# US 2019/0318356 A1 · Offline storage system and method of use

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- Number: US 2019/0318356 A1
- Document ID: US20190318356A1
- Status: Published application
- Kind: A1
- Application number: 16/386,786
- Filed: 2019-04-17
- Published: 2019-10-17
- Earliest claimed: 2018-04-17
- Assignees: Coinbase, Inc.
- Applicants: Coinbase, Inc.
- Inventors: Philip Martin; Julian Borrey; Yolanda Liu; Zachary Blacher; Robert John Kearney
- Web page: https://patentgrep.com/patent/US20190318356A1

## CPC

- G06F 21/6209: {to a single file or object, e.g. in a secure envelope, encrypted and accessed using a key, or with access control rules appended to the object itself}
- G06Q 20/401: {Transaction verification}
- G06Q 20/065: {using e-cash}
- G06Q 20/3829: {involving key management}
- H04L 9/0637: {Modes of operation, e.g. cipher block chaining [CBC], electronic codebook [ECB] or Galois/counter mode [GCM]}
- H04L 9/0822: {using key encryption key}
- H04L 9/0825: {using asymmetric-key encryption or public key infrastructure [PKI], e.g. key signature or public key certificates}
- H04L 9/085: {Secret sharing or secret splitting, e.g. threshold schemes}
- H04L 9/14: using a plurality of keys or algorithms
- H04L 9/3073: {involving pairings, e.g. identity based encryption [IBE], bilinear mappings or bilinear pairings, e.g. Weil or Tate pairing}
- H04L 9/3247: {involving digital signatures}
- H04L 9/50: {using hash chains, e.g. blockchains or hash trees}
- G06Q 2220/00: Business processing using cryptography
- H04L 9/0897: {involving additional devices, e.g. trusted platform module [TPM], smartcard or USB}
- H04L 9/3226: {using a predetermined code, e.g. password, passphrase or PIN (network architectures or network communication protocols for supporting authentication of entities using passwords in a packet data network H04L63/083)}
- H04L 9/3239: {involving non-keyed hash functions, e.g. modification detection codes [MDCs], MD5, SHA or RIPEMD}
- H04L 9/3271: {using challenge-response}
- H04L 2209/56: Financial cryptography, e.g. electronic payment or e-cash
- H04L 2209/80: Wireless
- G06F: ELECTRIC DIGITAL DATA PROCESSING (computer systems based on specific computational models G06N)
- G06Q: INFORMATION AND COMMUNICATION TECHNOLOGY [ICT] SPECIALLY ADAPTED FOR ADMINISTRATIVE, COMMERCIAL, FINANCIAL, MANAGERIAL OR SUPERVISORY PURPOSES; SYSTEMS OR METHODS SPECIALLY ADAPTED FOR ADMINISTRATIVE, COMMERCIAL, FINANCIAL, MANAGERIAL OR SUPERVISORY PURPOSES, NOT OTHERWISE PROVIDED FOR
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

A method for storing a cryptocurrency private key offline, including: encrypting the cryptocurrency private key using a primary encryption key; sharding the encrypted cryptocurrency private key into a plurality of alpha shards; generating beta shards by encrypting the alpha shards with secondary encryption keys; and storing representations of the beta shards offline. The method can additionally or alternatively include: retrieving the representations of the beta shards from the offline storage; decrypting the beta shards into the alpha shards based on the secondary encryption keys; reconstructing the encrypted cryptocurrency private key by recombining the alpha shards; and decrypting the encrypted cryptocurrency private key with the primary encryption key.

## Claims

### Claim 1

A method for offline storage of a cryptocurrency private key of a cryptocurrency public-private key pair for a cryptographic currency, the method comprising:
encrypting the cryptocurrency private key of the cryptocurrency public-private key pair using a primary encryption key;
sharding the encrypted cryptocurrency private key into a plurality of alpha shards;
generating beta shards by encrypting the alpha shards with secondary encryption keys; and
storing representations of the beta shards offline.

