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+# ADR 008: PrivValidator
+
+## Context
+
+The current PrivValidator is monolithic and isn't easily reuseable by alternative signers.
+
+For instance, see https://github.com/tendermint/tendermint/issues/673
+
+The goal is to have a clean PrivValidator interface like:
+
+``
+type PrivValidator interface {
+	Address() data.Bytes
+	PubKey() crypto.PubKey
+
+	SignVote(chainID string, vote *types.Vote) error
+	SignProposal(chainID string, proposal *types.Proposal) error
+	SignHeartbeat(chainID string, heartbeat *types.Heartbeat) error
+}
+```
+
+It should also be easy to re-use the LastSignedInfo logic to avoid double signing.
+
+## Decision
+
+Tendermint node's should support only two in-process PrivValidator implementations:
+
+- PrivValidatorUnencrypted uses an unencrypted private key in a "priv_validator.json" file - no configuration required (just `tendermint init`).
+- PrivValidatorSocket uses a socket to send signing requests to another process - user is responsible for starting that process themselves.
+
+The PrivValidatorSocket address can be provided via flags at the command line -
+doing so will cause Tendermint to ignore any "priv_validator.json" file and to attempt
+to connect over the socket.
+
+In addition, Tendermint will provide implementations that can be run in that external process.
+These include:
+
+- PrivValidatorEncrypted uses an encrypted private key persisted to disk - user must enter password to decrypt key when process is started.
+- PrivValidatorLedger uses a Ledger Nano S to handle all signing.
+
+What follows are descriptions of useful types
+
+### Signer
+
+```
+type Signer interface {
+	Sign(msg []byte) (crypto.Signature, error)
+}
+```
+
+Signer signs a message. It can also return an error.
+
+### ValidatorID
+
+
+ValidatorID is just the Address and PubKey
+
+```
+type ValidatorID struct {
+	Address data.Bytes    `json:"address"`
+	PubKey  crypto.PubKey `json:"pub_key"`
+}
+
+### LastSignedInfo
+
+LastSignedInfo tracks the last thing we signed:
+
+```
+type LastSignedInfo struct {
+	Height    int64            `json:"height"`
+	Round     int              `json:"round"`
+	Step      int8             `json:"step"`
+	Signature crypto.Signature `json:"signature,omitempty"` // so we dont lose signatures
+	SignBytes data.Bytes       `json:"signbytes,omitempty"` // so we dont lose signatures
+}
+```
+
+It exposes methods for signing votes and proposals using a `Signer`.
+
+This allows it to easily be reused by developers implemented their own PrivValidator.
+
+### PrivValidatorUnencrypted
+
+```
+type PrivValidatorUnencrypted struct {
+	ID             types.ValidatorID `json:"id"`
+	PrivKey        PrivKey           `json:"priv_key"`
+	LastSignedInfo *LastSignedInfo   `json:"last_signed_info"`
+}
+```
+
+Has the same structure as currently, but broken up into sub structs.
+
+Note the LastSignedInfo is mutated in place every time we sign.
+
+### PrivValidatorJSON
+
+The "priv_validator.json" file supports only the PrivValidatorUnencrypted type.
+
+It unmarshals into PrivValidatorJSON, which is used as the default PrivValidator type.
+It wraps the PrivValidatorUnencrypted and persists it to disk after every signature.
+
+## Status
+
+Proposed.
+
+## Consequences
+
+### Positive
+
+- Cleaner separation of components enabling re-use.
+
+### Negative
+
+- More files - led to creation of new directory.
+
+### Neutral
+