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clarified byte-encoding of the ik public key when using it as a fingerprint

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Tim Henkes 2020-03-15 15:44:01 +01:00
parent 966b37c164
commit 302c56f064
1 changed files with 1 additions and 1 deletions
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xep-0384.xml
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@ -638,7 +638,7 @@
</section1>
<section1 topic='Security Considerations' anchor='security'>
<p>Clients MUST NOT use a newly built session to transmit data without user intervention. If a client were to opportunistically start using sessions for sending without asking the user whether to trust a device first, an attacker could publish a fake device for this user, which would then receive copies of all messages sent by/to this user. A client MAY use such "not (yet) trusted" sessions for decryption of received messages, but in that case it SHOULD indicate the untrusted nature of such messages to the user.</p>
<p>When prompting the user for a trust decision regarding a key, the client SHOULD present the user with a fingerprint in the form of a hex-string, QR code, or other unique representation, such that it can be compared by the user. To ensure interoperability between clients and older versions of OMEMO, the fingerprint SHOULD be chosen to be the public part of the IdentityKey in its Curve25519 form (see the <link url="#protocol-key_exchange">notes on XEdDSA in the X3DH protocol section</link> for details). When displaying the fingerprint as a hex-string, one way to make it easier to compare the fingerprint is to split the hex-string into 8 substrings of 8 chars each, then coloring each 8-char group using &xep0392;. Lowercase letters are recommended when displaying the fingerprint as a hex-string.</p>
<p>When prompting the user for a trust decision regarding a key, the client SHOULD present the user with a fingerprint in the form of a hex-string, QR code, or other unique representation, such that it can be compared by the user. To ensure interoperability between clients and older versions of OMEMO, the fingerprint SHOULD be chosen to be the public part of the IdentityKey in its byte-encoded Curve25519 form (see the notes on XEdDSA and the byte-encoding of public keys in the <link url="#protocol-key_exchange">X3DH protocol section</link> for details). When displaying the fingerprint as a hex-string, one way to make it easier to compare the fingerprint is to split the hex-string into 8 substrings of 8 chars each, then coloring each 8-char group using &xep0392;. Lowercase letters are recommended when displaying the fingerprint as a hex-string.</p>
<p>While it is RECOMMENDED that clients postpone private key deletion until after message catch-up, the X3DH standard mandates that clients should not use duplicate-PreKey sessions for sending, so clients MAY delete such keys immediately for security reasons. For additional information on potential security impacts of this decision, refer to <note>Menezes, Alfred, and Berkant Ustaoglu. "On reusing ephemeral keys in Diffie-Hellman key agreement protocols." International Journal of Applied Cryptography 2, no. 2 (2010): 154-158.</note>.</p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>