commit 186e6c9ba1ec9a83472cdce6acd41c2f0215f1ae
parent 647fde5bc6f043150fa424cf20e977644a308358
Author: Mikolai Gütschow <mikolai.guetschow@tu-dresden.de>
Date: Mon, 8 Jul 2024 11:03:43 +0200
crypto primitives: refine HKDF-Mod
Diffstat:
2 files changed, 59 insertions(+), 67 deletions(-)
diff --git a/draft-guetschow-taler-protocol.md b/draft-guetschow-taler-protocol.md
@@ -170,7 +170,7 @@ Based on the HKDF defined in {{hkdf}}, this function returns an OKM that is smal
HKDF-Mod(N, salt, IKM, info) -> OKM
Inputs:
- N big number; Nbits denotes the length of N in bits
+ N big number; Nbit denotes the length of N in bits
salt optional salt value (a non-secret random value);
if not provided, it is set to a string of 64 zeros.
IKM input keying material
@@ -181,18 +181,14 @@ Output:
OKM output keying material (smaller than N)
~~~
-The output OKM is calculated as follows:
+The final output `OKM` is determined iteratively based on a counter initialized at zero,
+where `c` denotes the two least significant octets of the counter in network-byte order
~~~
-Nlen = ceil(Nbits / 8)
-while true:
- counter = 0
- c = 2 least significant octets of counter in network-byte order
- x = HKDF(salt, IKM, info | c, NLen)
- reset all but lower Nbits bits in x
- if x < N:
- OKM = x
- break
+counter = 0
+do until OKM < N:
+ x = HKDF(salt, IKM, info | c, ceil(Nbits/8))
+ OKM = Nbits least significant bits of x
counter += 1
~~~
diff --git a/draft-guetschow-taler-protocol.xml b/draft-guetschow-taler-protocol.xml
@@ -189,7 +189,7 @@ OKM = HKDF-Expand(PRK, info, L) with Hash = SHA-256 (HashLen = 32)
HKDF-Mod(N, salt, IKM, info) -> OKM
Inputs:
- N big number; Nbits denotes the length of N in bits
+ N big number; Nbit denotes the length of N in bits
salt optional salt value (a non-secret random value);
if not provided, it is set to a string of 64 zeros.
IKM input keying material
@@ -200,18 +200,14 @@ Output:
OKM output keying material (smaller than N)
]]></artwork></figure>
-<t>The output OKM is calculated as follows:</t>
+<t>The final output <spanx style="verb">OKM</spanx> is determined iteratively based on a counter initialized at zero,
+where <spanx style="verb">c</spanx> denotes the two least significant octets of the counter in network-byte order</t>
<figure><artwork><![CDATA[
-Nlen = ceil(Nbits / 8)
-while true:
- counter = 0
- c = 2 least significant octets of counter in network-byte order
- x = HKDF(salt, IKM, info | c, NLen)
- reset all but lower Nbits bits in x
- if x < N:
- OKM = x
- break
+counter = 0
+do until OKM < N:
+ x = HKDF(salt, IKM, info | c, ceil(Nbits/8))
+ OKM = Nbits least significant bits of x
counter += 1
]]></artwork></figure>
@@ -334,7 +330,7 @@ while true:
-<?line 221?>
+<?line 217?>
<section anchor="change-log"><name>Change log</name>
@@ -352,50 +348,50 @@ Education and Research (BMBF) within the project Concrete Contracts.</t>
</back>
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