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---
v: 3
title: "The GNU Taler Protocol"
docname: draft-guetschow-taler-protocol
category: info
ipr: trust200902
workgroup: independent
stream: independent
keyword:
- taler
- cryptography
- ecash
- payments
# venue:
# repo: https://git.gnunet.org/lsd0009.git/
# latest: https://lsd.gnunet.org/lsd0009/
author:
-
name: Mikolai Gütschow
org: TUD Dresden University of Technology
abbrev: TU Dresden
street: Helmholtzstr. 10
city: Dresden
code: D-01069
country: Germany
email: mikolai.guetschow@tu-dresden.de
normative:
RFC2104:
RFC5869:
RFC6234:
HKDF: DOI.10.1007/978-3-642-14623-7_34
informative:
--- abstract
\[ TBW \]
--- middle
# Introduction
\[ TBW \]
Beware that this document is still work-in-progress and may contain errors.
Use at your own risk!
# Notation
- `a | b` denotes the concatenation of a with b
# Cryptographic Primitives
## Cryptographic Hash Functions
### SHA-256 {#sha256}
Taler uses SHA-256 as defined in Section 5.1 of [RFC6234].
### SHA-512 {#sha512}
Taler uses SHA-512 as defined in Section 5.2 of [RFC6234].
### Truncated SHA-512 {#sha512-trunc}
## Key Derivation Functions
### HKDF {#hkdf}
The Hashed Key Derivation Function (HKDF) used in Taler is an instantiation of [RFC5869]
with two different hash functions for the Extract and Expand step as suggested in [HKDF].
HMAC-SHA512 (HMAC [RFC2104] instantiated with SHA-512, cf. {{sha512}}) is used for `HKDF-Extract`.
HMAC-SHA256 (HMAC [RFC2104] instantiated with SHA-256, cf. {{sha256}}) is used for `HKDF-Expand`.
~~~
HKDF(salt, IKM, info, L) -> OKM
Inputs:
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
info optional context and application specific information
(can be a zero-length string)
L length of output keying material in octets
(<= 255*32 = 8160)
Output:
OKM output keying material (of L octets)
~~~
The output OKM is calculated as follows:
~~~
PRK = HKDF-Extract(salt, IKM) with Hash = SHA-512, HashLen = 64
OKM = HKDF-Expand(PRK, info, L) with Hash = SHA-256, HashLen = 32
~~~
### HKDF-Mod
Based on the HKDF defined in {{hkdf}}, this function returns an OKM that is smaller than a given big number N.
~~~
HKDF-Mod(N, salt, IKM, info) -> OKM
Inputs:
N big number; Nbits 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
info optional context and application specific information
(can be a zero-length string)
Output:
OKM output keying material (smaller than N)
~~~
The output OKM is calculated as follows:
~~~
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 += 1
~~~
## Non-Blind Signatures
### Ed25519
## Blind Signatures
### FDH-RSA
### Clause-Schnorr
# The Taler Crypto Protocol
## Withdrawal
# Security Considerations
\[ TBD \]
# IANA Considerations
None.
--- back
# Change log
# Acknowledgments
{:numbered="false"}
\[ TBD \]
This work was supported in part by the German Federal Ministry of
Education and Research (BMBF) within the project Concrete Contracts.
|
