docs(adr): ADR-008: Change the plain text encoding of links to hex (#642

)

## Description

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This PR introduces the draft of the 8th ADR related to Change the plain text encoding of links to hex in #636.

---

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docs/architecture/adr-008-change-the-plain-text-encoding-the-links-to-hex.md

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+# ADR 008: Change the plain text encoding of links to hex
+
+## Changelog
+
+- October 5th 2021: Initial draft;
+- October 7th 2021: Moved from DRAFT to PROPOSED;
+- October 10th 2021: First review;
+
+## Status
+
+PROPOSED
+
+## Abstract
+
+Currently, when verifying an application link or a chain link we assume that their plain text values have been encoded using UTF-8.
+However, there is a major problem with the UTF-8 encoding: it does not support all bytes properly. For this reason, we SHOULD change 
+the encoding of the plain text from UTF-8 to HEX in order to avoid any possible encoding problem that would result in a signature 
+that it's impossible to verify correctly. 
+
+## Context
+
+The `x/profiles` module gives users the possibility to link their profile to some external account(s), either centralized applications 
+(eg. GitHub, Reddit, Twitter) or other blockchains (eg. Cosmos, Solana, Polkadot). When linking a Desmos profile to any of these accounts, 
+we use a signature-based authentication process in order to make sure that the user controls such accounts. In both the applications 
+and chains links, we currently expect the user to use the UTF-8 encoding when sending over the plain text used to create the signature. 
+However, since the UTF-8 encoding is not able to correctly represent all bytes, there might be cases in which we end up with a signature 
+that it's impossible to verify. For example, this is what happens if the original plain text was encoded before being signed with another 
+encoding such as UTF-16, Unicode, etc.
+
+## Decision
+
+We propose to change the encoding of the plain text of both application link and chain link to hex.
+
+### Chain link implementation
+
+When saving a `ChainLink`, we use the `Proof` object in order to verify the signature. To make sure it supports the HEX encoding 
+instead of the UTF-8 one, we need to change how the `Validate` method checks for the validity of such proof:
+```go
+// Validate checks the validity of the Proof
+func (p Proof) Validate() error {
+
+    ...
+
+    _, err = hex.DecodeString(p.PlainText)
+    if err != nil {
+        return fmt.Errorf("invalid hex-encoded plain text")
+    }
+
+	return nil
+}
+```
+Second, we need to change how the `Proof#Verify` method verifies the signature provided in order to make sure that it deserializes
+the plain text as an HEX value instead of an UTF-8 one:
+```go
+// Verify verifies the signature using the given plain text and public key.
+// It returns an error if something is invalid.
+func (p Proof) Verify(unpacker codectypes.AnyUnpacker, address AddressData) error {
+	var pubkey cryptotypes.PubKey
+	err := unpacker.UnpackAny(p.PubKey, &pubkey)
+	if err != nil {
+		return fmt.Errorf("failed to unpack the public key")
+	}
+
+	value, err := hex.DecodeString(p.PlainText)
+	if err != nil {
+		return fmt.Errorf("invalid hex-encoded plain text")
+	}
+
+	sig, err := hex.DecodeString(p.Signature)
+	if err != nil {
+		return fmt.Errorf("invalid hex-encoded signature")
+	}
+
+	if !pubkey.VerifySignature(value, sig) {
+		return fmt.Errorf("failed to verify the signature")
+	}
+
+	valid, err := address.VerifyPubKey(pubkey)
+	if err != nil {
+		return err
+	}
+
+	if !valid {
+		return fmt.Errorf("invalid address and public key combination provided")
+	}
+
+	return nil
+}
+```
+
+Finally, we need to modify the `generateChainLinkJSON` function to return a HEX encoded plain text:
+```go
+// generateChainLinkJSON returns build a new ChainLinkJSON instance using the provided mnemonic and chain configuration
+func generateChainLinkJSON(mnemonic string, chain chainlinktypes.Chain) (profilescliutils.ChainLinkJSON, error) {
+
+    ...
+
+	sig, pubkey, err := keyBase.Sign(keyName, []byte(value)) 
+	if err != nil {
+		return profilescliutils.ChainLinkJSON{}, err
+	}
+
+	return profilescliutils.NewChainLinkJSON(
+		profilestypes.NewBech32Address(addr, chain.Prefix),
+		profilestypes.NewProof(pubkey, hex.EncodeToString(sig), hex.EncodeToString([]byte(value))),
+		profilestypes.NewChainConfig(chain.Name),
+	), nil
+}
+```
+
+### Application link implementation
+
+While dealing with application links, we use the `Result_Success_` type to identify a successfully verified link. In order to force 
+the plain text to be HEX encoded, we need to modify the `Validate` function to perform such check:
+
+```go
+// Validate returns an error if the instance does not contain valid data
+func (r Result_Success_) Validate() error {
+
+    ...
+
+    if _, err := hex.DecodeString(r.Value); err != nil {
+        return fmt.Errorf("invalid hex-encoded plain text")
+    }
+
+	return nil
+}
+```
+
+Besides, we also need to change the function that is currently used by users to generate the signature using the Desmos CLI to 
+make sure it returns the plain text using the HEX encoding:
+```go
+// GetSignCmd returns the command allowing to sign an arbitrary value for later verification
+func GetSignCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "sign [value]",
+		Short: "Allows to sign the given value using the private key associated to the address or key specified using the --from flag",
+		RunE: func(cmd *cobra.Command, args []string) error {
+
+            ...
+
+			value := []byte(args[0])
+			bz, pubKey, err := txFactory.Keybase().Sign(key.GetName(), value)
+			if err != nil {
+				return err
+			}
+
+			// Build the signature data output
+			signatureData := SignatureData{
+				Address:   strings.ToLower(pubKey.Address().String()),
+				Signature: strings.ToLower(hex.EncodeToString(bz)),
+				PubKey:    strings.ToLower(hex.EncodeToString(pubKey.Bytes())),
+				Value:     hex.EncodeToString(value),
+			}
+
+			// Serialize the output as JSON and print it
+			bz, err = json.Marshal(&signatureData)
+			if err != nil {
+				return err
+			}
+
+			return clientCtx.PrintBytes(bz)
+		},
+	}
+
+	flags.AddTxFlagsToCmd(cmd)
+
+	return cmd
+}
+```
+## Consequences
+
+### Backwards Compatibility
+
+With this approach there SHOULD not be any problem with old chain and application links since since the signature was 
+verified during the creation process and this ADR only targets the new links that will be created. However, in order to 
+make sure that clients can verify all the links at the same way, we SHOULD keep the on-chain data consistent using a migration script 
+that transforms all currently stored plain texts from being UTF-8 encoded strings into HEX encoded strings.
+As a result, this feature is backwards compatible.
+
+### Positive
+
+* Ensure the signing and verification process for chain and application links work properly with arbitrary bytes
+
+### Negative
+
+(none known)
+
+### Neutral
+
+(none known)
+
+## Further Discussions
+
+## Test Cases [optional]
+
+## References
+
+- Issue [#636](https://github.com/desmos-labs/desmos/issues/636)
+- [The basics of UTF-8](https://www.codeguru.com/cplusplus/the-basics-of-utf-8/)