### Claim 2 (depends on 1)

The method of claim 1, wherein the alpha shards are sharded using a secret sharing algorithm.

### Claim 3 (depends on 2)

The method of claim 2, wherein the alpha shards are sharded using Shamir's Secret Sharing Scheme.

### Claim 4 (depends on 1)

The method of claim 1, wherein each alpha shard is encrypted using a different secondary encryption key.

### Claim 5 (depends on 1)

The method of claim 1, wherein a secondary encryption key used to encrypt an alpha shard is used to encrypt a second alpha shard of a second private key from a second cryptocurrency public-private key pair.

### Claim 6 (depends on 1)

The method of claim 1, wherein the physical representations of the beta shards comprise QR codes.

### Claim 7 (depends on 1)

The method of claim 1, wherein each secondary encryption key comprises a public secondary encryption key of a different secondary encryption key pair.

### Claim 8 (depends on 1)

The method of claim 1, wherein the secondary encryption keys are each associated with a different key holder, wherein the key holders are geographically dispersed.

### Claim 9 (depends on 8)

The method of claim 8, wherein each secondary encryption key comprises a public key of a different secondary encryption key pair, wherein the private keys of each secondary encryption key pair are distributed to the key holders.

### Claim 10 (depends on 1)

The method of claim 1, wherein the physical repository is air-gapped.

### Claim 11 (depends on 1)

The method of claim 1, further comprising:
retrieving the representations of the beta shards from the offline storage;
decrypting the beta shards into the alpha shards based on the secondary encryption keys;
reconstructing the encrypted cryptocurrency private key by recombining the alpha shards; and
decrypting the encrypted cryptocurrency private key with the primary encryption key.

### Claim 12

The method of claim ii, wherein determining alpha shards based on the set of secondary encryption keys comprises:
transmitting each beta shard to a key holder associated with the respective beta shard, wherein the beta shard is decrypted into an alpha shard using the respective secondary encryption key; and
receiving the alpha shards from the key holders.

### Claim 13 (depends on 12)

The method of claim 12, wherein the secondary encryption key comprises an asymmetric key pair, wherein the beta shard is generated using the public key of the asymmetric key pair, and wherein the beta shard is decrypted into the alpha shard using the private key of the asymmetric key pair.

### Claim 14 (depends on 12)

The method of claim 12, further comprising:
prior to transmitting the beta shard to the key holder, authenticating the key holders using multi-factor authentication; and
transmitting the beta shard in response to key holder authentication.

### Claim 15

The method of claim ii, further comprising:
receiving a transaction request from a client service, the transaction request comprising a transaction requiring a signature by the cryptocurrency private key; and
signing the transaction with the decrypted cryptocurrency private key.

### Claim 16 (depends on 15)

The method of claim 15, further comprising transmitting the signed transaction to a blockchain.

### Claim 17 (depends on 1)

The method of claim 1, wherein each secondary encryption key is stored on a different hardware security module (HSM).

### Claim 18

A method for cryptocurrency private key retrieval from offline storage, comprising:
retrieving representations of beta shards, associated with the cryptocurrency private key, from offline storage;
facilitating decryption of the beta shards into alpha shards using secondary encryption keys;
reconstructing an encrypted version of the cryptocurrency private key from the alpha shards; and
decrypting the encrypted cryptocurrency private key using a primary encryption key.

### Claim 19 (depends on 18)

The method of claim 18, wherein the secondary encryption keys comprise private keys of asymmetric key pairs, wherein the beta shards were generated by encrypting the alpha shards with the corresponding public keys, and the primary encryption key comprises a symmetric key.

### Claim 20 (depends on 18)

The method of claim 18, wherein the encrypted cryptocurrency private key was sharded into the alpha shards using a secret sharing algorithm, wherein the alpha shards are recombined into the encrypted cryptocurrency private key using the secret sharing algorithm.