Accounts

Synopsis

This document describes the in-built account and public key system of the Cosmos SDK.

Pre-requisite Readings

• Anatomy of a Cosmos SDK Application

Account Definition

In the Cosmos SDK, an account designates a pair of public key PubKey and private key PrivKey. The PubKey can be derived to generate various Addresses, which are used to identify users (among other parties) in the application. Addresses are also associated with messages to identify the sender of the message. The PrivKey is used to generate digital signatures to prove that an Address associated with the PrivKey approved of a given message.

For HD key derivation the Cosmos SDK uses a standard called BIP32. The BIP32 allows users to create an HD wallet (as specified in BIP44) - a set of accounts derived from an initial secret seed. A seed is usually created from a 12- or 24-word mnemonic. A single seed can derive any number of PrivKeys using a one-way cryptographic function. Then, a PubKey can be derived from the PrivKey. Naturally, the mnemonic is the most sensitive information, as private keys can always be re-generated if the mnemonic is preserved.

     Account 0                         Account 1                         Account 2

+------------------+              +------------------+               +------------------+
|                  |              |                  |               |                  |
|    Address 0     |              |    Address 1     |               |    Address 2     |
|        ^         |              |        ^         |               |        ^         |
|        |         |              |        |         |               |        |         |
|        |         |              |        |         |               |        |         |
|        |         |              |        |         |               |        |         |
|        +         |              |        +         |               |        +         |
|  Public key 0    |              |  Public key 1    |               |  Public key 2    |
|        ^         |              |        ^         |               |        ^         |
|        |         |              |        |         |               |        |         |
|        |         |              |        |         |               |        |         |
|        |         |              |        |         |               |        |         |
|        +         |              |        +         |               |        +         |
|  Private key 0   |              |  Private key 1   |               |  Private key 2   |
|        ^         |              |        ^         |               |        ^         |
+------------------+              +------------------+               +------------------+
         |                                 |                                  |
         |                                 |                                  |
         |                                 |                                  |
         +--------------------------------------------------------------------+
                                           |
                                           |
                                 +---------+---------+
                                 |                   |
                                 |  Master PrivKey   |
                                 |                   |
                                 +-------------------+
                                           |
                                           |
                                 +---------+---------+
                                 |                   |
                                 |  Mnemonic (Seed)  |
                                 |                   |
                                 +-------------------+

In the Cosmos SDK, keys are stored and managed by using an object called a Keyring.

Keys, accounts, addresses, and signatures

The principal way of authenticating a user is done using digital signatures. Users sign transactions using their own private key. Signature verification is done with the associated public key. For on-chain signature verification purposes, we store the public key in an Account object (alongside other data required for a proper transaction validation).

In the node, all data is stored using Protocol Buffers serialization.

The Cosmos SDK supports the following digital key schemes for creating digital signatures:

• secp256k1, as implemented in the Cosmos SDK's crypto/keys/secp256k1 package.
• secp256r1, as implemented in the Cosmos SDK's crypto/keys/secp256r1 package,
• tm-ed25519, as implemented in the Cosmos SDK crypto/keys/ed25519 package. This scheme is supported only for the consensus validation.

	Address length in bytes	Public key length in bytes	Used for transaction authentication	Used for consensus (cometbft)
secp256k1	20	33	yes	no
secp256r1	32	33	yes	no
tm-ed25519	-- not used --	32	no	yes

Addresses

Addresses and PubKeys are both public information that identifies actors in the application. Account is used to store authentication information. The basic account implementation is provided by a BaseAccount object.

Each account is identified using Address which is a sequence of bytes derived from a public key. In the Cosmos SDK, we define 3 types of addresses that specify a context where an account is used:

• AccAddress identifies users (the sender of a message).
• ValAddress identifies validator operators.
• ConsAddress identifies validator nodes that are participating in consensus. Validator nodes are derived using the ed25519 curve.

These types implement the Address interface:

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/types/address.go#L126-L134

Address construction algorithm is defined in ADR-28. Here is the standard way to obtain an account address from a pub public key:

sdk.AccAddress(pub.Address().Bytes())

Of note, the Marshal() and Bytes() method both return the same raw []byte form of the address. Marshal() is required for Protobuf compatibility.

For user interaction, addresses are formatted using Bech32 and implemented by the String method. The Bech32 method is the only supported format to use when interacting with a blockchain. The Bech32 human-readable part (Bech32 prefix) is used to denote an address type. Example:

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/types/address.go#L299-L316

	Address Bech32 Prefix
Accounts	cosmos
Validator Operator	cosmosvaloper
Consensus Nodes	cosmosvalcons

Public Keys

Public keys in Cosmos SDK are defined by cryptotypes.PubKey interface. Since public keys are saved in a store, cryptotypes.PubKey extends the proto.Message interface:

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/crypto/types/types.go#L8-L17

A compressed format is used for secp256k1 and secp256r1 serialization.

• The first byte is a 0x02 byte if the y-coordinate is the lexicographically largest of the two associated with the x-coordinate.
• Otherwise the first byte is a 0x03.

This prefix is followed by the x-coordinate.

Public Keys are not used to reference accounts (or users) and in general are not used when composing transaction messages (with few exceptions: MsgCreateValidator, Validator and Multisig messages). For user interactions, PubKey is formatted using Protobufs JSON (ProtoMarshalJSON function). Example:

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/client/keys/output.go#L23-L39

Keyring

A Keyring is an object that stores and manages accounts. In the Cosmos SDK, a Keyring implementation follows the Keyring interface:

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/crypto/keyring/keyring.go#L57-L105

The default implementation of Keyring comes from the third-party 99designs/keyring library.

