Blind Signatures

The current version of this protocol relies on the vanilla RSA blind signature scheme, for which we provide an open-source reference implementation. Shell-script 402-Receipts tools are not appropriate for production use, but will serve for proof-of-concept and demonstration of the required behavior.

The Client will need to generate a “client secret” (a precursor to the blinding factor). This is basically just a cryptographically-randomly generated number, except that it must be checked for appropiateness with respect to the Notary’s public key.

bash@client$ cat scratch/public.pem
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAwZ8xGol2RiBQsExz9dc9
hDMwSbRoc+3y38SxlpI7rrge+ww6mpxbxXhstflJonhZVUOMICdFPbg/eXg4g5Y8
lUsclwtLHsc/7mFfUK5uYvZZvjhm5RWIizz+Kma9svYiUZCDwi8XJywaAk0Sif4s
5KqQNV2a4gc7uTWm62RGCPDaOn2UyBYbAUwElV+nS7YXVO7UfxPOiOhJ04L70fIl
xzorJlddRLGOb2JFQVOiU9MkYwATHt5Qnwcq2l+LxHPrVnqu80pyyfkYyDl0FBZV
J+xztfeesfj6824zDHC/qB6JKpOhXWtftFFXgoRaE7P9DM1Gp9Dv7FdMrr86bkfD
rQIDAQAB
-----END PUBLIC KEY-----
bash@client$ bin/blsig_get_client_secret scratch/public.pem 
b5e8104c3d2b8a763e1825823fb2514c45ecdad9cd7f4941390846049bfc00c0cab91b1eefe93793178bd0dc0a93ea3f6d7f9708b80d3355956153a5b207a0f9e7c99a3b87a2d55a112ca458e540301839af91bc4e3c192173d09dcdef3de8c064fcf68013d1b866026189f3a99f5a627305288ab130404df007bd24cfa4a762799d2a0195ceaa9702a848887a10272c68ead42e4b53572cac3205a72ccf741c82f746bd6f5f90b747f0d1991ecf99e5696c857a1ccce3a3834fc47c27bf68057d6b9ef8f100deeb17bd93b4b51ecfe344df736ea26a93c48262ea839eed994d1ce7d8cef32704e0875fa4ec0f36ae4f42d68236227c2e9cdd55cbf92206e90dh

The client secret should be hidden from all other parties, and should be kept only until the final signature has been obtained.

The serialized Receipt representation used for the Notary Signature is SHA512 hashed and then blinded with the client secret and public key.

bash@client$ bin/blsig_get_blinded_hash 'domain"https://www.example.com/"item"This is technically a valid item string."signer"https://receipts.dmn.network/path"time1557944008units"USD"amount0.0000050000001uuidbf9c1367958941ff8f74134877341cce' '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' scratch/public.pem
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

This value (the “blinded hash”) is sent by the Client to the Notary. The Notary signs it and sends the resulting value back to the Client. This “blinded signature” is incomplete, and without the corresponding client secret it’s meaningless and useless.

bash@notary$ cat scratch/private.pem 
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
bash@notary$ bin/blsig_get_blind_signature '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' scratch/private.pem 
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

The Client unblinds the signature and verifies that the signature is valid. (It happens that the Client is able to do the verification before the unblinding.) The resulting signature, as hex bytes, is the Notary Signature , and is used as the signature in the Receipt .

bash@client$ bin/blsig_get_unblinded_signature '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' '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' 'b5e8104c3d2b8a763e1825823fb2514c45ecdad9cd7f4941390846049bfc00c0cab91b1eefe93793178bd0dc0a93ea3f6d7f9708b80d3355956153a5b207a0f9e7c99a3b87a2d55a112ca458e540301839af91bc4e3c192173d09dcdef3de8c064fcf68013d1b866026189f3a99f5a627305288ab130404df007bd24cfa4a762799d2a0195ceaa9702a848887a10272c68ead42e4b53572cac3205a72ccf741c82f746bd6f5f90b747f0d1991ecf99e5696c857a1ccce3a3834fc47c27bf68057d6b9ef8f100deeb17bd93b4b51ecfe344df736ea26a93c48262ea839eed994d1ce7d8cef32704e0875fa4ec0f36ae4f42d68236227c2e9cdd55cbf92206e90dh' scratch/public.pem 
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

When the Host (and later the Notary) receives the Receipt, they can re-serialize data and validate the signature.

bash@host$ bin/blsig_verify_unblinded_signature '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' 'domain"https://www.example.com/"item"This is technically a valid item string."signer"https://receipts.dmn.network/path"time1557944008units"USD"amount0.0000050000001uuidbf9c1367958941ff8f74134877341cce' scratch/public.pem 
true