Saving private key to file

[Vineet77]

Module keys
Branch save_key

Now that we have the ability to generate public and private keys. We need the ability to save the private key to a file. We will save the key in a PEM file format.

The header will be PRIVATE KEY.

The function is savePrivateKeyToFile. The function will take a private key and a name, and it will say that key (PEM encoded) in a file <name>.pem. The file should be located under the resources directory.

In order to save to the resources directory, you will need to look into Java's classloader behavoir.
Here are a couple of links to give you a way forward:

• https://stackoverflow.com/questions/14089146/file-loading-by-getclass-getresource
• https://docs.oracle.com/javase/8/docs/technotes/guides/lang/resources.html
• https://stackoverflow.com/a/24954082/5327161 (make sure you read this answer, it might prove useful later on).

Unit tests for the function are already provided, so in other for the issue to be complete, they must pass.

In order to encode a private key to a PEM file, you must encode in into the DER format which, for the Elliptic Curve we use, have the following format:

Let says we have the following private key
0xf01daae1f05fab90ff1fd1c5ca7d6e4cbc2f4e63e6c12cddd4d4a0e0198c564f

And the following public key:
Public key X: 0x91b6287f3393e22f0f3c5f54f192e942f4927d594e6f23404b334beb968cc569
Public key Y: 0x0ba2dbd69c4941b0b4f2c3cb8acd8599bb2715873ed607a4db321d4c431404d9

Then the DER encoding would be:

0x30 -First Byte, declares the start of an ASN.1 sequence
0x77 -Second Byte, length of following sequence
0x02 -Third Byte, declares the start of an integer
0x01 -Fourth Byte, length of integer in bytes (1 byte)
0x01 -Fifth Byte,value of integer (1)
0x04 -Sixth Byte, declares the start of an octet string
0x20 -Seventh Byte, length of string to follow (32 bytes)
 -Eight Byte to 39th byte, private key: 
0xf01daae1f05fab90ff1fd1c5ca7d6e4cbc2f4e63e6c12cddd4d4a0e0198c564f
0xa0 -40st Byte, declares the start of context-specific tag 0
0x0a -41nd Byte, length of context-specific tag
0x06 -42rd Byte, declares the start of an object ID
0x08 -43th Byte, length of object ID to follow 
 -44th Byte to 51th byte, object ID of prime256v1: 
0x2a8648ce3d030107
0xa1 -52nd Byte, declares the start of context-specific tag 1
0x44 -53rd Byte, declares the length of context-sepcifc tag (68 bytes)
0x03 -54th Byte, declares the start of a bit string
0x42 -55th Byte, length of bit string to follow (66 bytes)
0x00 -56th Byte, padding?
0x04 -57th Byte, Indicating uncompressed public key
 -58th Byte to 89th byte, public key x: 
0x91b6287f3393e22f0f3c5f54f192e942f4927d594e6f23404b334beb968cc569
 -90th Byte to 121th byte, public key y: 
0x0ba2dbd69c4941b0b4f2c3cb8acd8599bb2715873ed607a4db321d4c431404d9

Then you take that binary encoding, encode in Base64. Insert a newline character every 64 characters and then add -----BEGIN EC PRIVATE KEY----- and -----END EC PRIVATE KEY----- at the beginning and end respectively.

The end result will be something like this:

-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIPAdquHwX6uQ/x/Rxcp9bky8L05j5sEs3dTUoOAZjFZPoAoGCCqGSM49
AwEHoUQDQgAEkbYofzOT4i8PPF9U8ZLpQvSSfVlObyNASzNL65aMxWkLotvWnElB
sLTyw8uKzYWZuycVhz7WB6TbMh1MQxQE2Q==
-----END EC PRIVATE KEY-----

And then you save this to the file.

Here are some helpful links to understand this stuff better:

• https://www.cryptologie.net/article/260/asn1-vs-der-vs-pem-vs-x509-vs-pkcs7-vs/
• https://tls.mbed.org/kb/cryptography/asn1-key-structures-in-der-and-pem
• https://bitcoin.stackexchange.com/questions/66594/signing-transaction-with-ssl-private-key-to-pem/66622#66622
• https://stackoverflow.com/questions/48101258/how-to-convert-an-ecdsa-key-to-pem-format

If you have any problems with this issues, feel free to contact me.