Future - Advice on reset link entropy
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Account recovery is an important aspect of identity management. There are many situations where a user should be provided with a link or one-time-password/code in order to access a credential reset mechanism.
Since this reset mechanism is effectively a bearer token which depending on implementation may need to be human-typeable, it may be possible to brute force it during the time window that the recovery link is available.
As a result, it would be good to see advice on the amount of entropy required for such a link to reduce the likelihood of it being guessed.
Account recovery isn't called out separately because it is just an alternative way to authenticate. As such, account recovery techniques fall under the general authentication guidelines.
Certain authenticators, particularly look-up secrets, are particularly well suited to account recovery situations and have specified entropy and verifier storage requirements. If the user is provided with an authentication secret (e.g., link or one-time authentication secret), it would be considered to be an out-of-band authenticator, which requires 20 bits of entropy. Note that 800-63B section 5.1.3.1 does not permit out-of-band secrets to be sent via email or using a VoIP connection because the intent is to prove possession of a specific device, and these mechanisms do not accomplish that.
This repository is not closely monitored, so in the future please direct questions to dig-comments@nist.gov.
@jimfenton I think it deserves a call out, or at least key words included in the document, because trying a text search I was unable to find this information in the document :(
See also the discussion of account recovery in the SP 800-63 Implementation Resources at https://pages.nist.gov/800-63-3-Implementation-Resources/63B/Lifecycle/