A few notes on the Keyring methods:

• Sign(uid string, msg []byte) ([]byte, types.PubKey, error) strictly deals with the signature of the msg bytes. You must prepare and encode the transaction into a canonical []byte form. Because protobuf is not deterministic, it has been decided in ADR-020 that the canonical payload to sign is the SignDoc struct, deterministically encoded using ADR-027. Note that signature verification is not implemented in the Cosmos SDK by default, it is deferred to the anteHandler.

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/proto/cosmos/tx/v1beta1/tx.proto#L50-L66

• NewAccount(uid, mnemonic, bip39Passphrase, hdPath string, algo SignatureAlgo) (*Record, error) creates a new account based on the bip44 path and persists it on disk. The PrivKey is never stored unencrypted, instead it is encrypted with a passphrase before being persisted. In the context of this method, the key type and sequence number refer to the segment of the BIP44 derivation path (for example, 0, 1, 2, ...) that is used to derive a private and a public key from the mnemonic. Using the same mnemonic and derivation path, the same PrivKey, PubKey and Address is generated. The following keys are supported by the keyring:

• secp256k1

• ed25519

• ExportPrivKeyArmor(uid, encryptPassphrase string) (armor string, err error) exports a private key in ASCII-armored encrypted format using the given passphrase. You can then either import the private key again into the keyring using the ImportPrivKey(uid, armor, passphrase string) function or decrypt it into a raw private key using the UnarmorDecryptPrivKey(armorStr string, passphrase string) function.

Create New Key Type

To create a new key type for using in keyring, keyring.SignatureAlgo interface must be fulfilled.

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/crypto/keyring/signing_algorithms.go#L10-L15

The interface consists in three methods where Name() returns the name of the algorithm as a hd.PubKeyType and Derive() and Generate() must return the following functions respectively:

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/crypto/hd/algo.go#L28-L31

Once the keyring.SignatureAlgo has been implemented it must be added to the list of supported algos of the keyring.

For simplicity the implementation of a new key type should be done inside the crypto/hd package. There is an example of a working secp256k1 implementation in algo.go.

Implementing secp256r1 algo

Here is an example of how secp256r1 could be implemented.

First a new function to create a private key from a secret number is needed in the secp256r1 package. This function could look like this:

// cosmos-sdk/crypto/keys/secp256r1/privkey.go

// NewPrivKeyFromSecret creates a private key derived for the secret number
// represented in big-endian. The `secret` must be a valid ECDSA field element.
func NewPrivKeyFromSecret(secret []byte) (*PrivKey, error) {
    var d = new(big.Int).SetBytes(secret)
    if d.Cmp(secp256r1.Params().N) >= 1 {
        return nil, errorsmod.Wrap(errors.ErrInvalidRequest, "secret not in the curve base field")
    }
    sk := new(ecdsa.PrivKey)
    return &PrivKey{&ecdsaSK{*sk}}, nil
}

After that secp256r1Algo can be implemented.

// cosmos-sdk/crypto/hd/secp256r1Algo.go

package hd

import (
    "github.com/cosmos/go-bip39"
    
    "github.com/cosmos/cosmos-sdk/crypto/keys/secp256r1"
    "github.com/cosmos/cosmos-sdk/crypto/types"
)

// Secp256r1Type uses the secp256r1 ECDSA parameters.
const Secp256r1Type = PubKeyType("secp256r1")

var Secp256r1 = secp256r1Algo{}

type secp256r1Algo struct{}

func (s secp256r1Algo) Name() PubKeyType {
    return Secp256r1Type
}

// Derive derives and returns the secp256r1 private key for the given seed and HD path.
func (s secp256r1Algo) Derive() DeriveFn {
    return func(mnemonic string, bip39Passphrase, hdPath string) ([]byte, error) {
        seed, err := bip39.NewSeedWithErrorChecking(mnemonic, bip39Passphrase)
        if err != nil {
            return nil, err
        }

        masterPriv, ch := ComputeMastersFromSeed(seed)
        if len(hdPath) == 0 {
            return masterPriv[:], nil
        }
        derivedKey, err := DerivePrivateKeyForPath(masterPriv, ch, hdPath)

        return derivedKey, err
    }
}

// Generate generates a secp256r1 private key from the given bytes.
func (s secp256r1Algo) Generate() GenerateFn {
    return func(bz []byte) types.PrivKey {
        key, err := secp256r1.NewPrivKeyFromSecret(bz)
        if err != nil {
            panic(err)
        }
        return key
    }
}

Finally, the algo must be added to the list of supported algos by the keyring.

// cosmos-sdk/crypto/keyring/keyring.go

func newKeystore(kr keyring.Keyring, cdc codec.Codec, backend string, opts ...Option) keystore {
    // Default options for keybase, these can be overwritten using the
    // Option function
    options := Options{
        SupportedAlgos:       SigningAlgoList{hd.Secp256k1, hd.Secp256r1}, // added here
        SupportedAlgosLedger: SigningAlgoList{hd.Secp256k1},
    }
...

Hereafter to create new keys using your algo, you must specify it with the flag --algo :

simd keys add myKey --algo secp256r1