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Enable pre-blinding the private key #28

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12 changes: 9 additions & 3 deletions draft-bradleylundberg-cfrg-arkg.md
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Expand Up @@ -432,14 +432,15 @@ This function may be invoked any number of times with the same private seed,
in order to derive the same or different private keys any number of times.

~~~pseudocode
ARKG-Derive-Private-Key((sk_kem, sk_bl), kh, info) -> sk'
ARKG-Derive-Private-Key((sk_kem, sk_bl), bf, kh, info) -> sk'
ARKG instance parameters:
BL A key blinding scheme.
KEM A key encapsulation mechanism.

Inputs:
sk_kem A key encapsulation private key.
sk_bl A key blinding private key.
bf An initial blinding factor to apply.
kh A key handle output from ARKG-Derive-Public-Key.
info An octet string containing optional context
and application specific information
Expand All @@ -450,14 +451,19 @@ ARKG-Derive-Private-Key((sk_kem, sk_bl), kh, info) -> sk'

The output sk' is calculated as follows:

sk_bl' = BL-Combine(sk_bl, bf)
pk_bl' = pk(sk_bl')

info_pk = BL-Serialize-Public-Key(pk_bl')

info_kem = 'ARKG-Derive-Key-KEM.' || info
info_bl = 'ARKG-Derive-Key-BL.' || info
info_bl = info_pk || 'ARKG-Derive-Key-BL.' || info

tau = KEM-Decaps(sk_kem, kh, info_kem)
If decapsulation failed:
Abort with an error.

sk' = BL-Blind-Private-Key(sk_bl, tau, info_bl)
sk' = BL-Blind-Private-Key(sk_bl', tau, info_bl)
~~~

Errors in this procedure are typically unrecoverable.
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