DTMF-based PIN's.
> >
> > We need something that works at the lowest common denominator as well,
> because as available as the Internet is worldwide, it's not yet at a level
> that one would consider "basic access".
> >
> > Mark.
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