Hi all,
I have a question about the alignment between the text in Section 3.1 of
draft-ietf-emu-bootstrapped-tls-01 and RFC 9258.
RFC 9258 describes how to import external PSKs for use with TLS 1.3.
It does so by defining a function with three inputs, namely an external
identity, an EPSK, and an optional context. The output is then a derived epsk
and an imported psk (ipsk). The identity of the ipsk is the serialized
ImportedIdentity structure.
Section 5.1 of RFC 9258 defines the functions as follows:
epskx = HKDF-Extract(0, epsk)
ipskx = HKDF-Expand-Label(epskx, "derived psk",
Hash(ImportedIdentity), L)
The epsk in RFC 9258 is defined as the a tuple of (Base Key, External Identity,
Hash). I assume that the epsk parameter, which is input to the HKDF-Extract, is
the private key.
Now, coming to draft-ietf-emu-bootstrapped-tls-01.
Here the derivations are defined as follows:
epsk = HKDF-Expand(HKDF-Extract(<>, bskey),
"tls13-imported-bsk", L)
epskid = HKDF-Expand(HKDF-Extract(<>, bskey),
"tls13-bspsk-identity", L)
where:
- epsk is the EPSK Base Key
- epskid is the EPSK External Identity
- <> is a NULL salt
- bskey is the DER-encoded ASN.1 subjectPublicKeyInfo
representation of the BSK public key
- L is the length of the digest of the underlying hash
algorithm
Since the functions are different I am wondering whether the idea is to create
another derivation before applying those inputs to the RFC 9258-defined
functions. Is this the idea?
Ciao
Hannes
PS: I noticed that in an earlier IETF presentation a point to a Github repo was
provided. I looked at that code, which has now been reverted, and it did not
match the content of the draft. Is there an implementation of this draft
available somewhere?
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