Method: ECIES::Crypt#decrypt

Defined in:
lib/ecies/crypt.rb

#decrypt(key, encrypted_message) ⇒ String

Decrypts a message with a private key using ECIES.

Parameters:

  • key (OpenSSL::EC:PKey) —

    The private key.

  • encrypted_message (String) —

    Octet string of the encrypted message.

Returns:

  • (String) —

    The plain-text message.


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# File 'lib/ecies/crypt.rb', line 87

def decrypt(key, encrypted_message)
  key.private_key? or raise "Must have private key to decrypt"
  @cipher.reset

  ephemeral_public_key_length = key.group.generator.to_octet_string(:compressed).bytesize
  ciphertext_length = encrypted_message.bytesize - ephemeral_public_key_length - @mac_length
  ciphertext_length > 0 or raise OpenSSL::PKey::ECError, "Encrypted message too short"

  ephemeral_public_key_octet = encrypted_message.byteslice(0, ephemeral_public_key_length)
  ciphertext = encrypted_message.byteslice(ephemeral_public_key_length, ciphertext_length)
  mac = encrypted_message.byteslice(-@mac_length, @mac_length)

  ephemeral_public_key = OpenSSL::PKey::EC::Point.new(key.group, OpenSSL::BN.new(ephemeral_public_key_octet, 2))

  shared_secret = key.dh_compute_key(ephemeral_public_key)

  key_pair = kdf(shared_secret, @cipher.key_len + @mac_length, ephemeral_public_key_octet)
  cipher_key = key_pair.byteslice(0, @cipher.key_len)
  hmac_key = key_pair.byteslice(-@mac_length, @mac_length)

  computed_mac = OpenSSL::HMAC.digest(@mac_digest, hmac_key, ciphertext + @mac_shared_info).byteslice(0, @mac_length)
  computed_mac == mac or raise OpenSSL::PKey::ECError, "Invalid Message Authenticaton Code"

  @cipher.decrypt
  @cipher.iv = IV
  @cipher.key = cipher_key

  @cipher.update(ciphertext) + @cipher.final
end