commit f9ad03dcf8b27b42dd27d7fa9a9c1674089b9c98
parent 186e6c9ba1ec9a83472cdce6acd41c2f0215f1ae
Author: Mikolai Gütschow <mikolai.guetschow@tu-dresden.de>
Date: Mon, 8 Jul 2024 11:50:09 +0200
crypto primitives: add RSA-FDH
Diffstat:
2 files changed, 97 insertions(+), 47 deletions(-)
diff --git a/draft-guetschow-taler-protocol.md b/draft-guetschow-taler-protocol.md
@@ -198,7 +198,29 @@ do until OKM < N:
## Blind Signatures
-### FDH-RSA
+### RSA-FDH
+
+~~~
+RSA-FDH(msg, pubkey) -> fdh
+
+Inputs:
+ msg message
+ pubkey RSA public key consisting of modulus N and public exponent e
+
+Output:
+ fdh full-domain hash of msg over pubkey.N
+~~~
+
+`fdh` is calculated based on HKDF-Mod from {{hkdf-mod}} as follows:
+
+~~~
+info = 0x5253412d46444120465470735721 ("RSA-FDA FTpsW!" encoded as UTF-8)
+salt = length(pubkey.N) | length(pubkey.e) | pubkey.N | pubkey.e
+fdh = HKDF-Mod(pubkey.N, salt, msg, info)
+~~~
+
+The resulting `fdh` can be used to test against a malicious RSA pubkey
+by verifying that the greatest common denominator (gcd) of `fdh` and `pubkey.N` is 1.
### Clause-Schnorr
diff --git a/draft-guetschow-taler-protocol.xml b/draft-guetschow-taler-protocol.xml
@@ -221,7 +221,29 @@ do until OKM < N:
</section>
<section anchor="blind-signatures"><name>Blind Signatures</name>
-<section anchor="fdh-rsa"><name>FDH-RSA</name>
+<section anchor="rsa-fdh"><name>RSA-FDH</name>
+
+<figure><artwork><![CDATA[
+RSA-FDH(msg, pubkey) -> fdh
+
+Inputs:
+ msg message
+ pubkey RSA public key consisting of modulus N and public exponent e
+
+Output:
+ fdh full-domain hash of msg over pubkey.N
+]]></artwork></figure>
+
+<t><spanx style="verb">fdh</spanx> is calculated based on HKDF-Mod from <xref target="hkdf-mod"/> as follows:</t>
+
+<figure><artwork><![CDATA[
+info = 0x5253412d46444120465470735721 ("RSA-FDA FTpsW!" encoded as UTF-8)
+salt = length(pubkey.N) | length(pubkey.e) | pubkey.N | pubkey.e
+fdh = HKDF-Mod(pubkey.N, salt, msg, info)
+]]></artwork></figure>
+
+<t>The resulting <spanx style="verb">fdh</spanx> can be used to test against a malicious RSA pubkey
+by verifying that the greatest common denominator (gcd) of <spanx style="verb">fdh</spanx> and <spanx style="verb">pubkey.N</spanx> is 1.</t>
</section>
<section anchor="clause-schnorr"><name>Clause-Schnorr</name>
@@ -330,7 +352,7 @@ do until OKM < N:
-<?line 217?>
+<?line 239?>
<section anchor="change-log"><name>Change log</name>
@@ -348,50 +370,56 @@ Education and Research (BMBF) within the project Concrete Contracts.</t>
</back>
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