Exercises: EdDSA Sign and Verify

In this exercise we shall sign and verify messages using the EdDSA digital signature algorithm and the edwards25519 curve, following the technical specification from RFC 8032. The Ed25519 digital signature algorithm can be found as library for the most programming languages.

The Ed25519 private key is encoded as 64 hex digits (32 bytes). The corresponding Ed25519 public key is encoded also as 64 hex digits (32 bytes). The EdDSA-Ed25519 signature {R, s} consists of 32 + 32 bytes (64 bytes, 128 hex digits).

EdDSA-Ed25519: Sign Message

Write a program to sign given text message with given private key. The input consists of 2 text lines. The first line holds the input message for signing. The second line holds the private key as hex string. Print the output as JSON document, holding the input message + the public key of the signer (as hex string, uncompressed) + the Ed25519 digital signature (as hex string).

Sample input:

Message for Ed25519 signing
de6d730f36a8607b8bfdaa79b3b1127291f1d50552c2fe05c5254a9719105c4a

Sample output:

{
  "msg": "Message for Ed25519 signing",
  "pubKey":"7721a5832cb70cce1a960cf236d50a0e862555ccad400b5fee0bcf777f7ab476",
  "signature":"6c4adbba332b5db520c0ec95433ea136f70fe2d50e8955a7049d216626a3491c0e5cbfefb8d779687cc9811311ccaf7cd07a0e96a570fb3a4b680a4ead60c602"
}

EdDSA-Ed25519: Verify Signature

Write a program to validate the Ed25519 digital signature, created by the previous exercise. The input comes as JSON document, holding the message + the public key (uncompressed, hex string) + the signature. Print as output a single word: "valid' or "invalid".

Sample input (correctly signed message):

{
  "msg": "Message for Ed25519 signing",
  "pubKey":"7721a5832cb70cce1a960cf236d50a0e862555ccad400b5fee0bcf777f7ab476",
  "signature":"6c4adbba332b5db520c0ec95433ea136f70fe2d50e8955a7049d216626a3491c0e5cbfefb8d779687cc9811311ccaf7cd07a0e96a570fb3a4b680a4ead60c602"
}

Sample output:

valid

Sample input (tampered message):

{
  "msg": "Tampered msg",
  "pubKey":"7721a5832cb70cce1a960cf236d50a0e862555ccad400b5fee0bcf777f7ab476",
  "signature":"6c4adbba332b5db520c0ec95433ea136f70fe2d50e8955a7049d216626a3491c0e5cbfefb8d779687cc9811311ccaf7cd07a0e96a570fb3a4b680a4ead60c602"
}

Sample output:

invalid