[ADR] Proposal for encoding symmetric cryptography

docs/architecture/adr-015-symmetric-crypto.md

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+# ADR 015: Need for symmetric cryptography
+
+## Context
+
+We require symmetric ciphers to handle how we encrypt keys in the sdk,
+and to potentially encrypt `priv_validator.json` in tendermint.
+
+Currently we use AEAD's to support symmetric encryption,
+which is great since we want data integrity in addition to privacy and authenticity.
+We don't currently have a scenario where we want to encrypt without data integrity,
+so it is fine to optimize our code to just use AEAD's.
+Currently there is not a way to switch out AEAD's easily, this ADR outlines a way
+to easily swap these out.
+
+### How do we encrypt with AEAD's
+
+AEAD's typically require a nonce in addition to the key. 
+For the purposes we require symmetric cryptography for,
+we need encryption to be stateless.
+Because of this we use random nonces. 
+(Thus the AEAD must support random nonces)
+
+We currently construct a random nonce, and encrypt the data with it. 
+The returned value is `nonce || encrypted data`.
+The limitation of this is that does not provide a way to identify
+which algorithm was used in encryption.
+Consequently decryption with multiple algoritms is sub-optimal. 
+(You have to try them all)
+
+## Decision
+
+We should create the following two methods in a new `crypto/encoding/symmetric` package: 
+```golang
+func EncryptSymmetric(aead cipher.AEAD, plaintext []byte) (ciphertext []byte, err error)
+func DecryptSymmetric(key []byte, ciphertext []byte) (plaintext []byte, err error)
+func RegisterSymmetric(aead cipher.AEAD, algo_name string, NewAead func(key []byte) (cipher.Aead, error)) error
+```
+
+This allows you to specify the algorithm in encryption, but not have to specify
+it in decryption. 
+This is intended for ease of use in downstream applications, in addition to people
+looking at the file directly.
+One downside is that for the encrypt function you must have already initialized an AEAD,
+but I don't really see this as an issue. 
+
+If there is no error in encryption, EncryptSymmetric will return `algo_name || nonce || aead_ciphertext`. 
+This requires a mapping from aead type to name. 
+We can achieve this via reflection. 
+```golang
+func getType(myvar interface{}) string {
+    if t := reflect.TypeOf(myvar); t.Kind() == reflect.Ptr {
+        return "*" + t.Elem().Name()
+    } else {
+        return t.Name()
+    }
+}
+```
+Then we maintain a map from the name returned from `getType(aead)` to `algo_name`. 
+
+In decryption, we read the `algo_name`, and then instantiate a new AEAD with the key.
+Then we call the AEAD's decrypt method on the provided nonce/ciphertext.
+
+`RegisterSymmetric` allows a downstream user to add their own desired AEAD to the symmetric package.
+
+## Status
+
+Proposed.
+
+## Consequences
+
+### Positive
+* Allows us to support new AEAD's, in a way that makes decryption easier
+* Allows downstream users to add their own AEAD
+
+### Negative
+
+### Neutral
+* Caller has to instantiate the AEAD with the private key.
+However it forces them to be aware of what signing algorithm they are using, which is a positive.