Programmable Envelopes

Programmable envelopes contain predefined fields to allow different types of messages to be sent according to specific use case requirements. Mailchain is flexible and extensible, adding a messaging layer to blockchain protocols, supporting different types of messages to be sent. Blockchain protocols typically allow sending data as a byte array, which is efficient but not descriptive. Programmable Envelopes allow descriptive implementations of messaging use cases that can be encoded to a byte array.
Each envelope is implemented as code, describing the encoding method, fields, and interface functions. 
Programmable Envelopes are preferred over adding optional fields. They remove some of the complexity, the need for additional documentation, are extensible in a modular sense, and create an improved developer experience.
Backwards and forwards compatibility can be achieved by never modifying an envelope after deployment.
Envelopes
0x00 - Reserved for unset value
Zero value is reserved to identify that it has not been set and should be treated like an unset envelope identifier. 
0x01 - Private Message Stored with MLI
The Private Message Stored with MLI (Message Location Identifier) is optimized for secure, verifiable messaging with minimal bytes, URL and ContentsHash are encrypted with a publicly verified IntegrityHash. 0x01 uses MLI to reduce the bytes needed to store the URL, the IntegrityHash is optional reducing the required number of bytes.
Encoding
​Protocol buffers are used to define the structure of this message. This is the proto3 spec. 
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// Use hosted location where the decryptedhash is the same as the location. Location, decrypted hash are encrypted so only receipient can location and verify the message.
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message ZeroX01 {
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    bytes UIBEncryptedLocationHash = 1;
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    bytes encryptedHash = 2;
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}
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Fields
Name
Type
Notes
UIBEncryptedLocationHash
bytes
Required - Bytes are stored are represented with UInt64Bytes.
encryptedHash
bytes
Optional - Bytes of hash
0x02 - Private Message Stored on IPFS
The Private Message Stored on IPFS (InterPlanetary File System) is optimized for secure, verifiable messaging with minimal bytes, URL and IntegrityHash are encrypted with a verified ContentsHash. 0x02 uses an ipfs:// URL, the ContentsHash is required.
Encoding
​Protocol buffers are used to define the structure of this message. This is the proto3 spec. 
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// Use hosted location where the encrypted hash is the same as the location. Location and decrypted hash are encrypted so only receipient can location and verify the message.
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message ZeroX02 {
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    bytes UIBEncryptedLocationHash = 1;
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    bytes decryptedHash = 2;
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}
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Fields
Name
Type
Notes
UIBEncryptedLocationHash
bytes
Required - Bytes are stored are represented with UInt64Bytes.
decryptedHash
bytes
Required - Bytes of hash
0x50 - Alpha
Implementation
Encoding
The first byte will describe the envelope implementation and encoding method used.
Byte Value
Notes
0x00
Reserved for unset value
0x01
​Private message stored with MLI​
0x50
​Alpha envelope​
0xFF
Reserved to extend beyond 255 envelope definitions
While Protocol buffers are used by the first implemented envelopes, this is not a requirement. Each envelope describes it encoding method in the definition.
Interface
Each envelope is required to implement the data interface. The interface enables each different envelope to define its implementation of the methods, this allows for different combinations of fields or logic to return a valid response for each of these methods. 
This interface is defined as:
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type Data interface {
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	URL(decrypter cipher.Decrypter) (url *url.URL, err error)
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	IntegrityHash(decrypter cipher.Decrypter) (hash []byte, err error)
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	ContentsHash(decrypter cipher.Decrypter) (hash []byte, err error)
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	Valid() error
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}
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Methods
URL
Addressable location of the message, the URL may be encrypted requiring decrypter to be supplied to decrypt URL.
Parameters:
Name
Type
Notes
decrypter
​cipher.decrypter​
Used to decrypt the URL where the message can be found. Decrypter is required but is only used if the url is stored encrypted.
Returns:
Name
Type
Notes
url
​*url.URL​
Address of the message. Nil if error.
err
error
Error object describing the cause.
IntegrityHash
Hash of the encrypted content. This can be used to validate the integrity of the envelope contents before decrypting. 
Parameters:
Name
Type
Notes
decrypter
​cipher.decrypter​
Used to decrypt the hash, where only the intended recipient can validate the contents. Decrypter is required but is only used if the hash is stored encrypted.
Returns:
Name
Type
Notes
hash
​[]byte​
Hash of the contents to validate integrity. Nil if error.
err
error
Error object describing the cause.
Integrity Hash is not intended to be cryptographically secure, use ContentsHash for a secure cryptographic hash.
ContentsHash
Hash of the decrypted content. This can be used to verify the contents of the message have not been tampered with.
Parameters:
Name
Type
Notes
decrypter
​cipher.decrypter​
Used to decrypt the hash, where only the intended recipient can verify the contents. Decrypter is required but is only used if the hash is stored encrypted.
Returns:
Name
Type
Notes
hash
​[]byte​
Hash of the contents to validate the message contents. Nil if error.
err
error
Error object describing the cause.
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