Class: ECIES::Crypt

Inherits:
Object
  • Object
show all
Defined in:
lib/ecies/crypt.rb

Overview

Provides functionality for encrypting and decrypting messages using ECIES. Encapsulates the configuration parameters chosen for ECIES.

Constant Summary collapse

DIGESTS =

The allowed digest algorithms for ECIES.

%w{SHA224 SHA256 SHA384 SHA512}
CIPHERS =

The allowed cipher algorithms for ECIES.

%w{AES-128-CBC AES-192-CBC AES-256-CBC AES-128-CTR AES-192-CTR AES-256-CTR}
IV =

The initialization vector used in ECIES. Quoting from sec1-v2: "When using ECIES, some exception are made. For the CBC and CTR modes, the initial value or initial counter are set to be zero and are omitted from the ciphertext. In general this practice is not advisable, but in the case of ECIES it is acceptable because the definition of ECIES implies the symmetric block cipher key is only to be used once.

("\x00" * 16).force_encoding(Encoding::BINARY)

Instance Method Summary collapse

Constructor Details

#initialize(cipher: 'AES-256-CTR', digest: 'SHA256', mac_length: :half, kdf_digest: nil, mac_digest: nil, kdf_shared_info: '', mac_shared_info: '') ⇒ Crypt

Creates a new instance of ECIES::Crypt.

Parameters:

  • cipher (String) (defaults to: 'AES-256-CTR') —

    The cipher algorithm to use. Must be one of CIPHERS.

  • digest (String, OpenSSL::Digest) (defaults to: 'SHA256') —

    The digest algorithm to use for HMAC and KDF. Must be one of DIGESTS.

  • mac_length (:half, :full) (defaults to: :half) —

    The length of the mac. If :half, the mac length will be equal to half the mac_digest's digest_legnth. If :full, the mac length will be equal to the mac_digest's digest_length.

  • kdf_digest (String, OpenSSL::Digest, nil) (defaults to: nil) —

    The digest algorithm to use for KDF. If not specified, the digest argument will be used.

  • mac_digest (String, OpenSSL::Digest, nil) (defaults to: nil) —

    The digest algorithm to use for HMAC. If not specified, the digest argument will be used.

  • kdf_shared_info (String) (defaults to: '') —

    Optional. A string containing the shared info used for KDF, also known as SharedInfo1.

  • mac_shared_info (String) (defaults to: '') —

    Optional. A string containing the shared info used for MAC, also known as SharedInfo2.



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# File 'lib/ecies/crypt.rb', line 38

def initialize(cipher: 'AES-256-CTR', digest: 'SHA256', mac_length: :half, kdf_digest: nil, mac_digest: nil, kdf_shared_info: '', mac_shared_info: '')
  @cipher = OpenSSL::Cipher.new(cipher)
  @mac_digest = OpenSSL::Digest.new(mac_digest || digest)
  @kdf_digest = OpenSSL::Digest.new(kdf_digest || digest)
  @kdf_shared_info = kdf_shared_info
  @mac_shared_info = mac_shared_info

  CIPHERS.include?(@cipher.name) or raise "Cipher must be one of #{CIPHERS}"
  DIGESTS.include?(@mac_digest.name) or raise "Digest must be one of #{DIGESTS}"
  DIGESTS.include?(@kdf_digest.name) or raise "Digest must be one of #{DIGESTS}"
  [:half, :full].include?(mac_length) or raise "mac_length must be :half or :full"

  @mac_length = @mac_digest.digest_length
  @mac_length /= 2 if mac_length == :half
end

Instance Method Details

#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 88

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

  group_copy = OpenSSL::PKey::EC::Group.new(key.group)
  group_copy.point_conversion_form = :compressed

  ephemeral_public_key_length = group_copy.generator.to_bn.to_s(2).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_text = 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(group_copy, OpenSSL::BN.new(ephemeral_public_key_text, 2))

  shared_secret = key.dh_compute_key(ephemeral_public_key)

  key_pair = kdf(shared_secret, @cipher.key_len + @mac_length)
  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

#encrypt(key, message) ⇒ String

Encrypts a message to a public key using ECIES.

@param key [OpenSSL::EC:PKey] The public key.

Parameters:

  • message (String) —

    The plain-text message.

Returns:

  • (String) —

    The octet string of the encrypted message.



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# File 'lib/ecies/crypt.rb', line 59

def encrypt(key, message)
  key.public_key? or raise "Must have public key to encrypt"
  @cipher.reset

  group_copy = OpenSSL::PKey::EC::Group.new(key.group)
  group_copy.point_conversion_form = :compressed
  ephemeral_key = OpenSSL::PKey::EC.new(group_copy).generate_key

  shared_secret = ephemeral_key.dh_compute_key(key.public_key)

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

  @cipher.encrypt
  @cipher.iv = IV
  @cipher.key = cipher_key
  ciphertext = @cipher.update(message) + @cipher.final

  mac = OpenSSL::HMAC.digest(@mac_digest, hmac_key, ciphertext + @mac_shared_info).byteslice(0, @mac_length)

  ephemeral_key.public_key.to_bn.to_s(2) + ciphertext + mac
end

#kdf(shared_secret, length) ⇒ String

Key-derivation function, compatible with ANSI-X9.63-KDF

Parameters:

  • shared_secret (String) —

    The shared secret from which the key will be derived.

  • length (Integer) —

    The length of the key to generate.

Returns:

  • (String) —

    Octet string of the derived key.



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# File 'lib/ecies/crypt.rb', line 127

def kdf(shared_secret, length)
  length >=0 or raise "length cannot be negative"
  return "" if length == 0

  if length / @kdf_digest.digest_length >= 0xFF_FF_FF_FF
    raise "length too large"
  end

  io = StringIO.new(String.new)
  counter = 0

  loop do
    counter += 1
    counter_bytes = [counter].pack('N')

    io << @kdf_digest.digest(shared_secret + counter_bytes + @kdf_shared_info)
    if io.pos >= length
      return io.string.byteslice(0, length)
    end
  end
end